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Physics > 3.7 Fields and their consequences > Flashcards

3.7 Fields and their consequences Flashcards

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1
Q

What is a force field?

A

A region in which a body experiences a non-contact force

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2
Q

What is newton’s law of gravitation?

A

Magnitude of gravitational force between two masses is ​directly proportional to the product of the masses​, and is ​inversely proportional to the square of the distance between them

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3
Q

What is meant by gravitational field strength?

A

Force per unit mass exerted by a gravitational field on an object

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4
Q

Gravitational field strength equation: general

A

g = F/m

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5
Q

Gravitational field strength equation: radial field

A

g = GM/r²

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6
Q

What is meant by gravitational potential?

A

The work done required in bringing a unit mass from infinity to the point

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7
Q

What is meant by gravitational potential difference?

A

The energy required to move a unit mass between two points

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8
Q

Why is gravitational potential energy negative?

A
  • Gravitational potential energy ​is zero at infinity
  • As an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced
  • Therefore gravitational potential energy is always negative
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9
Q

How to derive T² = k r^3

A

Centripetal force = Gravitational force

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10
Q

What is meant by escape velocity?

A

Minimum velocity an object must travel at in order to escape the gravitational field of a mass

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11
Q

How to derive escape velocity = sqrt(2GM/r)?

A

kinetic energy = gravitational potential energy

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12
Q

What is a synchronous orbit?

A

When the orbital period of the satellite is equal to the rotational period of the object that it is orbiting

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13
Q

What is a geostationary orbit? What are its features?

A
  • A synchronous orbit (24 hours) and they always stay above the same point on the Earth, because they orbit directly above the equator
  • Only one particular orbit height is possible
  • Travels west to east
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14
Q

Application of low-orbit satellites?

A

Monitoring weather and military applications

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15
Q

What is meant by electric field strength?

A

Force per unit positive charge experienced by an object in an electric field

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16
Q

E (electric field strength in general) equation

A

E = F/Q (by definition!)

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17
Q

E (electric field strength for uniform fields formed by parallel plates) equation

A

E = V/D

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18
Q

E (electric field strength for radial fields) equation

A

E = kQ/r²

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19
Q

What is k in electric fields?

A

8.99E+9 (1/4piEpsilon0)

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20
Q

What do electric field lines show?

A

The direction of the force acting on a positive charge, and the strength go the field depending on the density of the field lines

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21
Q

What do fields lines look like in a uniform field?

A

Equally spaced and parallel

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22
Q

Why can use 1/4piEpsilon0 when in air?

A

Air can be treated as a vacuum when calculating force between charges

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23
Q

Derivation of ΔW= Q ΔV for moving charges between parallel plates

A
  • F = EQ
  • d = ΔV/E
  • ΔW= F x d = (EQ) x (ΔV/E) = Q ΔV
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24
Q

Path shape of charged particle fired at right angles through an electric field

A

Parabola (constant force -> constant acceleration)

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25
Q

What is absolute electric potential?

A

Potential energy per unit charge of a positive point charge at a point in the field

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26
Q

Where is absolute electric potential greatest?

A

At surface of the charge

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27
Q

Where is absolute electric potential greatest?

A

At surface of the charge

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28
Q

What is electric potential difference?

A

Work done needed to move a unit charge between two points

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29
Q

Capacitance equation

A

Q = CV

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30
Q

Define capacitance

A

The charge stored by a capacitor per unit potential difference across it

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31
Q

Equation for the dielectric constant

A

permittivity of medium / permittivity of free space

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32
Q

How do dielectrics work?

A
  • Polar molecules align with their positive side facing the negative plate (and vice versa)
  • Producing a counter electric field, reducing the field strength between the places
  • The pd V decreases between the capacitor but charge Q remains the same
  • So capacitance Q/V increases
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33
Q

What is Coulomb’s law?

A

Magnitude of the electrostatic force between two point charges in a vacuum is inversely proportional the the product of their charges and inversely proportional to the square of the distances between the charge

34
Q

Capacitance calculation (using distance between plates etc)?

A

C = Ae0eR/d

35
Q

Energy stored by a capacitor

A

1/2 QV

36
Q

Capacitor: Area under current time graph

A

Charge

37
Q

Capacitor: Gradient of charge time graph

A

Current

38
Q

Capacitor: Time to half

A

ln2 * RC

39
Q

Capacitor charging : Current-time graph

A

Start high and then decay

40
Q

Capacitor charging : Voltage across capacitor-time graph

A

Start low and then asymptote high

41
Q

Capacitor charging : Charge-time graph

A

Start low and then asymptote high

42
Q

Capacitor discharging : Current-time graph

A

Start high and then decay

43
Q

Capacitor discharging : Voltage against both resistor and capacitor-time graph

A

Start high and then decay

44
Q

Capacitor discharging : Charge-time graph

A

Start high and then decay

45
Q

Capacitance in terms of area and distance between plates

A

C = A epsilon0 epsilonR / d

46
Q

When does F=BIL apply?

A

When current is perpendicular to magnetic field

47
Q

When does 1T mean?

A

A force of 1N of 1m of a wire carrying 1A of current perpendicular to a magnetic field

48
Q

Magnetic fields, what is B?

A

Magnetic flux density / T

49
Q

One application of cyclotrons

A

Producing ion beams for radiotherapy

50
Q

When does Φ = BA apply?

A

When B is perpendicular to A

51
Q

Magnetic flux linkage for coil of wire at an angle

A

NΦ = BAN cos θ

52
Q

Magnitude of emf for a rotating coil of wire

A

EMF = BAN ω sin ωt

53
Q

What does Faraday’s law state?

A

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

54
Q

What does Lenz’s Law state?

A

The induced emf is in such a direction as to oppose the motion/change the caused it

55
Q

How to change the scale of Y-axis on oscilloscope

A

Can select the number of volts per division using the Y-gain control dial

56
Q

What type of currents do transformers work with?

A

AC

57
Q

Transformer equation

A

Ns/Np = Vs/Vp

58
Q

What does a step-up transformer do?

A

Increases the input voltage by having more turns on the secondary coil than the primary

59
Q

What are by ‘eddy currents’ in transformers? How are they reduced?

A
  • Eddy currents are induced by the alternating magnetic field in the primary coil, and form a loop
  • Due to Lenz’s law, they oppose the field that produced them, reducing the field’s flux density
  • They also generate heat, which causes energy to be lost through I²R heating
  • Eddy current can be reduced by using a laminated iron core, as eddy currents cannot pass through the insulator so their amplitude is reduced
60
Q

3 causes of inefficiencies in transformers and how to solve these

A
  • Eddy currents / use laminated iron core
  • Resistance in coils / Using thick wire, which has a low resistance
  • Core not easily magnetised / Use a soft iron core
61
Q

SI unit for gravitational field strength

A

N.kg^-1 (not m.s^-2)

62
Q

Why do orbiting astronauts feel weightless?

A
  • Both astronaut and vehicle have same centripetal acceleration
  • And same orbital speed
  • So there is no reaction force between astronaut and vehicle
63
Q

What is the purpose of the iron core in a transformer?

A

Provides a magnetic path to channel flux, ensuring flux generated by the primary coil is linked to the secondary coil

64
Q

Explain why an orbiting satellite’s speed doesn’t change even though it is acted on by a force

A
  • The force acts in towards centre of Earth, perpendicular to direction of motion
  • There is no movement in the direction of force
  • so no work done, so no change in kinetic energy, so no change in speed
65
Q

What do gravitational field lines show?

A

The direction of force on a mass

66
Q

3 features of a low polar orbit

A
  • Orbital period is a few hours
  • Earth moves relative to orbit
  • Many orbits with different radii and periods are possible
67
Q

3 advantages of low polar orbit (over geostationary) for monitoring Earth

A
  • Satellite scans the whole surface of Earth
  • Gives access to every point on Earth’s surface every day
  • Lower signal strength required that that for geostationary satellite
68
Q

Explain why a magnet Q falling through a copper tubes falls slowly

A
  • Falling magnet means there is a change of flux in the tube
  • There is a rate of change of flux, so an emf is induced in the tube
  • This emf causes a current
  • This current produces a magnetic field
  • Lenz’s law means the field opposes the motion of Q (producing an upwards force when Q enters the tube, and again when it leaves the tube)
69
Q

How do equipotentials cross field lines?

A

Always at right angles! (Label the right angle if needed)

70
Q

Similarities between electric fields and gravitational fields

A
  • Both have inverse-square relationships with r
  • Both have the concepts of field strength, potential, equipotential surfaces, work done - formulas have the same layout
  • Both can be represented using field lines
71
Q

Key differences between electric and gravitational fields

A
  • Masses always attract
  • Charges may repel or attract
  • At the subatomic level, gravity can be pretty much ignored, as it is much weaker than the electrostatic force
72
Q

What is gravity?

A

The universal attractive force which acts between all matter

73
Q

Electric field lines always go from _________________ to _________________

A

Electric field lines always go from positive charge to negative charge (NE positive/negative)

74
Q

How many time constants until the capacitor can be considered to be fully charged?

A

5

75
Q

3 desired features of materials for a moderator in a fission reactor

A
  • small/light nuclei
  • should not become radioactive when bombarded with electrons
  • available in large amounts
76
Q

Describe how a transformer works

A
  1. A primary coil is wrapped around an iron core
  2. An alternating current in the coil creates an alternating magnetic field in the core
  3. The core channels the flux generated in the primary coil, ensuring the primary coil and secondary coil are linked
  4. The alternating magnetic field induces an emf in secondary coil
  5. Which creates an alternating current in the secondary coil
77
Q

Purpose of step-up and step-down transformer in the power grid

A

step-up: to increase voltage to transport power with minimal energy losses (reduces I²R heating losses)
step-down: to decrease the voltage to save levels for households and buildings

78
Q

Describe what it would look like if you turned off the time base on an oscilloscope connected to an alternating current

A

There would be a straight vertical line on the voltage axis

79
Q

What does it mean for the transformer core to be soft? Why do we want this?

A
  • A magnetically soft material is one that magnetises and demagnetises easily (e.g. magnetisation disappears when current in removed)
  • Energy is needed to magnetise and demagnetise the core, and this energy is wasted as it heats the core
  • So need to use a magnetically soft material
80
Q

What is a drawback of increasing the voltage of electricity for power transmission cables?

A

More insulation/isolation difficulties

Physics (11 decks)

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