Magnetism & electromagnetism

1. Magnetism & electromagnetism

2. 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

3. 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

4. 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.]

5. 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)

6. 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.

7. 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

8. Describing a magnetic field 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 symbolB, unit tesla. Bar magnet

9. 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.]

10. 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.

11. Domain theory Microscopic structure demagnetised Electron spin, inside atoms, is the main cause of ferromagnetism. magnetised

12. 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

13. 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.

14. Magnetic field of a straight wire NB: Here field lines are closed loops. 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.

15. Magnetic field of a solenoid S N Right hand grip rule:Wrap fingers around solenoid in direction of current; thumb indicates N pole.

16. Note the similarity

17. 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.

18. 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

19. Fleming’s left hand rule Catapult effect Force on a current-carrying wire in a B-field. Compare AC to DC.

20. Simple DC motor

21. Westminster kit motor http://www.nuffieldfoundation.org/practical-physics/electric-motor Model loudspeakerhttp://www.nuffieldfoundation.org/practical-physics/model-loudspeaker Motors & loudspeakers homopolar motor

22. Parallel currents parallel - attract anti-parallel - repel Force per unit length, at spacing r,

23. 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

24. 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 Labphet.colorado.edu/en/simulation/faraday

25. Lenz’s law illustrated

26. AC generator

27. Motor/generator SEP unit

28. Transformer

29. the ‘turns-ratio equation’

30. Ideal transformer power in primary coil = power in secondary coil How a transformer works:micro.magnet.fsu.edu/electromag/java/transformer/index.html

32. 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

33. 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)

34. 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

35. 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. EandBfields can exist without a circuit and test magnetic dipole.

36. Electromagnetic waves 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 108 m/s. E and B are always perpendicular to each other, and perpendicular to the direction of propagation.

37. Em fields are real ‘The electromagnetic field is, for the modern physicist, as real as the chair on which he sits.’ Einstein and Infeld, 1938

38. Support, references talkphysics.org SPT 11-14 Electricity & magnetism David Sang (ed., 2011) Teaching secondary physics ASE / Hodder Practical Physics website: Electromagnetism topic http://www.nuffieldfoundation.org/practical-physics/electromagnetism PhET simulation Faraday’s Electromagnetic Lab