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A2 Physics > Section 7- Fields > Flashcards

Section 7- Fields Flashcards

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1
Q

What is meant by the strength of a gravitational field?

A

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

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2
Q

Define radial field and uniform field.

A

Radial field: field lines look like spokes of a wheel

Uniform fields: perpendicular field lines towards mass

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3
Q

What is gravitational potential?

A

The energy of an object due to its position in a gravitational field.

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4
Q

Where would an object have to be placed for its gravitational potential energy to be zero?

A

At infinity

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5
Q

How does gravitational attraction vary with distance?

A

Proportional to 1/r^2 and the mass of each object. It is always an attractive force.

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6
Q

Describe the shape of a graph of g against r for points outside the surface of a planet.

A

Inverse square law curve.

Increases with straight line before decreasing exponentially.

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7
Q

What is significant about the gradient of the V against r graph?

A

At any point it is equal to -g

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8
Q

What is the condition needed for a satellite to be in a stable orbit?

A

E=- GMm/ 2r

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9
Q

What happens to the speed of a satellite if it moves closer to earth?

A

V decreases

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10
Q

Why must a geostationary satellite be in an orbit above the equator?

A

It will have a time period of 24h so that it will stay at exactly the same place in relation to earth.

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11
Q

How do you charge a metal object?

A

The metal must be isolated from the earth then can be charged by direct contact with a charged object.

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12
Q

What does the direction of an electric field line show?

A

The path of a free positive test charge.

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13
Q

Describe how to measure the strength of an electric field.

A

E=F/Q measure the force and charge

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14
Q

Is electric field strength a scalar of vector?

A

A vector from positive to negative.

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15
Q

What is electrics potential at a certain position in an electric field defined as?

A

The work done per unit positive charge on a positive test charge when it is moved from infinity to that position.

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16
Q

The electric field strength is equal to…

A

The negative of the potential gradient.

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17
Q

How does the force between two point charges depend on distance?

A

F is proportional to 1/r^2 because F=kQ1Q2/r^2

18
Q

State the equation that gives the electric field strength near a point charge.

A

F=(1/4piEo)*(Qq/r^2)

19
Q

Why is E equal to zero inside a charged sphere?

A

Because the charge is assumed to be in the centre of the sphere.

20
Q

What are the main similarities between gravitational and electric fields?

A

The line of force/ field lines are found with a free test mass/ charge.
In uniform fields g and E are the same everywhere and the field lines are parallel and equally spaced.

21
Q

What are the main differences between gravitational and electric fields?

A

The force attracts only in a gravitational field whereas the charges can attract or repel in an electrostatic field.

22
Q

What happens when a capacitor charges?

A

A capacitor is connected to a battery and on conductor will gain electrons from the battery, the other conductor will lose electrons to it.

23
Q

Relate the pd across the plates of a capacitor to the charge on its plate.

A

It is a straight line through the origin- it is proportional.

24
Q

What are capacitors used for?

A

Smoothing circuits, backup power supplies, timing circuits, pulse producing circuits, tuning circuits, filter circuits.

25
Q

Why does a capacitor store energy as it’s being charged?

A

Because electrons are forced into one plate and taken off the other. The energy is stored as electric potential energy.

26
Q

What circuit components would you change to make a capacitor charge/ discharge slower?

A

Increase the resistance or capacitance.

27
Q

How does a dielectric affect a capacitor?

A

More charge is stored on the plates because the positive side of the dielectric attracts more electrons from the battery on the negative plate and the negative side pushes electrons back to the battery.

28
Q

What is the relative permittivity?

A

The ratio of charge stored with the dielectric to the charge stored without the dielectric.

29
Q

Describe the action of a simple polar molecule rotating in an electric field.

A

Polar dipoles rotate with the negative end towards the positive plate. In an alternating electric field the dipoles with rotate back and forth.

30
Q

What factors do the magnitude of a current carrying wire depend on?

A

The angle of the wire to magnetic field (greatest at right angles).
The current
The length of the wire

31
Q

What happens to a charged particle in a magnetic field?

A

They experience a force and follows a circular path.

32
Q

Why does a force act on a wire in a magnetic field when a current flows along the wire?

A

Because the electrons moving along the wire are pushed to one side by the force of the field. If the electrons are confined to the wire then the wire itself must move.

33
Q

What equation is used to find the force of a moving charge?

A

F=BQv

34
Q

Why do moving charges move in a path that is circular?

A

Because the magnetic force is always perpendicular to velocity.

35
Q

What factors affect the radius of the circular path of a charge in a magnetic field?

A

R decreases if:
B is increased
v is decreased
Particles with a larger specific charge are used

36
Q

What must happen to a conductor for electricity to be generated?

A

A magnet is moved near a wire and a current passes through it. This causes an induced emf which forces electrons around a circuit.

37
Q

What factors increase induced emf?

A

Moving the wire faster
Using a stronger magnet
Make the wire into a coil

38
Q

What is lenz’s law?

A

The direction of the induced current is always such as to oppose the change that causes the current.

39
Q

What is meant by the rms value of an alternating current?

A

The value of direct current that would give the same heating effect as the alternating current in the same resistor.

40
Q

Explain the purpose of transformers.

A

Transmission of electrical power over long distances is much more efficient at high voltage than at low voltage.

41
Q

How is the efficiency of transformers improved by design?

A

Low resistance windings to reduce power lost from heat.
Laminated core- layers of iron separates by insulator to reduce eddy currents (induced currents in the core).
Soft iron is easily magnetised and demagnetised- stops energy being wasted.

A2 Physics (4 decks)

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