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A Level Physics > C23 (Magentic Fields) > Flashcards

C23 (Magentic Fields) Flashcards

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1
Q

How can you create an electromagnetic field

A

Moving wire in a magnetic field
Movement of a magnetic

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2
Q

Unit of magnetic flux

A

Weber (Wb)

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3
Q

Definition of magnetic flux

A

Defined as the product of the component of the magnetic flux density is perpendicular to the area at the cross-sectional area

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4
Q

Magnetic flux linkage definition

A

As the product of the number of turns in the coil (N) times by the magnetic flux

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5
Q

Equation for flux linkage

A

No. Turns x magnetic flux

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6
Q

Step-up transformers

A

Step-up transformers has more turns on secondary than on primary coil, Vs > Vp (vise versa for Step-down transformers).

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7
Q

What do transformers not work on

A

DC supplies (there’s no alternating current)

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8
Q

Transformers symbol equation

A

Ns / Np = Vs / Vp

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9
Q

In a transformer the more rapid the change in flux the…

A

Larger the induced e.m.f

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10
Q

What are transformers used for

A

increase / decrease e.m.f. Transformers which increase, called step-up transformers, decrease called set-down.

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11
Q

What do AC generators obey

A

Faraday’s law and Lenz’s law, as the induced current is in the opposite direction to the change in flux

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12
Q

Faraday’s law of electromagnetic induction

A

The magnitude of the induced emf is directly proportional to rate of change of magnetic flux linkage

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13
Q

How can you change the direction of the induced emf and hence the current

A

When a magnet is pulled away from a coil instead of being pushed into the coil

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14
Q

Lenz’s law

A

The direction of any induced emf or current is always in a direction that opposes the change that’s producing it

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15
Q

Which 2 laws do you combine

A

Lenz’s law
Faraday’s law

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16
Q

For an alternating current generator, the induced emf is at maximum when…

A

the flux linkage is zero

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17
Q

What are the 2 types generators can you have, to produce induced emf

A

Can have magnet or coil rotating

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18
Q

In an alternating current generator, when is flux linkage at its maximum

A

When the emf is zero

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19
Q

What condition must be met for 100% efficient transformers

A

the output power from secondary is equal to output power from primary coil.

20
Q

Equation for 100% efficient transformers

A

Vs x Is = Vp x Ip

21
Q

Reasons why transformers don’t have 100% efficiency

A

Due to heating of primary and secondary coil (increasing resistance)

Due to eddy currents (current induced in core, set up in the core of a transformer).

22
Q

Reduce energy losses in the transformer by:

A

Increase inefficiency by using low-resistance windings (ensure coils are thick as possible), reduce power loss due to heating effect of the current.

Making a laminated core with layers of iron separated by an insulator helps minimise currents induced in the core itself (eddy currents). This too minimises lose due to heating.

23
Q

What does the fact that core of transformers are made of soft iron mean

A

Very easy to magnetise and demagnetise, helps improve overall efficiency of transformer.

24
Q

Transformers are commonly used in the national grid to…

A

step-up and down e.m.f.

Electricity is transmitted across the national grid at a high voltage and low current to reduce energy lost as heat in the wires.

25
Q

What are the 3 natural magnetic elements?

A

Iron
Nickel
Cobalt

26
Q

Magnetic materials can only be…

A

Attracted

27
Q

How can you magnetise a material?

A

By passing it through a magnetic field

28
Q

Earth has a magnetic field (which direction)?

A

From south towards north
(Magnetic field due to the molten iron core).

29
Q

Magnetic field definition:

A

The region around a permanent magnet or a current carrying conductor, in which other magnetic objects will experience a force.

30
Q

In what direction are the lines of a magnetic field drawn in?

A

In the direction a ‘free’ north pole would move (north to south).

31
Q

Compare between the 3 types of fields what the (field is produced by):

A

Grav, mass of an object

Electric, charged objects

Magnetic, moving charges

32
Q

Compare between the 3 types of fields what the (Field acts on):

A

Grav, test mass (other masses)

Electric, charges

Magnetic, moving charges

33
Q

Compare between the 3 types of fields what’s the (field strength defines as):

A

Grav, F = -GM / r or g = F / m

Electric, E = F / Q

Magnetic, B = F / I x L x sin(theta)

34
Q

What does equally spaced and parallel magnetic field lines represent?

A

A uniform field (the strength of the magnetic field doesn’t vary).

35
Q

Magnetic field is stronger when…

A

Lines are closer, I.e for bar magnetic, filed strongest at its north and south poles.

36
Q

When a charged particle moves, what does it create?

A

A magnetic field, and therefore when a current oases through a wire, a magnetic field is produced.

37
Q

How do you explain the magnetic field in a bar magnet?

A

Created by electrons whizzing around the iron nuclei.

38
Q

(Magnetic field of a current carrying conductor)
How can you work out the direction of the current and magnetic field?

A

RIGHT hand grip rule: current is direction of thumb, other fingers show direction of the magnetic field.

39
Q

How can you represent current (in a wire) coming out of the page?

A

Circle with DOT

40
Q

How can you represent current (in a wire) going into the page?

A

Circle with CROSS

41
Q

If the magnetic field and current are parallel then that’s the magnitude of the force?

A

There is no force (that will be experienced).

42
Q

Fleming’s Left Hand Rule (definition):

A

A current-carrying conductor is surrounded by its own magnetic field. When the conductor is placed in an external magnetic field, the 2 fields interact. These 2 magnets experience equal and opposite forces.

43
Q

Fleming’s Left-hand rule:
(What does each finger represent)?

A

Thumb, direction of motion (force) on the wire

Index finger, direction of external magnetic field

Middle finger, direction of the conventional current

ALL PERPENDICULAR TO EACH OTHER

44
Q

What’s the SI unit for magnetic flux density (B)

A

Tesla (T)

45
Q

What’s is one Tesla (1T)?

A

The magnetic field density when a wire of length 1m carrying current 1A at right angles to the field, experiences a force of 1N.

46
Q

Magentic flux density equation:
(What does the size of the force on the current-carrying wire depend on)?

A

F = B x I x L x sin(theta)

Depends on:
- Strength magnetic field, B (magnetic flux density)
- Current in wire , I
- Length of wire in the field, L
- Sin(theta), where theta is the angle between the magnetic field and current direction.

A Level Physics (28 decks)

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