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AQA A-Level Physics > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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1
Q

What are flux lines?

A

Magnetic fields are represented by flux lines (field lines).

Flux lines go from the north to the south pole of a magnet.

The closer together the lines, the stronger the field is.

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2
Q

What kind of field is induced around a wire when a current is flowing through it?

A

When current flows in a wire or any other long straight conductor, a magnetic field is induced around the wire.

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3
Q

What is magnetic flux density?

A

Magnetic flux density:
The flux per metre^2. Measured in Tesla or Wbm^-2

Sometimes also referred to as the magnetic field strength.

(Measure of the strength of a magnetic field)

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4
Q

What equation can you use when a current is 90° to the magnetic field?

A

When current is at 90° to the magnetic field,

F = BIL

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5
Q

What is Fleming’s Left-Hand Rule?

A

Fleming’s Left-Hand Rule:

thuMb - Motion (force)
First finger - magnetic Field
seCond finger - Current

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6
Q

Why do current-carrying wires experience a force in a magnetic field?

A

A force acts on a charged particle moving in a magnetic field.

This is why a current-carrying wire experiences a force in a magnetic field - electric current in a wire is the flow of negatively charged electrons.

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7
Q

What kind of path do charged particles take when defelected in a magnetic field?

A

Charged particles in a magnetic field are deflected in a circular path.

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8
Q

What equation can you use when a charged particle is being deflected in a magnetic field?

A

Charged particles in a magnetic field are deflected in a circular path - meaning that there is a centripetal force/acceleration.

F = mv^2/r
F = BQv
mv^2/r = BQv

So, for charged particles in a magnetic field in a circular orbit:

r = mv/BQ not given in exam

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9
Q

What does the equation r = mv/BQ for charged particles in a magnetic field being deflected tell us about radius?

A

r = mv/BQ tell us that:

  1. The radius increases (i.e. particle is deflected less) if the mass or velocity of the particle increases.
  2. The radius decreases (i.e. particle is deflected more) if the strength of the magnetic field or the charge on the particle increases.
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10
Q

What is a cyclotron?

A

A cyclotron is a particle accelerator made up of two D shaped electrodes positioned opposite each other.

The electric field changes direction each time a particle moves from one electrode to the other, causing the particle to accelerate.

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11
Q

What uses do cyclotrons have?

A

Cyclotrons can be used in medicine:
Used to produce radioactive tracers
or
High-energy beams of radiation for use in radiotherapy

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12
Q

How do cyclotrons work?

A

Cyclotrons

  1. Charged particles are fired into one of the electrodes at the centre. Magnetic field makes them follow a semicircular path.
  2. An applied potential difference between the electrodes accelerates the particles across the gap until they enter the next electrode.
  3. Because the particle’s speed is slighter higher, it will follow a circular path with a larger radius before leaving the electrode again.
  4. Potential difference is reversed so the particle is accelerated again before entering the next electrode. Process is repeated as particle spirals outwards, accelerating, before finally exiting the cyclotron.
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13
Q

What is magnetic flux linkage?

A

Magnetic flux linkage is the magnetic flux multiplied by the number of turns, N, of a coil:

NΦ = BAN

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14
Q

What is electromagnetic induction?

A

When a ​conducting rod moves relative to a magnetic field​, the electrons in the rod will experience a force (as they are charged particles), and build up on one side of the rod, causing an e.m.f. to be induced​ in the rod.

This is known as ​electromagnetic induction​.

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15
Q

What is Faraday’s Law?

A

Faraday’s Law: The induced e.m.f. is directly proportional to the rate of change of flux linkage.

ε = N(ΔΦ/Δt)

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16
Q

What is Lenz’s Law?

A

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

17
Q

What is a cyclotron made of?

A

A cyclotron is made up of two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes.

18
Q

Cyclotrons

  1. Charged particles are fired into one of the electrodes at the _____. A _____ _____ makes them follow a semicircular path.
  2. An applied _____ _____ between the electrodes accelerates the particles across the gap until the enter the next electrode. This is an _____ field.
  3. Because the particle’s speed is slighter higher, it will follow a circular path with a _____ _____ before leaving the electrode again.
  4. Potential difference is _____ so the particle is accelerated again before entering the next electrode. Process is repeated as particle _____ _____ , _____, before finally exiting the cyclotron.
A

Cyclotrons

  1. Charged particles are fired into one of the electrodes at the (centre). A (magnetic field) makes them follow a semicircular path.
  2. An applied (potential difference) between the electrodes accelerates the particles across the gap until the enter the next electrode. This is an (electric) field.
  3. Because the particle’s speed is slighter higher, it will follow a circular path with a (larger radius) before leaving the electrode again.
  4. Potential difference is (reversed) so the particle is accelerated again before entering the next electrode. Process is repeated as particle (spirals outwards, accelerating), before finally exiting the cyclotron.
19
Q

How can you induce an e.m.f. in a flat coil or a solenoid (coiled wire)?

A

You can induce an e.m.f. in a flat coil or solenoid by:

Moving the coil towards or away from the poles of a magnet

Moving a magnet towards or away from the coil

20
Q

What are alternators?

A

An alternator is a generator of alternating current.

It converts kinetic energy into electrical energy by inducing an electric current by rotating a coil in a magnetic field.

21
Q

What are transformers used for?

A

Transformers are devices that make use of electromagnetic induction to change the size of the voltage for an alternating current.

22
Q

How do transformers work?

A

How Transformers Work:

  1. An alternating current flowing in the primary coil produces magnetic flux.
  2. The changing magnetic field is passed through the iron core to the secondary coil where it induces an alternating voltage of the same frequency as the input voltage.
23
Q

How do step-up transformers work?

A

Step-up transformers increase the voltage by having more turns on the secondary coil than the primary.

Ns/Np = Vs/Vp

24
Q

Where is the transformer equation derived from?

A

From Faraday’s Law, we know that induced e.m.f. is:

ε = N (ΔΦ/Δt) given in exam

So

Vp = Np (ΔΦ/Δt) not given in exam

and

Vs = Ns (ΔΦ/Δt) not given in exam

So

Ns/Np = Vs/Vp given in exam
or
Np/Ns = Vp/Vs

25
Q

What are eddy currents?

A

Eddy currents are looping currents induced by the changing magnetic flux in the core of a transformer.

They create a magnetic field that acts against the field that induced them, reducing the field strength.

They also dissipate energy by generating heat.

26
Q

How can you reduce the effect of eddy currents creating losses of power in transformers?

A

The effect of eddy currents can be reduced by laminating the core with layers of insulation.

27
Q

How can you reduce heat being generated by resistance in coils of transformers?

A

To minimise heat being generated by resistance in coils of transformers, thick copper wire is used because it has a low resistance.

28
Q

How can you find the efficiency of a transformer?

A

To find the efficiency of a transformer, find the ratio of power in to power out.

IsVs/IpVp

Gives a decimal - multiply by 100 to give a percentage.

29
Q

At what voltage is electricity transmitted though the national grid?

A

Electricity is transmitted at around 400,000V through the national grid.

30
Q

At what voltage is electricity for domestic use?

A

For domestic use, electricity is 230V.

31
Q

Why does the national grid transmit electricity at the lowest possible current?

A

The national grid transmits electricity at the lowest possible current because the power losses due to the resistance of the cables equal to P = I^2R

So, if you double the current, you quadrouple the power lost.

32
Q

What is the equation for a force felt by a moving charge in a magnetic field?

A

Force felt by a moving charge in a magnetic field:

F = BQv

F = force, N
B = magnetic flux density, T
Q = charge on the particle, C
v = velocity of the particle, ms^-1
33
Q

What is the flux linkage of a rectangular coil rotating through a magnetic field?

A

Flux linkage of a rectangular coil rotating through a magnetic field:

NΦ = BANcosθ

AQA A-Level Physics (14 decks)

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