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Physics > Unit 7 Fields and their consequences > Flashcards

Unit 7 Fields and their consequences Flashcards

1
Q

What is a 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

States that the gravitational force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of the distances between them.

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3
Q

What is gravity?

A

A universal attractive force that acts between all matter.

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4
Q

What is a point mass?

A

A mass which behaves as if all its mass is concentrated at its centre

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5
Q

How can force fields be represented?

A

Vectors using field lines or lines of force on a diagram

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6
Q

What kind of field is the Earth’s gravitational field?

A

Radial overall, but can be considered uniform close to the Earth’s surface

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7
Q

Define the gravitational field strength at a point in a gravitational field.

A

Work done per unit mass to move a small object from infinity to that point.

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8
Q

Why are the values of gravitational potential negative?

A

Because at an infinite distance away from the mass causing the field, the gravitational potential is zero. Work must be done against the gravitational attraction to move the masses apart , so an object gains potential energy as it moves towards infinity but since the max value of potential energy can have is 0, all the points in the field must have a negative value for gravitational potential.

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9
Q

What is the gravitational potential difference?

A

The difference between the gravitational potential at two points in a gravitational field.

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10
Q

What is an equipotential surface?

A

Surface where the gravitational potential is the same at all points.

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11
Q

Give the equation for a satellite’s total energy?

A

E(total) = Potential energy + kinetic energy = -(GMm/2r)

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12
Q

What is a gravitational field?

A

The region surrounding an object in which it exerts a gravitational force on any other object.

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13
Q

What is the strength of a gravitational field defined as?

A

The force per unit mass on a small test mass placed in the field.

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14
Q

What is a radial field?

A

A field in which the field lines are straight and converge or diverge as if from a single point.

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15
Q

What is a uniform field?

A

A region where the field strength is the same in magnitude and direction at every point in the field ie. The gravitational field of the Earth is uniform over a region which is small compared to the scale of the Earth.

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16
Q

What do field lines indicate?

A

The direction of a line of force or a field line indicates the direction of the force. The gravitational field lines of a single mass point towards that mass.

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17
Q

What is Kepler’s 1st Law?

A

All planets move in elliptical orbits with the sun at one focus.

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18
Q

What is Kepler’s 2nd Law?

A

States that a line segment joining the Sun to a planet sweeps out equal areas in equal time intervals

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19
Q

What is Kepler’s 3rd Law?

A

States that the square of the orbital time period T is directly proportional to the cube of the orbital radius r.

Can be mathematically written as T^2/r^3 = k

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20
Q

What is the escape velocity?

A

The minimum speed an object must be given to escape the body’s gravitational field when launched from the surface.

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21
Q

What are some properties of a geostationary satellite?

A

Has a synchronous orbit where the time period is equal to the rotational period of the object it is orbiting

It remains above the same point on the Earth’s equator

Orbits in the equatorial plane

Moves in the same direction that the Earth rotates

Orbits 36000 km above the surface of the Earth with an orbital radius of about 42000 km

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22
Q

Why are geostationary satellites useful?

A

They are useful as communication satellites because transmitters and receivers on Earth can point in a fixed direction.

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23
Q

What are some properties of a low orbit/ polar satellite?

A

Orbit at less than 2000 km above the Earth

Satellites with a polar orbit move in a plane that is 90 degrees to the equatorial plane and pass over every point on the Earth as it rotates beneath them.

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24
Q

State some similarities between the gravitational and electric fields?

A

Magnitude of both of these fields follow the inverse square law relationship

Both fields rely on the product of two properties ie. m1 x m2 and q1 x q2

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25
Q

State some differences between gravitational and electric fields?

A

Gravity is always an attractive force whereas electric could be attractive or repulsive

They both use different constants ie. G and permittivity of free space

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26
Q

Is work done when moving along an equipotential surface?

A

No work is done.

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27
Q

What is an electric field?

A

A region of space in which any object with charge will experience a force.

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28
Q

Where do electric fields exist?

A

Around all objects with charge

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29
Q

How can a uniform electric field be produced?

A

Applying a potential difference between two parallel metal plates.

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30
Q

Give the properties of a uniform electric field.

A

Field strength and direction are the same at every point.

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31
Q

What kind of trajectory will a charged particle entering a uniform field in a directional initially perpendicular to the field have?

A

Parabolic trajectory

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32
Q

Define the absolute electric potential at a point in an electric field.

A

Work done per unit charge to bring a small positive test charge from infinity to that point.

33
Q

What is the electric potential difference?

A

The difference between the electric potential at two points in an electric field.

34
Q

What is an equipotential surface in an electric field?

A

Surface where the electric potential is the same at all points.

35
Q

How are equipotential surfaces arranged relative to field lines?

A

Perpendicular to field lines

36
Q

What shape are the equipotential surfaces in a radial field and a uniform field?

A

Spheres and flat planes

37
Q

How can the electric field strength at any point in an electric field be found from the graph of V against r?

A

Gradient of the graph at that point.

38
Q

How can the potential difference between two points in a field be found from the graph of E against r?

A

Area under the graph between those two points.

39
Q

What is a capacitor?

A

A device designed to store charge.

40
Q

What is the capacitance of a capacitor?

A

The charge stored per unit potential difference.

41
Q

What does a simple capacitor consist of?

A

Two parallel metal plates opposite each other with a gap between them.

42
Q

What effect does inserting a dielectric between the plates have on a capacitor?

A

Increases its capacitance

43
Q

Why is the energy stored by a capacitor only half the energy supplied by the power supply?

A

Rest of the energy is lost to the resistance of the circuit and the internal resistance of the power supply.

44
Q

What is the area under a current time graph equal to?

A

The charge that flowed off the capacitor

45
Q

What is the gradient of the tangent of a charge time graph at any point equal to?

A

Current at that time.

46
Q

What is time constant if a capacitor discharge circuit?

A

Time taken for the charge, current and potential difference to fall to 1/e (approximately 0.37 or 37%) of its original value.

47
Q

How can the time constant be found from the graphs of lnl, lnV and lnQ against time?

A

Time constant = -1/gradient of the graph

48
Q

What is a magnetic field?

A

A region of space in which objects with magnetic properties experience a force.

49
Q

Where are magnetic fields formed?

A

Around all moving charged objects

50
Q

What does the separation of field lines indicate?

A

The strength of the field

51
Q

How can the direction of the field around a current carrying straight wire be found?

A

Using the corkscrew rule

52
Q

How can the direction of the field around a solenoid be found?

A

Using the right hand grip rule.

53
Q

Define magnetic flux density.

A

Force per unit length per unit current on a current carrying conductor at right angles to the direction of the magnetic field, in tesla (Φ = BA, in weber)

54
Q

Give the magnetic flux linkage for a coil of wire with cross-sectional area A and N turns.

A

NΦ = BAN

55
Q

How is magnetic flux linkage denoted?

A

NΦ - this is one quantity, you cannot cancel N.

56
Q

State the unit of magnetic flux linkage.

A

Weber-turn, Wb turn

57
Q

Give the equation for the flux through a single loop of wire if the magnetic field is at an angle θ to the normal at the face of the loop.

A

Φ = BAcosθ

58
Q

Give the equation for the size of the force on a current carrying wire at 90 degrees to a magnetic field.

A

F = BIL

59
Q

Give the equation for the magnitude of the force on a charged particle moving at right angles to a magnetic field.

A

F = BQv

60
Q

Give the equation for the speed of particles of charge Q and mass m, upon leaving a cyclotron of radius R.

A

v = BQR / m

61
Q

Give the equation for the time taken by a particle to complete a full circle in a cyclotron.

A

T = 2πm/BQ

62
Q

Give the equation for the frequency of the alternating potential difference in a cyclotron.

A

1/T = BQ/2πm

63
Q

What is electromagnetic induction?

A

An e.m.f is induced in a conductor that experiences a change in magnetic flux.

64
Q

What can cause a change in magnetic flux through a conductor?

A

Relative movement between conductor and field; change in strength of field.

65
Q

State Lenz’s law.

A

Direction of induced current or e.m.f will always oppose the change in flux producing it.

66
Q

State Faraday’s law.

A

Magnitude of induced e.m.f in a conductor is equal to the rate of change of flux it experiences or the rate of change of flux linkage in a coil.

67
Q

Give Faraday’s law in symbol form.

A

ε = -N (ΔΦ/Δt)

68
Q

Give the formula for the e.m.f induced in a straight conductor moving at right angles through a magnetic field

A

ε = BLv

69
Q

How does the induced e.m.f in a simple alternating current generator vary with time?

A

Sinusoidally

70
Q

Give the equation for the angular frequency of a coil spinning with a frequency f.

A

⍵ = 2πf

71
Q

Give the equation for the flux linkage in a coil rotating in a magnetic field.

A

NΦ = BANcos⍵t

72
Q

Give the equation for the e.m.f induced in a coil rotating in a magnetic field.

A

ε = -BAN⍵sin⍵t

73
Q

Give the equation for the maximum induced e.m.f.

A

Maximum induced e.m.f = BAN⍵

74
Q

Give the equation for the peak to peak value of the e.m.f.

A

Peak to peak value of the e.m.f = 2BAN⍵

75
Q

What is an oscilloscope?

A

Voltmeter showing how voltage varies with time.

76
Q

What is the root mean square speed (rms) value of the current equivalent to?

A

Value of the direct current that would have the same heating effect

77
Q

State the properties of a step up transformer.

A

More turns on the secondary coil than on the primary coil; steps up the voltage and steps down the current

78
Q

State the properties of a step down transformer.

A

Fewer turns on the secondary coil than on the primary coil: steps down the voltage and steps up the current

Physics (10 decks)

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