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Year 13 Dr Aslam > Electric and Gravitational FIelds > Flashcards

Electric and Gravitational FIelds Flashcards

1
Q

What is a force field?

A

A region where an object will experience a non-contact force.

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2
Q

What do force fields cause?

A

Interactions between objects or particles.

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3
Q

What is a gravitational field?

A

A region where objects with mass will experience an attractive force.

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4
Q

How can a force field be represented?

A

Using field lines (or “lines of force”) that show the direction of the force that would be exerted on an object in a given position.

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5
Q

How are field lines used to show the strength of a field?

A

The further apart the lines are, the weaker the field.

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6
Q

Describe the gravitational field of the Earth.

A
  • It is radial, so the field lines meet at the centre of the Earth like a spiderweb* Close to the surface, the field can be considered almost uniform since the field lines are almost parallel and equally spaced
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7
Q

What will the force experienced by an object in a gravitational field always be?

A

Attractive

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8
Q

the gravitational force between two point masses (Newton’s Law of Gravitation) is found on the data sheet. what do the symbols stand for? F = Gm₁m₂ / r²

A

F = Gm₁m₂ / r²Where:• F = Force (N)• G = Gravitational constant = 6.67 x 10^-11 Nm²/kg² • m = Mass (kg)• r = Distance between centres of two point masses (m)

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9
Q

Where do we assume all the weight is concentrated for objects that experience a force?

A

In the centre - e.g. uniform spheres

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10
Q

When talking about a mass of an object in orbit, what is M and what is m

A 
M = mass of larger objectm = mass of smaller, orbiting object
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11
Q

How do you get the equation for speed of an object orbiting a larger object (e.g. a planet)? not found on data sheet

A

GMm/r² = mv² /r. equating centripetal force equation to gravity equationThe smalls m’s cancel and one of the r’s cancel.V = √GM/r

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12
Q

What is the equation of the time period of earths orbit?

A

Time = distance/speed.T = 2πr/v

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13
Q

What type of law is Newton’s Law of Gravitation and how can this be symbolised?

A
  • Inverse square law| * F ∝ 1 / r²
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14
Q

If the distance between 2 point masses is doubled, what happens to the magnitude of the gravitational force between them?

A

It is one quarter of the original.

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15
Q

What has a bigger impact on the size of the gravitational force, the distance between them or the mass?

A
  • The distance| * This can be seen with Newton’s Law of Gravitation
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16
Q

In gravitational calculations, what is G?

A
  • The gravitational constant* It is used in some equations* 6.67 x 10^-11 Nm²/kg²
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17
Q

What is gravitational field strength?

A

• The force per unit mass exerted at a given position in a gravitational field.OR• The acceleration of a mass in a gravitational field.

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18
Q

What is the symbol for gravitational field strength?

A

g

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19
Q

What are the units for gravitational field strength?

A

N/kg or m/s²

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20
Q

What is the equation that defines gravitational field strength?

A

g = F / mWhere:• g = Gravitational field strength (N/kg)• F = Force (N)• m = Mass (kg)

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21
Q

Is the value of g constant throughout a field?

A

No, its value depends on the where you are in the field.

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22
Q

What is the value of g at the Earth’s surface?

A

9.81 N/kg (or m/s²)

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23
Q

Because F is a vector, where is the direction of the force always towards? (common sense)

A

Towards the the centre of the mass which caused the gravitational force

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24
Q

Is g constant around the world?

A

The gravitational field is almost uniform at the Earth’s surface, so you can assume that g is a constant as long as you don’t go too high above the Earth’s surface.

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25
Q

The force on M1 due to M2 to equal and opposite to the force on…

A

M2 due to M1

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26
Q

In a radial field, how does g vary with the radius from the centre of the mass?

A

g is inversely proportional to r²

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27
Q

Describe the gravitational field around a point mass.

A

Radial

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28
Q

Give the equation for g around a point mass.

A

g = GM / r² ORg = -ΔV / Δr

Where:• g = Gravitational field strength (N/kg)• G = Gravitational constant (Nm²/kg²)• M = Point mass (kg)• r = Distance from centre (m)• V = Gravitational potential (J/kg)
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29
Q

What kind of law is the equation that gives g relative to the distance from a point mass?

A

Inverse square law (since g is inversely proportional to r²)

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30
Q

Describe the graph of g against r for a point mass.

A
  • Does not cross y-axis * Curve starts at its highest point at a certain x-value (RE - radius of the Earth)* It then curves like a 1/x² graph and never quite reaches the x-axis
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31
Q

Remember to practise drawing out the graph of g against r for a point mass.

A

See diagram of 121 of revision guide.

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32
Q

What is gravitational potential?

A

The gravitational potential energy that a unit mass would have at that point in a gravitational field.

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33
Q

What is the symbol for gravitational potential?

A

V

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34
Q

What is the equation for gravitational field strength with F and M?

A

g = F/M

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35
Q

What are the units for gravitational potential?

A

J/kg

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36
Q

g= F/M, what is the F?

A

Force experienced by a mass in the field

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37
Q

What is the difference between gravitational potential energy and gravitational potential?

A
  • Gravitational potential -> GPE that a unit mass would have at a given point in a gravitational field* Gravitational potential energy -> The energy that a mass has due to its position in a gravitational field
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38
Q

Why do we assume g is constant

A

Almost uniform field near earth’s surface

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39
Q

What is the equation for gravitational potential?

A

V = -GM / rWhere: • V = Gravitational potential (J/kg)• G = Gravitational constant = 6.67 x 10^-11 Nm²/kg² • M = Mass of point mass (kg)• r = Distance from centre of point mass (m)

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40
Q

What is unusual about gravitational potential and GPE? Why?

A
  • They are negative, since you can think of it of as negative energy since work has to be done to move an object out of the field* They becomes less negative with distance from the point mass* At infinite distance, the gravitational potential is 0J/kg and GPE is 0J
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41
Q

Which quantities in gravitational field questions are always negative?

A
  • Gravitational potential| * Gravitational potential energy (GPE)
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42
Q

Describe how gravitational potential (and GPE) changes with distance from a planet’s surface.

A
  • Most negative on the planet’s surface* Becomes less negative with distance from the planet* 0J/kg at infinite distance
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43
Q

At infinite distance from a planet, what is the gravitational potential and GPE?

A
  • Gravitational potential (0J/kg)| * GPE (0J)
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44
Q

Describe a graph of V against r for the Earth.

A
  • Does not cross y-axis * Curve starts at its most negative point at a certain x-value (RE - radius of the Earth)* It then curves like a -1/x graph and never quite reaches the x-axis
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45
Q

Because Gravitational fields are vectors, what can you do to them?

A

add up to find combined effect of more than 1 object

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46
Q

How can you work out the value of g at a certain point using a V-r graph for a point mass?

A
  • Find the gradient at any point| * This is because g = -ΔV / Δr
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47
Q

Describe a graph of g against r for the Earth.

A
  • Does not cross y-axis * Curve starts at its highest point at a certain x-value (RE - radius of the Earth)* It then curves like a 1/x graph and never quite reaches the x-axis
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48
Q

How do you work out ΔV using a g-r graph?

A
  • Area under the curve between two x-values| * Because -ΔV = g x Δr
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49
Q

Remember to practise drawing out all 3 gravitational field graphs. Also, practise finding different quantities from them.

A

Pgs 121 + 122 of revision guide

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50
Q

What is escape velocity?

A
  • The velocity at which an object’s kinetic energy is equal to minus its gravitational potential energy* It is the minimum velocity at which an object must travel in order to escape a gravitational field
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51
Q

Why is potential negative?

A

Have to do work against the field to move an object out of it.

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52
Q

What is an object’s total energy when it travels at escape velocity?

A
  • Zero| * Because the kinetic energy and GPE sum to 0 (since GPE is always negative)
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53
Q

What is the equation for escape velocity?

A

v = √(2GM/r)Where:• v = Escape velocity (ms⁻²)• G = Gravitational constant = 6.67 x 10^-11 Nm²/kg² • M = Mass of point mass (kg)• r = Distance from centre of point mass (m)NOTE: Not given in exam.

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54
Q

Derive the equation for escape velocity.

A
  • KE = 1/2mv²* GPE = -GMm/r* 1/2mv² = GMm/r* 1/2v² = GM/r* v² = 2GM/r* v = √(2GM/r)
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55
Q

What is the equation for GPE relative to G, M and r instead of mgh?

A

GPE = -GMm/r| This is derived from V = -GM/r

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56
Q

How do you derive GPE = -GMm/r

A

Work done = m x V.V = -GM/rreplace V with mV (which is work done) = mV = -GMm/r, which isGPE = -GMm/r

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57
Q

How do you find the change in kinetic energy of a satellite when it moves from and orbit of R1 to a lower orbit of R2?

A

(GPE lost = KE gained).v = √(GM / r).KE = 1/2 mv^2.KE = 1/2 m(√(GM / r))^2KE = GMm/2rChange in KE = GMm/2(R1) - GMm/2(R2)

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58
Q

Is escape velocity dependent on the mass of the object?

A

No, it is the same for all masses in a gravitational field.

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59
Q

What is gravitational potential difference?

A

The energy needed to move a unit mass between two gravity sonar potentials.

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60
Q

What is the equation for the work done when moving an object through a gravitational potential difference?

A

ΔW = mΔVWhere:• ΔW = Work fine (J)• m = Mass (kg)• ΔV = Gravitational potential difference (J/kg)

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61
Q

What are equipotentials?

A

Lines (in 2D) or surfaces (in 3D) that join all of the points with the same potential (V).If you travel along an equipotential, your potential doesn’t change.

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62
Q

How much work is done when moving an object along an equipotential?

A

0JChange in potential = 0Change in work done = Mass x change in potential.

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63
Q

Describe the equipotential around a uniform spherical mass.

A

Spherical surfaces

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64
Q

Describe how equipotential and field lines are related in gravitational fields.

A

They are perpendicular.

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65
Q

What force keeps an object undergoing circular motion in orbit?

A

Centripetal force

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66
Q

In the case of a satellite orbiting the Earth, what is the centripetal force?

A

Gravitational force.| They are kept in orbit by the gravitational “pull” of of the mass (Earth) they orbit.

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67
Q

Give the relationship between the time period and radius of an orbit.

A

• T² = 4π²r³ / GMSo• T² ∝ r³ (NOTE: Not given in exam)

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68
Q

Derive the relationship between the period and radius of an orbit.

A

Find two equations with force and velocity and find velocity, v.Then use the time period equation to change v into T:

• Centripetal force:F = mv² / r• Attraction due to gravity:F = GMm / r² • mv² / r = GMm / r²• v² = GMmr / r²m• v = √(GM / r)• Since one orbit is 2πr:v = 2πr / T• T = 2πr / v• T = 2πr / √(GM / r)• T = 2πr√r / √(GM)• T² = 4π²r³ / GM• Therefore:T² ∝ r³
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69
Q

How is the speed of a satellite related to its orbital radius?

A

• v = √(GM / r)So:• v ∝ 1 / √rSo greater radius = lower speed(NOTE: This comes from the first part of the T² ∝ r³ derivation.)

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70
Q

Remember to practise deriving the relationship between T and r for a satellite.

A

Pg 124 of revision guide

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71
Q

If T² ∝ r³, what can be said to be constant?

A

T² / r³ = Constant

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72
Q

What can be said about the energy of an orbiting satellite?

A

It is constant, since the kinetic and potential energy always sum to a constant value.

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73
Q

Why is a satellite’s energy constant in circular orbit?

A
  • Speed and distance above the Earth do not change* So the kinetic energy and potential energy are constant * So the total energy is always constant
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74
Q

Why is a satellite’s energy constant in elliptical orbit?

A
  • The satellite speeds up as it’s orbital radius decreases and slows down as orbital radius increases* So kinetic energy increases as potential energy decreases (and vice versa)* So the total energy remains constant
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75
Q

What is it important to remember about r?

A

It is measured from the centre of the orbit (or the centre of the point mass), not the surface of the Earth.

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76
Q

What is a synchronous orbit?

A

Where the orbital period is the same as the rotational period of the orbited object.

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77
Q

What are the two types of satellite?

A
  • Geostationary| * Low orbit
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78
Q

What are geostationary satellites?

A

Satellites that have the same angular speed as the Earth turns below them, so that they stay in the same position above the Earth.

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79
Q

Describe the orbit that geostationary satellites have.

A

Synchronous, along the equator.

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80
Q

What is the time period of orbit of a geostationary satellite?

A

1 day

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81
Q

What is the orbital radius of a geostationary satellite?

A

42,000km (about 36,000km above the Earth’s surface)

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82
Q

What are geostationary satellites used for?

A

Sending TV and telephone signals.

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83
Q

What are low orbit satellites?

A

Satellites that orbit between 180-2000km above the Earth, so that they do not stay in the same place relative to the Earth.

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84
Q

Describe the orbit that low-orbit satellites have.

A

Usually in a plane that includes the north and south pole.

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85
Q

Compare the advantages of low orbit satellites and geostationary satellites.

A

Low orbit• Cheaper to launch• Require less powerful transmitters since they are close to EarthGeostationary• Do not require multiple satellites to achieve constant reception in one area

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86
Q

How is T against r plotted?

A

Logarithmic scale:

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87
Q

At what height do low orbit satellites orbit?

A

180-2000km above the surface

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88
Q

What are low orbit satellites used for?

A
  • Communications -> Cheap to launch and do not require powerful transmitters, although many are required for constant coverage* Imaging and weather -> Due to being close enough to see surface in high detail
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89
Q

What type of satellite can be used to monitor the whole Earth and why?

A
  • Low orbit satellites| * Each orbit is over a different part of the Earth’s surface as the Earth rotates underneath
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90
Q

What kind of satellite is the ISS?

A

Low orbiting

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91
Q

State 2 reasons why rockets launched from the Earth’s surface do not need to achieve escape velocity to reach their orbit?

A

They don’t need to escape gravitational field, only need to reach the orbit = less energy required.Energy is added during the flight (with fuel) providing a continuous thrust.

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92
Q

Does any charged object have an electric field around it?

A

Yes

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93
Q

What is an electric field?

A

A region where charged objects will experience a non-contact force.

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94
Q

What is the unit for electric charge?

A

Coulombs (C)

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95
Q

What is the symbol for electric charge?

A

Q

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96
Q

Can charge be positive and negative?

A

Yes

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97
Q

Oppositely charged particles…

A

Attract

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98
Q

Like charges…

A

Repel

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99
Q

What happens when a charged object is placed in an electric field?

A

It experiences a force.

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100
Q

In electric field questions, what can he assumed about a charged object that is a sphere?

A

All of its charge is at its centre.

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101
Q

How can electric fields be represented?

A

Using field lines.

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102
Q

State Coulomb’s law.

A
  • The magnitude of the force between two charged objects is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.* F = 1/4πε₀ x Q₁Q₂/r²
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103
Q

Give the equation for Coulomb’s law.

A

F = 1/4πε₀ x Q₁Q₂/r²

Where:• F = Force (N)• ε₀ = Permittivity of free space = 8.85 x 10^-12 F/m• Q = Charge (C)• r = Distance between charges (m)
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104
Q

What type of law is Coulomb’s law?

A
  • Inverse square law| * Since F ∝ 1/r²
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105
Q

What is the significance of the ε in Coulomb’s law?

A
  • This is the permittivity of the material the charges are in* This affects the size of the force between the charges* If the system is in air, it can be considered the same as in a vacuum
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106
Q

What is electric field strength?

A

The force per unit positive charge exerted at a certain point in an electric field.

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107
Q

What is the symbol for electric field strength?

A

E

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108
Q

What is the unit for electric field strength?

A

N/C

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109
Q

What is the equation than defines electric field strength?

A

E = F/QWhere:• E = Electric Field Strength (N/C)• F = Force (N)• Q = Charge (C)

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110
Q

Is electric field strength a scalar or vector quantity?

A

Vector

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111
Q

Is electric field strength a constant?

A

No, it depends on where you are in the electric field (unless it is uniform).

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112
Q

What type of electric field does a point charge have?

A

Radial field

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113
Q

For 2 positive parallel plates, which way do the field lines point?

A

From the plate with more positive voltage to the plate with less positive voltage.

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114
Q

Give the equation for the electric field strength around a point charge.

A

E = 1/4πε₀ x Q/r²

Where:• E = Electric field strength (N/C)• ε₀ = Permittivity of free space = 8.85 x 10^-12 F/m• Q = Charge of point charge• r = Distance from the point charge
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115
Q

What type of equation is the equation that is used to find the electric field strength around a point charge?

A
  • Inverse square law| * Since E ∝ 1/r²
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116
Q

What happens to the field lines as you get further away from a point charge?

A

They get further apart.

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117
Q

Describe the graph for E against r for an electric field around a point charge.

A

1/x² graph.

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118
Q

When will a charged not follow the inverse square law?

A

If it isn’t a point charge (e.g. a metal sphere)

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119
Q

How can a uniform electric field be produced?

A

Connecting two parallel plates to opposite poles of a battery.

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120
Q

What can be said about electric field strength in a uniform electric field?

A

It is the same at all points.

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121
Q

What is the equation that defines electric field strength in a uniform electric field?

A

E = V/dWhere:• E = Electric field strength (N/C or V/m)• V = Potential difference (change) between plates (V)• d = Distance between plates (m)

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122
Q

What is an alternative unit for electric field strength in a uniform field?

A

V/m

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123
Q

What can a uniform electric field be used for? How?

A
  • Determining whether a particle is charged.* If a particle curves in the same direction as the field lines, it is positively charged* If a particle curves in the opposite direction as the field lines, it is negatively charged
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124
Q

What angle will a charged particle that enters an electric field feel a constant force parallel to the electric field lines?

A

enters the field at right angles

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125
Q

What is a particle’s curved path in an electric field called?

A

Parabola

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126
Q

What is absolute electric potential?

A

The electric potential energy that a unit positive charge would have at a point in an electric field.

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127
Q

What effects electric potential?

A

Size of charge creating the electric field and distance from the charge.

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128
Q

What is the symbol for electric potential?

A

V

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129
Q

What are the units for electric potential?

A

Volts (V)

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130
Q

Give the equation for electric potential around a point charge.

A

V = 1/4πε₀ x Q/r

Where:• V = Electric potential (V)• ε₀ = Permittivity of free space = 8.85 x 10^-12 F/m• Q = Charge of point charge• r = Distance from the point charge
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131
Q

When is V positive around a point charge?

A

When Q is positive. Force is repulsive

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132
Q

When is V negative around a point charge?

A

When Q is negative. Force is attractive

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133
Q

When is the magnitude of the electric potential around a point charge the greatest?

A

On the surface of the charge.

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134
Q

What is electric potential (V) equal to at infinite distance?

A

0V

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135
Q

Describe the graph of V against r around a point charge for a repulsive force.

A
  • 1/x² graph| * This is because a repulsive force must mean a positive point charge, so V is always positive.
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136
Q

Describe the graph of V against r around a point charge for an attractive force.

A
  • -1/x² graph| * This is because an attractive force must mean a negative point charge, so V is always negative.
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137
Q

What equation relates electric field strength with the change in electric potential around a point charge?

A

E = ΔV / ΔrWhere:• E = Electric field strength (N/C or V/m)• ΔV = Electric potential difference (V)• Δr = Change in distance from the charge (m)

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138
Q

How can electric field strength be found from a V-r graph around a point charge?

A
  • Gradient of tangent| * Because E = ΔV / Δr
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139
Q

How can potential difference between two points be found from an E-r graph around a point charge?

A
  • Area under graph between two points| * Because E = ΔV / Δr so ΔV = E x Δr
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140
Q

What is electric potential difference?

A

The energy needed to move a unit (positive(?)) charge between two points.

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141
Q

What equation gives the work required to move a charge through an electric potential difference?

A

ΔW = Q x ΔVWhere:• ΔW = Work done (J)• Q = Charge being moved (C)• ΔV = Electric potential difference (V)

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142
Q

What is the symbol for electric potential difference?

A

ΔV

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143
Q

Derive the formula for work done in moving a charge through an electric potential difference.

A
  • E = F / Q = ΔV / d* Fd = QΔV* ΔW = QΔV
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144
Q

What is the equation for the work done to move a mass through a gravitational field?

A

ΔW = mΔVWhere:• ΔW = Work done (J)• m = Mass (kg)• ΔV = Potential difference (ΔV)

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145
Q

Derive the equation for the work done to move a mass through a gravitational field.

A
  • g = -ΔV / Δr = F / m (since the gravitational field is considered near uniform near the Earth)* mΔV = -FΔr* ΔW = mΔV
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146
Q

What are equipotentials in electric fields?

A

Lines that show all points of equal potential in the electric field.

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147
Q

What shape are equipotentials around a point charge?

A

Spherical

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148
Q

Describe what equipotentials look like between parallels plates (in a uniform electric field).

A

They are parallel to each plate, with equal spacing.| Right angles to field lines

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149
Q

What are the Inverse square laws that are seen in both electric and gravitational fields?

A
  • Force between two masses / point charges| * Field strength around a mass / point charge
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150
Q

Describe how the electric and gravitational field equations differ.

A
  • Q is used instead of m (or M)| * 1/4πε₀ is used instead of G
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151
Q

Remember to practise listing all the similarities between electric and gravitational fields.

A

Pg 130 of revision guide or pg 300 of revision guide

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152
Q

What is the one important difference between electric and gravitational fields?

A

Gravitational fields are always attractive, whereas electric forces can be attractive or repulsive.

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153
Q

At sub-atomic level, does electrostatic or gravitational force have a greater effect and why?

A
  • Electrostatic* Because the masses are tiny, so the gravitational force is also tiny* NOTE: There are other forces that keep the nucleus stable
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154
Q

When can charge transfer between two objects?

A

When they slide past each other

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155
Q

When two objects slide past each other, how is charge transferred?

A

Electrons leave one surface and join the other

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156
Q

What is earthing?

A

When electrons move to or from the earth to balance charges on charged objects

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157
Q

What happens, when charge is transferred between objects, if one of the objects is an insulator?

A

The charge can build up

158
Q

Why does charge build up on the dome of a Van de Graaf generator?

A

The dome is metal, but charge builds up as it is isolated

159
Q

What happens when a charged polythene rod is brought near to uncharged objects?

A

It attracts the object

160
Q

How do you get charge onto an electroscope?

A

∙ charged object e.g. polythene rod is brought near the metal plate∙ gold leaf moves towards rod∙ electroscope is earthed so charge is stored

161
Q

How can electric fields be represented?

A

Field lines

162
Q

What do field lines show?

A

The direction of the force that would be felt by a small positive charge

163
Q

What do equally spaced field lines show?

A

A uniform field

164
Q

What practical can be done to plot field lines?

A

Electrolytic tanks and conducting paper∙ damp filter paper, potassium permanganate and 250V electrodes∙ plot equipotential lines using a point probe attached to a voltmeter (field lines plotted perpendicular to equipotential line)

165
Q

What are the rules about field lines?

A

∙ never start or stop in empty space∙ never cross∙ density of field lines shows strength of field

166
Q

Where do field lines stop and start?

A

Either on a charge or at infinity

167
Q

Why do field lines never cross?

A

If they did, a small positive charge place there would feel forces in different directions, which could be resolved into the one true direction of the field there

168
Q

What is a neutral point?

A

A point exactly between two like charges where no field exists

169
Q

What is the equation for electric field strength?

A

E = F / Q

170
Q

What is field strength?

A

The force per unit (positive) charge exerted by the field

171
Q

What are the units for electric field strength?

A

N C-1

172
Q

Summary of Coulomb’s law?

A

The force between two point charges is∙ directly proportional to each of the charges Q1 and Q2∙ inversely proportional to the square of their separation

173
Q

What is the equation for Coulomb’s law?

A

F = kQ1Q2 / r²

174
Q

What does k depend on in F=kQ1Q2 / r²?

A

The permittivity of the substance separating the charges

175
Q

What permittivity does every insulating material have?

A

Greater than the permittivity of free space

176
Q

What does it mean, in terms of charges, that the permittivity of water is about 80 times the permittivity of free space?

A

Makes the forces between charges 1/80th of value - when salt is put in water the forces are reduced and crystal structure collapses

177
Q

What equation do you get when the electric field and coulombs law equation are combined?

A

E = kQ / r²

178
Q

What does k equal in E = kQ / r²?

A

1 / 4πℇ

179
Q

How can the strength of the uniform field between parallel plates be made stronger?

A

∙ increase p.d. across plates∙ moving plates closer together

180
Q

Equation for strength of a uniform field?

A

E = V / d

181
Q

What can electric field strength be measured in?

A

Vm-1 or NC-1

182
Q

Why can electric field strength be measured in Vm-1 or NC-1?

A

Work is done moving an object, and work is done when a charge moves through a p.d.

183
Q

How are electrons used in TVs, oscilloscopes and X-ray machines?

A

∙ electron gun produces electrons by thermionic emission∙ then they are accelerated by an electric field∙ as electrons accelerate across field, they lose potential energy but gain kinetic (gain in Ek = loss in Ep)

184
Q

What happens in an electron deflection tube?

A

Moving electrons pass through an electric field between two plates

185
Q

What is the equation for the force on an electron in an electron beam, and why is this?

A

∙ F = e V / d(as E=V/d and force on an electron charge is F=Ee

186
Q

What is the shape of the path when electron beams are deflected?

A

Curved, as the force (F=eV/d) is constant

187
Q

What is the difference between electric and gravitational fields, in terms of what ‘feels’ the force?

A

∙ electric - charge q∙ gravitational - mass m

188
Q

What is the difference between electric and gravitational fields, in terms of their definition?

A

∙ electric - force per unit charge∙ gravitational - force per unit mass

189
Q

What is the difference between electric and gravitational fields, in terms of their constant of proportionality?

A

Both inversely proportional to r²

190
Q

What is the difference between electric and gravitational fields, in terms of their force equation?

A

∙ electric - F=Eq∙ gravitational - F=mg

191
Q

What is the difference between electric and gravitational fields, in terms of their direction of force?

A

∙ electric - like charges repel, unlike charges attract ∙ gravitational - all masses attract

192
Q

What is the difference between electric and gravitational fields, in terms of their relative strength?

A

∙ electric - strong at close range (responsible for chemical bonding)∙ gravitational - weak except for massive bodies (responsible for motion of planets)

193
Q

Do all points in an electric field have an absolute electric potential?

A

Yes

194
Q

What is absolute electric potential?

A

The electric potential energy that a unit positive charge would have at that point

195
Q

What is absolute electric potential calculated using?

A

V = 1/4πℇ x Q/r

196
Q

What does r equal in V = 1/4πℇ x Q/r?

A

Distance from the charge Q

197
Q

What does the sign of electric potential depend on?

A

The sign of Q (V is +ve when Q is +ve and the force is repulsive)

198
Q

When will V (electric potential) be zero?

A

When r is

199
Q

For a V-r graph, what does the gradient of a tangent to the graph give?

A

The field strength at that point

200
Q

When will two points in an electric field have an electric potential difference between them?

A

If they have a different absolute electric potential

201
Q

What is electric potential difference?

A

The energy required to remove a unit charge between two points in an electric field that have a different electric potential

202
Q

How can electric potential difference be found?

A

Using the area under the graph of E against r

203
Q

Where are field lines in relation to equipotential lines?

A

Perpendicular

204
Q

What happens in terms of energy when you travel along an equipotential line?

A

No work is done so no energy is transferred

205
Q

For a charged sphere, where may charge considered to be?

A

At the centre

206
Q

When is electric potential zero?

A

At infinity

207
Q

What equation links V to E?

A

E = ΔV/Δr

208
Q

What is G?

A

The universal gravitational constant

209
Q

RATE THIS 5

A

hh

210
Q

What does the formula F = Gm1m2/r2 apply to?

A

Point masses, but spherical masses can be treated as point masses with all their mass concentrated in the centre

211
Q

Is the force in F = Gm1m2/r2 attractive or repulsive?

A

Always attractive

212
Q

What is a field?

A

A region of space around an object where other bodies feel a force due to it

213
Q

Is your pull on the earth the same as the earth’s pull on you?

A

Yes, but your gravitational field is far weaker

214
Q

What is gravitational field strength ?

A

A point in a body’s field as the gravitational force exerted on an object placed at that point, per kg of the object’s mass

215
Q

If two objects of different masses are placed at the same point in a field will they experience the same field strength and gravitational force?

A

Will experience same field strength, but different gravitational forces

216
Q

When calculating gravitational field strength, what is the mass that matters?

A

The mass below you only

217
Q

Where does g = GM/r2 come from?

A

When objects are a distance from earth e.g. other planets* F = Gm1m2/r2* small mass m, and planet mass M - F = GMm/r2* but F=mg so mg = GMm/r2* therefore g = GM/r2

218
Q

What is Kepler’s third law?

A

The time of one orbit T, and the distance from the planet to the sun r, are related by T² ∝ r³

219
Q

How is T² ∝ r³ proven?

A
  • planet remains in orbit due to centripetal force (F=mv2/r)* F=Gm1m2/r2* GMm/r2 = mv2/r* but v = 2πr/T* so GM/r2 = 4π2r/T2* T2 = (4π2/GM) r3
220
Q

What are geostationary satellites used for?

A

Communications

221
Q

How many geostationary satellites would be able to cover the entire earth?

A

3 placed into orbit 120 degrees apart above the equator

222
Q

Why are the energy and costs required for launching a satellite into a geostationary orbit high?

A

Because they have to be launched so high

223
Q

Equation for GPE in a uniform field?

A

Change in GPE = mgh

224
Q

What is the value of GPE at infinite distance?

A

0

225
Q

What is GPE proportional to, in terms of r?

A

1/r

226
Q

What is the equation for GPE in a radial field?

A

GPE = –GMm/r| note this is not change in GPE

227
Q

When GPE = –GMm/r, what happens to GPE as you move closer to M?

A

GPE becomes more negative as at infinite distance GPE=0

228
Q

Why did someone come up with gravitational potential?

A

To get an expression involving energy which is independent of the mass placed in a field

229
Q

What are the two ways gravitational potential can be thought about?

A
  1. GP at a point in a field is the energy per unit mass (V=Ep/m so V=gh)2. GP is work done per unit mass in moving a small object from infinity to that point (ΔV=ΔE/m)
230
Q

Is gravitational potential a scalar or vector?

A

Scalar

231
Q

Is gravitational field strength vector or scalar?

A

Vector

232
Q

Is the force between two masses vector or scalar?

A

Vector

233
Q

Is gravitational potential energy vector or scalar?

A

Scalar

234
Q

What is the unit for gravitational potential?

A

J kg-1

235
Q

What do equipotentials do?

A

Join points of equal potential

236
Q

When V = -GM/r is used to calculate V at earth’s surface, V = -63 MJ kg-1. What does this mean?

A

63mJ of work needs to be done in order to move 1kg from the earth’s surface to infinity

237
Q

What type of graph is the graph of V against r?

A

A 1/r curve, not an inverse-square

238
Q

What is the derivation for the escape velocity equation?

A
  • Ep = -GMm/r and Ek=1/2mv²* energy is conserved so Ep+Ek=0* -GMm/r + 1/2mv² = 0* so v = √2GM/r* or √2gr as gr=GM/r
239
Q

What is a null point?

A

A point between two masses where the resultant g force is zero

240
Q

What are low earth orbits?

A
  • satellite systems used in telecommunications| * orbit between 400 and 1,000 miles above the earth’s surface
241
Q

What is a synchronous orbit?

A

An orbit where the satellite has a period equal to that of the body being orbited

242
Q

What is the radius of a geostationary orbit?

A

approx 42000km

243
Q

What do capacitors do?

A

Store electric charge

244
Q

Where are capacitors used?

A

In almost all electric circuits

245
Q

What does a capacitor consist of?

A

Two parallel metal plates separated by an insulator called a dielectric

246
Q

What is a dielectric?

A

The insulator in a capacitor that separates the two metal plates

247
Q

When will a capacitor have a greater capacitance?

A

When it can store more charge

248
Q

What is the capacitance of a capacitor?

A

The charge stored per unit of potential difference across it

249
Q

What is the equation for capacitance?

A

C = Q/V

250
Q

What is the unit of capacitance?

A

the farad, F| =1CV⁻¹

251
Q

Are farads large or small units?

A

Very large

252
Q

How are farads usually marked?

A

In pico or microfarads| 1 pF = 10⁻¹²

253
Q

What may a capacitor have marked on it?

A

A working voltage which must not be exceeded

254
Q

Roughly, how much electric charge do capacitors store?

A

Small amounts, providing power for a short amount of time

255
Q

Why can charged capacitors be dangerous?

A

They can discharge all of their charge in a fraction of a second

256
Q

Where can capacitors be used?

A
  • camera flash* back up power supply* to smooth dc power supplies
257
Q

What happens in terms of electrons when a capacitor is charged?

A
  • electrons are pushed onto one plate and off the other* the power supply does work on the electrons so the Ep increases* this Ep is stored in the electric field between the plates
258
Q

In a V-Q graph, what is the area underneath the graph?

A

Energy stored

259
Q

Explain how a capacitor ‘gets’ charge.

A
  • during charging electrons flow from -ve terminal of power supply to one plate of capacitor and from other plate to +ve terminal* switch closed, charging starts, rate of flow of charge is large (i.e. big current) and this decreases with time and plates become more charged so resist further charging
260
Q

What happens, when charging a capacitor, to the rate of flow of charge?

A
  • at first rate of flow of charge/current is large| * this decreases with time as the plates become more charged so resisting any further charging
261
Q

What effect would adding a resistor in the circuit when charging a capacitor have?

A

Only affects time taken for capacitor to become fully charged and not the eventual p.d. across it

262
Q

What does the gradient of the tangent to the curve at a point on a Q-t graph give?

A

The current at that time

263
Q

How to find current at a particular time using a Q-t graph?

A

Gradient of tangent at a certain point

264
Q

On a current-time graph, what does the area underneath the curve give?

A

The charge stored

265
Q

What is T½ on a Q-t graph?

A

The time for the charging current to half

266
Q

What actually happens in a discharge circuit?

A
  • as soon as switch is closed, ‘large’ current flows and p.d. across capacitor drops* as charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls * rate of decrease of p.d. also falls
267
Q

When is a capacitor fully discharged?

A

When the charge on the plates is zero and the current and p.d. are also zero

268
Q

How does a resistor affect the discharge of a capacitor?

A

The value of the resistor doesn’t affect the final p.d., only the time it takes to reach this value

269
Q

How does a larger resistor affect the time taken for a capacitor to discharge?

A

The bigger the resistor, the longer time taken to discharge

270
Q

What can a discharge curve apply to?

A

V, Q or I against time

271
Q

What is the area under the discharge curve of a I-t graph equal to?

A

The charge that has flowed

272
Q

What is the gradient of the tangent on a Q-t graph equal to?

A

The current at that point in time

273
Q

What is half life equal to?

A

0.69RC

274
Q

What is the time constant?

A

The time taken for the p.d. across the capacitor and the charge stored on the capacitor to drop to 1/e of their original values

275
Q

What is the gradient of the graphs of loge(current) etc. graphs?

A

All have same gradient; -1/RC

276
Q

What equation shows the factors affecting the capactiance of a parallel plate capacitor?

A

C = Aε0εr/d

277
Q

What does C equal in C = Aε0εr/d?

A

Capacitance (F)

278
Q

What does A equal in C = Aε0εr/d?

A

Cross sectional area of overlap of the plates (m²)

279
Q

What does ε0 equal in C = Aε0εr/d?

A

Permittivity of free space (Fm⁻¹)

280
Q

What does εr equal in C = Aε0εr/d?

A

Relative permittivity of the dielectric

281
Q

What does d equal in C = Aε0εr/d?

A

Distance between the plates (m)

282
Q

What is relative permittivity also known as in terms of capacitors?

A

The dielectric constant of the material

283
Q

What is εr calculated from?

A

εm / ε0 where εm is the permittivity of the material used as the dielectric

284
Q

What is the unit of εr?

A

No unit

285
Q

What is a dielectric material (dielectric for short)?

A

An electrical insulator that can be polarised by an applied electric field

286
Q

What happens when a dielectric is placed in an electric field?

A

Electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarisation

287
Q

When does dielectric polarisation occur?

A

When a dielectric is placed in an electric field and the electric charges shift from the average equilibrium positions

288
Q

What is the effect of dielectric polarisation?

A
  • +ve charges are displaced toward field and -ve shift in opposite direction* creates internal electric field that reduces overall field within dielectric so reducing p.d. across capacitor
289
Q

How is an internal electric field created in a capacitor?

A

The dielectric is placed in an electric field, dielectric polarisation occurs and creates an internal electric field

290
Q

What is the effect of an internal electric field in a dielectric?

A

It reduces the overall field within the dielectric itself, reducing the p.d. across the capacitor

291
Q

How can the p.d. return to its original value after dielectric polarisation?

A

Requires the addition of more charge onto the plates

292
Q

What effect does dielectric polarisation have on the capacitor overall?

A

The capacitor can store more charge for the same p.d. and increase its capacitance

293
Q

When will a wire in a magnetic field experience a force?

A

When it carries a current

294
Q

How can the direction of the force on a current carrying wire be worked out?

A

Using Fleming’s left hand rule

295
Q

What does each finger mean in Fleming’s left hand rule?

A

Thumb = Thrust/Force First finger = fieldSecond finger = Current

296
Q

What equation is used to work out the magnitude of force on a current-carrying wire?

A

F = BIL

297
Q

What does F mean in F=BIL?

A

Force (N)

298
Q

What does B mean in F=BIL?

A

Flux Density (T)

299
Q

What does L mean in F=BIL?

A

Length of conductor in field (m)

300
Q

What does I mean in F=BIL?

A

Current (A)

301
Q

How is the tesla defined using words?

A

One newton per amp per metre

302
Q

How is the tesla defined using the equation?

A

1T = 1 N/Am

303
Q

How can the strength of a magnetic field be measured?

A

By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field

304
Q

How can magnetic flux density B be measured?

A

By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field

305
Q

What does the right hand thumb rule determine?

A

The direction of the magnetic field in a direction of current

306
Q

What directions are field and current relative to each other in F=BIL?

A

Perpendicular

307
Q

How is a wire with the current coming towards you drawn?

A

A circle with a dot in

308
Q

How is a wire with the current travelling away from you drawn?

A

A circle with a cross in like an arrow

309
Q

What happens to a charged particle when it moves through a magnetic field?

A

It experiences a force

310
Q

When a charged particle moves through a magnetic field and experiences a force, what is this force proportional to?

A
  • B - magnetic flux density* Q - charge on the particle* v - velocity of the particle
311
Q

When a charge is moving in a magnetic field, how can the force be calculated?

A

F = BQv

312
Q

What condition must be true when F=BQv is used to calculate the force on a charge?

A

The charge must be moving at 90° to the field

313
Q

In the equation F=BQv, what is the direction of the force given by?

A

Fleming’s left hand rule

314
Q

When a charged particle moves at right angles a magnetic field, what is the constant force perpendicular to?

A

Both the velocity and the field

315
Q

Why does the constant force on a particle in a magnetic field change the particle’s direction of motion and has no effect on speed?

A

The constant force is perpendicular to both the velocity and field

316
Q

Does the force from a magnetic field on a charged particle affect the particle’s speed?

A

No

317
Q

Does the force from a magnetic field on a charged particle affect the particle’s direction of motion?

A

Yes

318
Q

What is the result when a constant force acts on a charged particle in a magnetic field?

A

The particle travels in a circular path

319
Q

What does it mean for a charged particle in a magnetic field, that the constant force is perpendicular to the velocity and field?

A

The particle travels in a circular path

320
Q

How is the equation for the radius of the circular path which a charged particle follows derived?

A
  • BQv = mv²/r| * r = mv/BQ
321
Q

What is the equation for the radius of the circular path which a charged particle follows?

A

r = mv/BQ

322
Q

How is the equation for the time period for an electron in a magnetic field to make one rotation derived?

A
  • time = distance/speed* time = length of circular path/speed of electron or T=2πr/v* since r = mv/BQ, T = 2πmv/vBQ* T = 2πm/BQ
323
Q

Equation for time period of an electron in a magnetic field to make one rotation?

A

T = 2πm/BQ

324
Q

In a magnetic field of constant flux density, does the time period of an electron depend on its speed?

A

No| a faster moving electron moves in a circle of larger radius, but takes the same time to make one revolution

325
Q

Where might the circular path of charged particles in a magnetic field be applied?

A

Cyclotron

326
Q

What is magnetic flux given by?

A

Φ = BA

327
Q

What does Φ mean in Φ=BA?

A

Magnetic flux (Wb)

328
Q

What does A mean in Φ=BA?

A

Cross sectional area (m2)

329
Q

What does B mean in Φ=BA?

A

Flux density (T)

330
Q

What is magnetic flux measured in?

A

weber (Wb)

331
Q

What is 1 weber defined as?

A

1 Wb = 1T m2

332
Q

What happens to the equation Φ = BA if the plane of the area is not perpendicular to the field?

A

Φ = BAcosΘ

333
Q

What is flux linkage?

A

The magnetic flux linking the coil

334
Q

What is flux linkage given by?

A

Flux linkage = NΦ

335
Q

What does Faraday’s law state?

A

The magnitide of the induced emf is equal to the rate of change of flux linkage

336
Q

What does N mean in flux linkage = NΦ?

A

The number of turns in a coil i.e. the number of turns cutting the flux

337
Q

What is the equation relating to Faraday’s law (words)?

A

Induced emf = change in flux linkage/time taken

338
Q

What is the equation relating to Faraday’s law (symbols)?

A

E = ΔNΦ / Δt

339
Q

What does Faraday’s law essentially tell us?

A

The size of the induced emf

340
Q

How can we find the direction of the induced emf described in Faraday’s law?

A

Using Lenz’s law

341
Q

What is Lenz’s law

A

The direction of the induced emf is such that it will try to oppose the change in flux that is producing it

342
Q

Equation that relates to Lenz’s law?

A

E = - ΔNΦ / Δt

343
Q

What does the minus sign in Lenz’s law equation show?

A

That the emf is always induced in a direction so as to oppose the change in flux

344
Q

What does a generator do?

A

Converts kinetic energy to electical energy

345
Q

What do the slip rings in a generator do?

A

Rotate with the coil and press against stationary carbon brushes

346
Q

In a generator, which side of the coil makes contact with which brush?

A

Each side of the coil always makes contact with the same brush

347
Q

In a generator, what happens to flux linkage as the coil rotates at a steady rate?

A

The flux linkage constantly changes

348
Q

In a generator, how much a.c. does one revolution of the coil give?

A

One revolution of the coil gives one cycle of a.c.

349
Q

Overall, what increases the peak emf of a generator?

A

By increasing the rate of change of flux linkage of the coil as it spins

350
Q

How can the rate of change of flux linkage of the coil in a generator be increased, in order to increase peak emf?

A
  • using a coil with more turns* using a coil with a larger cross-sectional area* increasing the strength of the magnetic field* increasing the frequency of rotation of the coil
351
Q

What will the frequency of rotation of the coil affect in a generator?

A

The rate of change of flux linkage of the coil, as well as the frequency of the a.c. signal

352
Q

In a generator, what is the relationship between the frequency of rotation of the coil and the frequency of the ac signal?

A

They are directly proportional

353
Q

What are the equations for flux linkage when looking at generators?

A
  • flux linkage = BANcosΘ* flux linkage = BANcosωt* flux linkage = BANωsinωt
354
Q

In the equation flux linkage = BANcosΘ, what does Θ depend on? What equation does this result in?

A

The angular speed ω of the coilgiving flux linkage = BANcosωt

355
Q

What does ω stand for?

A

Angular speed in rad s-1

356
Q

How does induced emf in a generator vary? Why is this?

A

Sinusoidally, as max. change happens when Θ=90°

357
Q

What type of current do cells and batteries supply?

A

Direct

358
Q

What is a direct current?

A

Current flowing in one direction only

359
Q

What type of current does mains supply?

A

Alternating

360
Q

What is the peak value of ac current or pd?

A

The maximum in either direction

361
Q

How can peak value of ac be measured?

A

From the wave as the amplitude

362
Q

How is peak current denoted?

A

I₀

363
Q

How is peak voltage denoted?

A

V₀

364
Q

What is the peak to peak value of current or pd?

A

The range of values - the distance from the peak above the zero line to the peak below the zero line

365
Q

What is the time period of an ac current?

A

The time taken for one complete cycle/wave

366
Q

What is the root mean squared?

A

A value of current that produces the same heating effect in a resistor as the equivalent dc

367
Q

Why, for an ac current, is it impossible to assign a value to pd and current for a fixed value of time? What is done instead?

A

Current and pd is constantly changing so average would be zero- root mean squared produces same heating effect in a resistor as the equivalent dc

368
Q

What is an oscilloscope used for?

A

To show the sizes of voltages and currents in both dc and ac circuits

369
Q

What does a dc trace on an oscilloscope look like? Why?

A

A straight line, as the current is constant, so the voltage is constant

370
Q

What does an ac trace on an oscilloscope look like? Why?

A

A wave, as the current is constantly changing from maximum flow in one direction to maximum flow in the other direction, so voltage does the same

371
Q

What controls do we use on an oscilloscope?

A
  • volts/div dial| * time base dial
372
Q

What does the volts/div dial on an oscilloscope allow you to do?

A

Change how much each vertical square is worth

373
Q

What does the time base dial on an oscilloscope allow you to do?

A

Change how much each horizontal square is worth

374
Q

How can you measure the voltage of a dc supply using an oscilloscope?

A

Counting number of vertical squares from origin to line and multiply by volts/div

375
Q

How can the time for one period be measured using an oscilloscope?

A

Counting how many horizontal squares one wavelength is and multiply by time base

376
Q

What does a transformer do?

A

Changes the value of ac voltage

377
Q

What do transformers consist of?

A

Two coils wound around a soft iron core

378
Q

How does a transformer work?

A
  • ac current flows in the primary coil* producing a changing magnetic flux in the soft iron core* meaning the flux linkage of the secondary coil is constantly changing* and so an ac voltage is induced across it
379
Q

What does a step-up transformer do? Why is this?

A

Increases ac voltage, as secondary coil has more turns than primary coil

380
Q

What does a step-down transformer do? Why is this?

A

Decreases ac voltage, as secondary coil has fewer turns than primary coil

381
Q

In a transformer, what is the ratio of voltages equal to?

A

The ratio of turns

382
Q

What equation shows the ratio of voltages in transformers?

A

Vs / Vp = Ns/ Np

383
Q

What happens in terms of energy losses in an ideal transformer?

A

No energy is lost

384
Q

What is the equation for power in an ideal transformer, and under what condition?

A

VpIp = VsIs| provided no energy is lost

385
Q

What are eddy currents?

A

Looping currents induced by the changing magnetic flux in the core

386
Q

What do eddy currents do?

A

Create a magnetic field that acts against the field that induced them

387
Q

How do eddy currents dissipate energy?

A

By generating heat

388
Q

How can the energy loss from eddy currents be reduced?

A

By laminating the core

389
Q

How can the efficiency of a transformer be calculated?

A

Ratio of power out to power in:E = IsVs / IpVp

390
Q

What is voltage stepped up to through the National Grid?

A

400 000 V

Year 13 Dr Aslam (13 decks)

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