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Physics.Y1_Imported > 23 - Magnetic Fields > Flashcards

23 - Magnetic Fields Flashcards

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1
Q

What causes a magnetic field?

A
  • Permanent magnets

- Moving charges

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2
Q

What do we use to map magnetic fields?

A

Magnetic field lines

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3
Q

Describe the magnetic field lines for a straight wire.

A

Concentric circles around the wire

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4
Q

Describe the magnetic field lines for a bar magnet

A

Loops from north pole to south pole

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5
Q

Describe the magnetic field lines for a ring (circle)

A

Concentric circles around ring obeying RHR

Field lines going through centre of ring

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6
Q

Describe the magnetic field lines for a solenoid.

A

Like bar magnet but with field lines going through core of solenoid.

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7
Q

What is the magnetic polarity of the Earth’s south pole (Antarctica)>

A

North

they are reversed

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8
Q

What do the fingers stand for in Flemming’s left hand rule?

A

First finger - Force
seCond finger - Current
thuMb - Motion

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9
Q

When a current carrying wire is placed in a magnetic field, what experiences a force?

A

The wire and the magnet creating the field both experience an equal and opposite force.

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10
Q

What does the equation F = BILsin0 give us?

Why does it contain sin0?

A

The force experienced by a current carrying wire in a magnetic field.

Because current and magnetic field must be perpendicular for a force. Sin0 gives us the perpendicular component of the current if it is not already 90, where sin90 = 1.

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11
Q

When do we use the equation F=BIL?

A

When the current and magnetic field are at right angles.

sin90 - 1

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12
Q

How can you experimentally determine the force exerted on a wire in a magnetic field and therefore find magnetic flux density?

A

Place opposite poles on a digital balance + set to zero.
Run wire through gap between poles.
Force on balance equal to force experienced by wire.
Mass on balance * g = F
B = F/IL (Measure current and length with ammeter + ruler)

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13
Q

What does B stand for?

A

Magnetic flux density

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14
Q

What is the unit for magnetic flux density?

A

T (Tesla)

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15
Q

What is the formula for the force on a single charged particle?

A

F = BQv

F=BIL
I=NQ/t
F=BNGL/t
distance / time (L/t) = v (velocity)
F=BNQv
Force per particle = /N
F=BQv
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16
Q

Describe and explain the path of a charged particle moving in a uniform magnetic field.

A

Circular motion.

Force will always be at right angle to both current (direction of travel) and magnetic field.
Therefore, force will act as a centripetal force, keeping the particle in circular motion.

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17
Q

How do we show something travelling into a plane (into paper)?

A

X

cross

18
Q

How do we show something travelling out of a plane (out of paper)?

A

.

dot

19
Q

How do we calculate the radius of circular motion of a particle in a magnetic field.

A

BQv = mv^2/r

r = mv/BQ

20
Q

Describe how a velocity selector works.

A

A magnetic field is placed across an electric field.
Charged particles are fired into the selector through a small gap with different velocities.
They experience a force in opposite directions due to the two different fields.
Because F = BQv, the force caused by the magnetic field is proportional to the velocity.

Particles travelling only at a certain velocity will experience an equal force from both fields and will travel in a straight line, emerging from the selector.

21
Q

What equations must we relate for a velocity selector?

A

BQv = EQ

v = E/B

22
Q

Describe briefly how mass spectrometers work.

Rough understanding for context

A

Velocity selector - all with same v
Enter magnetic field and experience circular motion.
Since r = mv/BQ, r is proportional to m.
All of the particles of different masses are deflected by a different amount onto a detector.
Particles can then be categorised by their masses.

23
Q

How can an emf be induced in a coil?

A

When there is a change in the magnetic flux linking the circuit.

24
Q

Define magnetic flux.

A

EQUATION

The product of the component of the magnetic flux density perpendicular to the area, and the cross sectional area.

Φ = BAcosθ

25
Q

When do we use Φ = BA?

A

When the magnetic field is normal to the area.

26
Q

What is flux linkage?

A

The magnetic flux multiplied by the number of turns in the coil.

NΦ

Unit Wb turns

27
Q

What is the unit for magnetic flux?

A

Webber (Wb)

28
Q

What can change in order to induce an emf?

A

Φ = BAcosθ

B, A or cosθ can be changed to change the magnetic flux and induce an emf.

29
Q

What is Faradays law?

A

Emf is directly proportional to the rate of change of magnetic flux linkage.

ℰ = -ΔΦ/Δt

30
Q

What is Lenz’s law?

A

The direction of the induced emf and current is always such as to oppose the change producing it.

(This is why the constant of proportionality in Faraday’s law is -1)

31
Q

Why must Lenz’s law be true?

A

Because of the conservation of energy.

In order to produce electrical energy, work must be done against the direction of motion.

32
Q

How does a simple AC generator work?

A

As a coil of wire rotates in the magnetic field, cosθ changes. This means that Φ changes and so an alternating emf is induced.

33
Q

How can we calculate the emf of an AC generator from a graph of magnetic flux linkage against time?

A

The gradient

Differentiate a cosine graph = sine graph

34
Q

What will happen to the voltage if there are double the number of output coils on a transformer?

A

The potential difference will double.

35
Q

How does a simple transformer work?

A

AC current supplied to primary coil.
Causes alternating magnetic flux linking the two coils.
Alternating magnetic flux linkage in the secondary coil induces an alternating emf and therefore current.

36
Q

Why do transformers have an iron core?

A

So that none of the magnetic flux linking the two coils is lost.

37
Q

What is a laminated iron core?

A

A core made up of layers of iron separated by an insulator.

38
Q

What does a laminated iron core achieve?

A

Prevents currents in the core itself where energy would be lost through heating.

Improves the efficiency of the transformer.

39
Q

Why are large voltages and low currents used for transmitting energy over the national grid.

A

To minimise heat loss.

40
Q

Define the Tesla.

A

The magnetic flux density when a wire of 1m, perpendicular to the magnetic field ,experiences a force of 1N when a current of 1A flows through it.

41
Q

Define magnetic flux density.

A

The force experienced by a wire perpendicular to a magnetic field, per unit length of the wire and per unit current through it.

Physics.Y1_Imported (16 decks)

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