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OCR Physics B Unit 5 > 5.1 Electromagnetic Machines > Flashcards

5.1 Electromagnetic Machines Flashcards

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1
Q

What is a magnetic field?

A

A region where a force is exerted on magnetic materials.

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2
Q

In which direction to field lines travel?

A

From north to south.

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3
Q

The strength of a magnetic field is represent by what?

A

How close together the field lines are.

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4
Q

What happens at a neutral point in a magnetic field?

A

Magnetic fields “cancel out”.

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5
Q

Around a current carrying wire there is always an…

A

Inducted magnetic field.

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6
Q

What can you use to determine the direction of the magnetic field around a current carrying wire?

A

The right hand grip rule.

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7
Q

What happens to a current carrying wire in a magnetic field?

A

It experiences a force.

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8
Q

Why does a current carrying wire experience a force in a magnetic field?

A

The magnetic field induced by the wire and the existing magnetic field interact, producing “contacted field lines” around one side of the wire. Since field lines tend to be short and straight, there is a resultant force.

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9
Q

If current is parallel to the direction of a magnetic field…

A

No force acts.

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10
Q

The direction of the force caused by a current carrying wire in a magnetic field acts in what direction?

A

Perpendicular to both the current direction and magnetic field.

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11
Q

Which rule can you use to find the force acting on a current carrying wire in a magnetic field?

A

Fleming’s left hand rule. Thumb: force. First finger: field. Second finger: current.

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12
Q

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

A

F = BIL

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13
Q

Define magnetic field strength, B.

A

The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.

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14
Q

Give another name for magnetic field strength.

A

Magnetic flux density.

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15
Q

What are the units for magnetic flux density?

A

Teslas, T.

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16
Q

What is tesla?

A

One weber per metre squared.

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17
Q

What can you add to a current carrying loop to turn it into an electric motor?

A

A split-ring commutator.

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18
Q

How do simple electric motors work?

A

A current carrying loop is in a magnetic field. The two sides of the loop have current flowing in opposite directions so the forces experienced are in opposite directions, causing the loop to rotate.

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19
Q

What does a split-ring commutator do?

A

Reverse the current in a loop each time it becomes vertical.

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20
Q

How do induction motors work?

A

They alternate the magnetic field around a coil of wire that is free to move. This induces a current in the wire, causing it to rotate.

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21
Q

What can magnetic flux be thought as?

A

The total number of field lines.

22
Q

Give the equation for magnetic flux, ɸ, passing through an area, A, perpendicular to a magnetic field, B.

A

ɸ=BA

23
Q

That magnetic flux passing though a coil with N turns is called…

A

Flux linkage, ɸN.

24
Q

What are the units of magnetic flux, ɸ?

A

The weber, Wb.

25
Q

If a counting rod moves though a magnetic field its electrons will experience a force, this means that…

A

The electrons will accumulate at one end of the rod, inducing an e.m.f.

26
Q

If the conducting rod moving through a magnetic field is part of a complete circuit…

A

An induced current will flow through it.

27
Q

What happens when flux lines are cut?

A

An e.m.f. is produced.

28
Q

When there is relative motion between a conductor and a magnet…

A

And electromotive force is induced.

29
Q

Flux cutting will only induce a current if…

A

The circuit is complete.

30
Q

What is Faraday’s Law?

A

The induced e.m.f. is directly proportional to the rate of change of flux linkage.

31
Q

Give the word equation for Faraday’s Law.

A

Induced e.m.f = flux change / time taken

32
Q

What feature of a ɸN vs time graph represents the size of the e.m.f.?

A

The gradient.

33
Q

How can you find the flux linkage from an e.m.f. vs time graph?

A

Find the area under the graph.

34
Q

What is Lenz’s Law?

A

The induced e.m.f. is always in such a direction as to oppose the change that caused it.

35
Q

Give an equation combining Faraday and Lenz’s laws.

A

Induced e.m.f. = -d(ɸN)/dt

36
Q

Why does e.m.f. produced act in a direction that opposes the change that causes it?

A

Conservation of energy. Energy used to pull a conductor through a magnetic field, against resistance caused by magnetic attraction is what produces the induced current.

37
Q

What do transformers do?

A

They change the size of a voltage for an alternating current.

38
Q

What principles do transformers use?

A

Electromagnetic induction: flux linking between two coils of wire.

39
Q

How do transformers work?

A

AC in the primary coil produces a changing magnetic flux, which passes through an iron core the the secondary could where the changing flux induces an alternating voltage of the same frequency.

40
Q

Give the equation for flux in the coil of a transformer.

A

V = N dɸ/dt

41
Q

Give the equation for an ideal transformer.

A

Vp/Vs = Np/Ns

42
Q

What is the permeance of an object?

A

The amount of flux induced in the object for a given number of current turns around it.

43
Q

Give the equation for permeance of a material.

A

Λ = μA/L

44
Q

What is the permeability of a material, μ?

A

The permeance per unit cross-sectional area of a unit length material.

45
Q

What are the compromises made when designing a transformer?

A

Maximising permeance requires a large cross sectional area but that causes a lower conductance in the copper coils, where you want a shorter length (and therefore radius) so a balance has to be struck.

46
Q

What happens if there is an air gap in a “magnetic circuit”?

A

Flux still flows but considerably less since air has a much lower permeability than iron.

47
Q

Give two reasons why transformers are not 100% efficient.

A

Eddy currents in the iron core and resistance in the copper wire.

48
Q

Why do eddy currents results in less than 100% efficiency?

A

Changing flux in the core induces a current within the core. By Lenz’s law the flux produced by eddy currents opposes the flux created by the primary coil, reducing the transformer’s efficiency.

49
Q

Why is voltage stepped up in the national grid?

A

Power losses due to resistance in the cables are proportional to I^2.

50
Q

What is an alternator?

A

A generator of alternating current.

51
Q

How do alternators work?

A

They induce an electric current by rotating a coil in a magnetic field. They use slip rings and brushes instead of a slit-ring commutator.

OCR Physics B Unit 5 (4 decks)

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