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13B > Fields Exam Questions > Flashcards

Fields Exam Questions Flashcards

1
Q

Explain what is meant by the gravitational potential at a point in a gravitational field (2)

A

Work done against the field per unit mass, when moved from infinity to the point

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2
Q

State Newton’s law of gravitation (2)

A

Force of attraction between two point masses is proportional to the product of the masses and inversely proportional to the square of the distance between them

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3
Q

Cavendish’s experiment to test Newton’s law of gravitation involved two unequal sized lead spheres, describe and explain the effect of doubling the radii on the force (3)

A

For the spheres mass is proportional to volume
Mass of either sphere would be 8x greater
This makes the force 64x greater
But separation would be doubled causing force to be 4x smaller
Net effect would make the force 16x greater

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4
Q

Describe the properties of the geostationary orbit and the advantages it offers for communications satellites (5)

A

Orbits westwards over the equator
Maintains a fixed position relative to surface of Earth
Period is 24 hours
Offers uninterrupted communication between transmitter and receiver
Steerable dish not necessary

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5
Q

Describe and explain what happens to the speed of a satellite in geostationary orbit when it moves in orbit closer to Earth (2)

A

Speed increases

Loses potential energy but gains kinetic energy as total energy must remain constant.

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6
Q

Why would geo stationary satellites not be suitable for use in tracking road vehicles? (2)

A

Signal would be too weak at large distance

Signal spreads out more the further it travels

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7
Q

What is the SI unit of gravitational potential?

Is it vector or scalar? (1)

A

J/kg

Scalar

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8
Q

What is the SI unit of electric field strength?

Is it vector or scalar? (1)

A

V/m

Vector

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9
Q

Define electric field strength at a point in an electric field (2)

A

Force per unit charge

Acting on a positive charge

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10
Q

A flash tube in a camera produces a flash of light when a capacitor is discharged. State and explain the effect on the photograph image of a moving object if a capacitor of greater capacitance is used (4)

A

Image would be less sharp because the discharge would last longer and the image would be photographed as it is moving
Image would be brighter because the capacitor stores more energy and therefore produces more light

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11
Q

Give two reasons why a capacitor is not a suitable source for powering a cordless phone (2)

A

Capacitor would be too large to fit in phone

Capacitor would need charging very frequently

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12
Q

A student had to design an experiment to measure the acceleration of a heavy cylinder as it rolled down an inclined slope of constant gradient. They used a capacitor discharge circuit to measure time taken to travel between two points on the slope.
Describe the procedure and include measurements to be made and how to calculate acceleration (6)

A

Initial pd across C(V0) from voltmeter before releasing roller
Distance s along slope between plungers
Final pd across C(V1) from voltmeter
Measurements to be repeated for a more reliable result
Time t found from V1 = V0 x e^(-t/RC)
From s=ut + 0.5at^2
Repeat and find average a from several results

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13
Q

Describe the trajectory of charged particles entering at right angles to a uniform electric field (1)

A

Parabolic

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14
Q

Describe the trajectory of charged particles entering at right angles to a uniform magnetic field (1)

A

Circular

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15
Q

Which quantities out of mass (m), charge (Q), magnetic flux density B with a speed v, if changed would affect the radius of the circular path? (1)

A

All four

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16
Q

Give the units of all quantities in F=BIl (1)

A

F - N
B - T
I - A
l - m

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17
Q

Under what conditions does F=BIl apply? (1)

A

I (current) must be perpendicular to B

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18
Q

If a 2+ copper ion is projected from the left into a magnetic field that is directed upwards, in what direction will the magnetic force act on the ion? (1)

A

Out of the plane of the diagram

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19
Q

If a 2+ copper ion is into a magnetic field, what is the effect on the ion’s path if the strength of the magnetic field is doubled? (1)

A

Radius halved

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20
Q

If a 2+ copper ion is projected into a magnetic field, what is the effect on the ion’s path if the ion is replaced by a 1+ ion instead? (1)

A

Radius doubles

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21
Q

The magnetic flux through a coil of N turns is increased uniformly from zero to a maximum value in a time t. An emf, E, is induced across the coil. What is the maximum value of the magnetic flux? (1)

A

Et/N

22
Q

How can a magnetic flux against time graph be transformed into an emf against time graph? (1)

A

Δφ/Δt = emf

23
Q

Why are high voltages and low currents used when transporting electricity on the National Grid? (1)

A

Heat losses are reduced

24
Q

A coil is connected to a centre zero ammeter, a student drops a magnet through the coil (North at the bottom).
Describe what the ammeter would show as the magnet falls through (2)

A

Deflects one way

Then the other way (after halfway through)

25
Q

State and explain how the acceleration of the magnet would be affected as it entered the coil (2)

A

Acceleration is less than g

Lenz’s law

26
Q

A magnet is passed through a coil connected to a voltmeter. Using Faraday’s law, explain why a larger emf would be induced if a stronger magnet moved at the same speed (3)

A

Greater flux linkage
Greater rate of change of flux linkage
Emf is proportional to rate of change of flux linkage

27
Q

State what is meant by root mean square voltage (1)

A

The square root of the mean of the squares of all the values of the voltage in one cycle

28
Q

State and explain the effect on the oscilloscope trace if the time base is switched off (2)

A

Vertical line is formed
Of length equal to twice the peak voltage
Because trace no longer moves horizontally

29
Q

An alternating current supply provides an output voltage of 12 V rms at a frequency of 50 Hz. Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and the frequency of the supply (6)

A

Power supply connected to oscilloscope input
Time base initially switched off
Y gain adjusted to get as long a line as possible
Length of line used to find peak to peak voltage
Rms voltage found
Time base switched on and adjusted to get several cycles on the screen
Use the time base setting to find period
Use period to find frequency
Compare values with stated values

30
Q

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

A

Energy continually added in flight

Rocket is not leaving the gravitational field

31
Q

Define the tesla (1)

A

Strength of magnetic flux density which produces 1N in a wire of length 1m with a current of 1A

32
Q

Explain why it is not possible for the magnetic field to alter the speed of a proton whilst it is in one of the dees of a cyclotron (1)

A

Force is perpendicular to velocity

33
Q

Which two force equations are used to derive the time taken for a proton to travel around a dee? (1)

A

F=BQv (where v=πr)

F=(mv^2)/r

34
Q

State what is represented by gravitational field lines (1)

A

Direction of gravitational force on a mass

35
Q

Why may field lines when considered on a uniform scale, converge over a small area? (2)

A

Field is stronger

Higher density than surroundings

36
Q

A ball travelling at constant speed, moves towards a position at which field lines converge, explain what happens to the ball’s speed (2)

A

Ball will accelerate

Because potential is lower at this position

37
Q

Field lines are shown moving away from an object, explain why these cannot be gravitational lines (1)

A

Gravitational lines are always attractive

38
Q

How is T^2 ∝ r^3 derived? (3)

A 
GMm/r^2 = mr(2π/T)^2
GM/r^2 = 4π^2 x r/T^2
GMT^2=4π^2 x r^3
T^2 = (4π^2/GM) x r^3
4π^2/GM is constant
Therefore T^2 ∝ r^3
39
Q

How does electric field strength vary moving between two plates of equal and opposite charge? (1)

A

E is constant

40
Q

A parallel-plate capacitor is fully charged and then disconnected from the power supply. A dielectric is then inserted between the plates. How do charge on the plates and electric field strength change? (1)

A

Charge stays the same

Electric field strength decreases

41
Q

State what is meant by a capacitance of 1 F (2)

A

For every unit potential difference across the capacitor, 1 C of charge is stored

42
Q

How is time taken for the charging current to fall to half for a capacitor calculated?

A

0.5I=I(1-e^-t/RC)

43
Q

State Lenz’s law (1)

A

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

44
Q

A bar magnet moves towards a coil and an ammeter connected to the coil detects an induced current, explain how this demonstrates Lenz’s law (2)

A

Induced current in same direction as induced emf

Induced emf in opposite direction to bar magnet motion

45
Q

A pd against charge graph for a charging capacitor has a constant gradient, what can be deduced from this?

A

Capacitance of capacitor is constant

46
Q

Explain whether work is done moving a proton from to infinity to a point P between two positive charges (2)

A

Work must be done on the positive proton because P is at a positive potential
The potential at infinity is zero

47
Q

A positively charged ball is attached to a string and hung in equilibrium at 30° from a positively charged wall. When the string is cut, explain the ball’s motion (2)

A

Ball experiences vertical and horizontal forces

Motion is at 30° to the vertical

48
Q

A charge particle is held at rest and then dropped through a uniform electric field. State and explain two reasons, why the horizontal acceleration of a particle is different for each particle (4)

A

Mass is not constant as the mass of a particle varies
Charge on a particle is not constant
Acceleration = Eq/m
E is constant but q and m are random variables so q/m will vary

49
Q

A charge particle is held at rest and then dropped through a uniform electric field. Explain why the time to fall vertically between the plates is independent of the mass of a particle (2)

A

Force on particle = mg and a = F/m so a = g

Time to fall a vertical distance depends only on distance and acceleration

50
Q

A capacitor has a material of relative permittivity that is different to air (1.0) between the plates. When the material is removed, how is the new capacitance worked out? (1)

A

Old capacitance/relative permittivity of the material

51
Q

A capacitor has a polythene sheet between its plates, which has a relative permittivity higher than that of air. Explain why there is an increase in energy stored by the capacitor when the polythene sheet is removed (2)

A

In the polar dielectric, molecules align in the field with positive charged end toward the negative plate
Work is done on the capacitor separating the positively charged surface of the dielectric from the negatively charged plate

52
Q

A coil, X, is in series with a switch and cell. This coil is another coil, Y, which is in series with an ammeter. When the switch is closed there is a current in coil X and the current is in a clockwise direction. Explain how Lenz’s law predicts the direction of the induced current when the switch is opened and when it is closed (4)

A

Induced current such as to oppose the change producing it
Switch on: current increases the flux through Y, current in opposite direction to create opposing flux
Switch off: flux through Y due to X decreases so current travels clockwise to create flux to oppose decrease

13B (22 decks)

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