Magnetism & electromagnetism

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Magnetism &

electromagnetism

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Learning outcomes

• describe and explain the behaviour of permanent magnets, including induced magnetism

• explain magnetisation and demagnetisation of ferromagnetic materials in terms of magnetic domains

• describe how magnetic fields arise from moving charges, e.g.

in current-carrying straight wires, plane coils and solenoids

• describe how a transformer works, in terms of transformer turns, currents & voltages

• describe the vectors involved in motor and dynamo effects

• explain why electricity is transmitted at high voltage

• experience relevant demonstration & class experiments

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Overview

• contexts for teaching about electromagnetism

• permanent magnets

• electromagnets

• catapult effect and motors

• electromagnetic induction and generators

• Lenz’s law

• transformers & high voltage transmission of electricity

Circus of experiments

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Misconceptions

• All metals are magnetic materials.

• Static charges interact with the poles of permanent magnets.

• Magnetic poles are located on the surface of a magnet.

[Careful observation shows that they are inside the magnet.]

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Teaching challenges

Magnetic fields

• cannot be seen directly

• are three-dimensional, though commonly represented by 2-D diagrams.

Some students find it hard to understand

• why permanent magnets can lose their strength

• that the geographic North pole must be a south magnetic pole

• that a current-carrying coil of wire induces (temporary) magnetism in the iron core of an electromagnet.

• the operation of motors and generators (incl left hand rule)

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A brief history

1600 William Gilbert, On magnetism; magnetic materials;

poles that attract & repel; Earth’s magnetic field, compass ‘dip’

1820 Hans Christian Oersted finds that an electric current deflects a compass needle.

1820 Andre Marie Ampère finds that parallel wires carrying current produce forces on each other.

1820s, 1830s Michael Faraday develops the concept of electric field and shows that

electric current + magnetism -> motion (motor effect)

motion + magnetism -> electric current (electromagnetic induction)

1860s James Clerk Maxwell (1831-1879) establishes a mathematical description of electromagnetism.

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Motors everywhere

lifts & escalators; fans, turbines, drills; wheelchairs; car windscreen wipers, starter motors, windows & side mirrors; motors in

electric cars, locomotives & conveyor belts; industrial robots, saws and blades in cutting and slicing processes; food mixers &

blenders, microwave ovens; hand power tools such as drills, sanders, routers; electric toothbrushes, shavers, hairdryers;

vacuum cleaners, sound systems, computers … using electricity supplied by power station generators

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Field lines indicate both direction and magnitude (strength) of a magnetic field. They end at poles.

A compass needle can be thought of as a test dipole.

Magnetic flux density (‘field strength’) has symbol B, unit tesla.

Describing a magnetic field

Bar magnet

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Common misconceptions

• All metals are magnetic materials.

• Static charges interact with the poles of permanent magnets.

• Magnetic poles are located on the surface of a magnet.

[Careful observation shows that they are inside the magnet.]

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Magnetic poles: always pairs

A permanent magnet can be split into two or more

magnets, each with N and S poles which cannot be isolated.

This suggests that the magnetic effect of a permanent

magnet comes from microscopic, circulating electric

currents.

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Electron spin, inside atoms, is the main cause of

ferromagnetism.

demagnetised

magnetised

Microscopic structure

Domain theory

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Magnetising & demagnetising

Make a magnet

• by stroking

• by using DC coil carrying current

• by tapping while aligned with the Earth’s field

Demagnetise a magnet

• by dropping or banging randomly

• by heating

• by applying a diminishing AC current

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Magnetic induction

A permanent magnet can induce temporary magnetism in a ‘soft’ magnetic material.

• This causes attraction, but cannot cause repulsion.

• Use repulsion to test if an object is already magnetised.

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Right hand screw rule, a.k.a. the ‘corkscrew’ or

‘pencil sharpener’ rule:

Place thumb in direction of current; fingers indicate direction of the magnetic field.

Magnetic field of a straight wire

NB: Here field lines are closed loops.

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Magnetic field of a solenoid

Right hand grip rule:

Wrap fingers around solenoid in direction of current; thumb indicates N pole.

N S

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Note the similarity

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A stronger electromagnet

Length of a solenoid is L

• Use iron or steel core

(increasing permeability, )

• Increase the current, I

• Increase wraps or turns of solenoid, N.

L I

B   N

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Uses of electromagnetism

• loudspeaker

• moving coil microphone

• motors of various designs

• electric bell or buzzer

(can be made in class, URLS below)

• moving coil galvanometer (ammeter)

• relay

(control circuit with small current switches a second, larger, current circuit)

Practical Physics website: model buzzer, model electric bell

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Catapult effect

Fleming’s left hand

rule

Force on a current-carrying wire in a B-field. Compare AC to DC.

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Simple DC motor

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Westminster kit motor

http://www.nuffieldfoundation.org/practical-physics/electric-motor

Model loudspeaker

http://www.nuffieldfoundation.org/practical-physics/model- loudspeaker

Motors & loudspeakers

homopolar motor

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Parallel currents

parallel - attract anti-parallel - repel

Force per unit length, at spacing r,

F  _oI₁I₂ 2πr

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The ampere defined

1 ampere: the steady current which, when flowing in two

straight parallel wires of infinite length and negligible cross-

section, separated by a distance of one metre in free space,

produces a force between the wires of

2 × 10

^-7

newtons per metre of length

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Electromagnetic induction

(‘Dynamo effect’)

Faraday’s law: Relative motion of a wire and a magnetic field will induce an e.m.f. (voltage). If there is a complete circuit, a current will be induced too.

– magnet stationary, coil moves – coil stationary, magnet moves,

– coil stationary, magnetic field lines changing

Induced EMF is proportional to ‘the rate at which field lines are cut’.

Lenz’s Law: The induced current always flows in such a direction as to oppose the change which causes it.

Faraday’s Electromagnetic Lab phet.colorado.edu/en/simulation/faraday

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Lenz’s law illustrated

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AC generator

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Motor/generator

V

_s

How a transformer works:

micro.magnet.fsu.edu/electromag/java/transformer/index.html

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High voltage transmission

Heating loss in a transmission cable:

Keep current small by making voltage large.

Grid voltages: 275 kV, 400 kV Model power line

www.electrosound.co.uk

R I IR

I IV

P   ( ) 

²

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A sustainable energy future

‘… much more energy demand will be met through the electricity system and generation will be added both centrally and

throughout the distribution system.’

‘Turning [carbon] emissions reduction targets into reality will

require more than political will: it will require nothing short of the biggest peacetime programme of change ever seen in the UK.’

(Royal Academy of Engineering report, March 2010, Generating the future)

‘Renewable generation, which by its nature will be widely distributed and mainly located in coastal and northern regions, will also require considerable investment in

electrical supply system infrastructures both in terms of local distribution systems and the national grid.’

(Royal Academy of Engineering, July 2006, Energy seminars report)

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Safety

Hazard with strongest rare earth (neodymium) magnets – swallow, shatter, pinch, interfere

• keep away from (>1m) any person who uses medical aids like a pacemaker

• only responsible students or yourself to handle largest ones, or more than one at a time

• wear safety spectacles and protective gloves when handling two or more of the largest, most powerful magnets – risk of

shattering or pinching

• keep away from (>1m) electronic devices like computer monitors, credit cards and memory sticks

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Electromagnetism: a summary

• The force, F, acting on charge q

has two components:

E, electric field due to stationary charge(s).

B, magnetic field due to moving charge(s) - currents - with relative velocity v.

• can be superposed e.g. E = E₁ + E₂ + …

• electric & magnetic fields store energy

• Maxwell’s equations: laws that describe the structure of the electromagnetic field. E and B fields can exist without a circuit and test magnetic dipole.

 ^E ^v ^B 

q

F   

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J. Clerk Maxwell (1865), ‘A Dynamical Theory of the Electromagnetic Field’ Phil. Trans. R. Soc. Lond.

A changing electric field induces a changing magnetic field, and vice versa. It therefore makes sense to talk of an

‘electromagnetic field’.

Electromagnetic waves propagate in free space at c = 3 x 10⁸m/s.

E and B are always perpendicular to each other, and perpendicular to the direction of propagation.