Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

A LEVEL - Physics > Section 9 - Gravitational and Electric Fields > Flashcards

Section 9 - Gravitational and Electric Fields Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

What is a force field?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What do force fields cause?

A

Interactions between objects or particles.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is a gravitational field?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Practice drawing out the Earth’s gravitational field.

A

See diagram pg 120 of revision guide.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is Newton’s Law of Gravitation?

A
  • An equation used to calculate the gravitational force between two point masses
  • F = Gm₁m₂ / r²
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the equation for the gravitational force between two point masses (Newton’s Law of Gravitation)?

A

F = Gm₁m₂ / r²

Where:
• 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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

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

A
  • Inverse square law

* F ∝ 1 / r²

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

In gravitational calculations, what is G?

A
  • The gravitational constant
  • It is used in some equations
  • 6.67 x 10^-11 Nm²/kg²
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the symbol for gravitational field strength?

A

g

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What are the units for gravitational field strength?

A

N/kg or m/s²

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the equation that defines gravitational field strength?

A

g = F / m

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Is the value of g constant throughout a field?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

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

A

9.81 N/kg (or m/s²)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Describe the gravitational field around a point mass.

A

Radial

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Give the equation for g around a point mass.

A

g = GM / r²

OR

g = -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)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
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

(See diagram pg 121 of revision guide)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

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

A

See diagram of 121 of revision guide.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What is gravitational potential?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What is the symbol for gravitational potential?

A

V

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What are the units for gravitational potential?

A

J/kg

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What is the equation for gravitational potential?

A

V = -GM / r

Where:
• 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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Which quantities in gravitational field questions are always negative?

A
  • Gravitational potential

* Gravitational potential energy (GPE)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

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

A
  • Gravitational potential (0J/kg)

* GPE (0J)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
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

(See diagram pg 122 of revision guide)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
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

(See diagram pg 122 of revision guide)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
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 velocity at which an object must travel in order to escape a gravitational field
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
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)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

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

A

GPE = -GMm/r

This is derived from V = -GM/r

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

Is escape velocity dependent on the mass of the object?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

What is gravitational potential difference?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

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

A

ΔW = mΔV

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

What are equipotentials?

A

Lines (in 2D) or surfaces (in 3D) that join all of the points with the same potential (V).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

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

A

0J

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

Describe the equipotentials around a uniform spherical mass.

A

Spherical surfaces

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

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

A

They are perpendicular.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

What force keeps an object undergoing circular motion in orbit?

A

Centripetal force

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

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

A

Gravitational force

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

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

A

• T² = 4π²r³ / GM
So
• T² ∝ r³

(NOTE: Not given in exam)

56
Q

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

A 
• 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
57
Q

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

A

• v = √(GM / r)
So:
• v ∝ 1 / √r

(NOTE: This comes from the first part of the T² ∝ r³ derivation.)

58
Q

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

A

Pg 124 of revision guide

59
Q

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

A

T² / r³ = Constant

60
Q

Do the practise question on pg 124 of revision guide.

A

Do it.

61
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.

62
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
63
Q

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

A
  • The satellite speeds up as it’s height decreases and slows down as height increases
  • So kinetic energy increases as potential energy decreases (and vice versa)
  • So the total energy remains constant
64
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.

65
Q

What is a synchronous orbit?

A

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

66
Q

What are the two types of satellite?

A
  • Geostationary

* Low orbit

67
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.

68
Q

Describe the orbit that geostationary satellites have.

A

Synchronous, along the equator.

69
Q

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

A

1 day

70
Q

What is the orbital radius of a geostationary satellite?

A

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

71
Q

What are geostationary satellites used for?

A

Sending TV and telephone signals.

72
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.

73
Q

Describe the orbit that low-orbit satellites have.

A

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

74
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 Earth
Geostationary
• Do not require multiple satellites to achieve constant reception in one area

75
Q

At what height do low orbit satellites orbit?

A

180-2000km above the surface

76
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
77
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

78
Q

Does any charged object have an electric field around it?

A

Yes

79
Q

What is an electric field?

A

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

80
Q

What is the unit for electric charge?

A

Coulombs (C)

81
Q

What is the symbol for electric charge?

A

Q

82
Q

Can charge be positive and negative?

A

Yes

83
Q

Oppositely charged particles…

A

Attract

84
Q

Like charges…

A

Repel

85
Q

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

A

It experiences a force.

86
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.

87
Q

How can electric fields be represented?

A

Using field lines.

88
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
89
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)
90
Q

What type of law is Coulomb’s law?

A
  • Inverse square law

* Since F ∝ 1/r²

91
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
92
Q

What is electric field strength?

A

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

93
Q

What is the symbol for electric field strength?

A

E

94
Q

What is the unit for electric field strength?

A

N/C

95
Q

What is the equation than defines electric field strength?

A

E = F/Q

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

96
Q

Is electric field strength a scalar or vector quantity?

A

Vector

97
Q

Is electric field strength a constant?

A

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

98
Q

What type of electric field does a point charge have?

A

Radial field

99
Q

Remember to revise the field line diagrams on the bottom of pg 126 of revision guide.

A

Do it.

100
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
101
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

102
Q

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

A

They get further apart.

103
Q

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

A

1/x² graph.

See diagram pg 127 of revision guide

104
Q

How can a uniform electric field be produced?

A

Connecting two parallel plates to opposite poles of a battery.

105
Q

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

A

It is the same at all points.

106
Q

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

A

E = V/d

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

107
Q

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

A

V/m

108
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
109
Q

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

A

Parabola

110
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.

111
Q

What is the symbol for electric potential?

A

V

112
Q

What are the units for electric potential?

A

Volts (V)

113
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
114
Q

When is V positive around a point charge?

A

When Q is positive.

115
Q

When is V negative around a point charge?

A

When Q is negative.

116
Q

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

A

On the surface of the charge.

117
Q

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

A

0V

118
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.

(See diagram pg 128 of revision guide)

119
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.

(See diagram pg 128 of revision guide)

120
Q

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

A

E = ΔV / Δr

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

121
Q

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

A
  • Gradient

* Because E = ΔV / Δr

122
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

123
Q

What is electric potential difference?

A

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

124
Q

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

A

ΔW = Q x ΔV

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

125
Q

What is the symbol for electric potential difference?

A

ΔV

126
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
127
Q

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

A

ΔW = mΔV

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

128
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
129
Q

What are equipotentials in electric fields?

A

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

130
Q

What shape are equipotentials around a point charge?

A

Spherical

131
Q

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

A

They are parallel to each plate, with equal spacing.

132
Q

Remember to practise drawing out equipotentials around a point charge or between two parallel plates.

A

Pg 129 of revision guide

133
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

134
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

135
Q

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

A

Pg 130 of revision guide

136
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.

137
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

A LEVEL - Physics (14 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions