2. 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.]
3. 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.
4. 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
5. 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
6. 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.]
7. 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.
8. Electron spin, inside atoms,
is the main cause of
ferromagnetism.
demagnetised
magnetised
Microscopic structure
Domain theory
9. 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
10. 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.
11. 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.
12. Magnetic field of a solenoid
Right hand grip rule: Wrap fingers around solenoid in
direction of current; thumb indicates N pole.
N S
14. 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.
I
L
N
B
15. 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
20. 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
21. 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.
27. power in primary coil = power in secondary coil
Ideal transformer
IpVp = IsVs
Is
Ip
=
Vp
Vs
28.
29. 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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30. 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)
31. 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
32. 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.
B
v
E
q
F
33. 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.
Electromagnetic waves
34. 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
![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.]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)