Download On magnetism

Magnetism &
electromagnetism
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
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
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.]
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)
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.
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
Describing a magnetic field
Field lines indicate both direction and magnitude
(strength) of a magnetic field. They end at poles.
Bar magnet
A compass needle can be thought of as a test dipole.
Magnetic flux density (‘field strength’) has symbol B, unit tesla.
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.]
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.
Domain theory
demagnetised
Microscopic structure
Electron spin, inside atoms,
magnetised
is the main cause of
ferromagnetism.
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
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.
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.
Magnetic field of a solenoid
N
S
Right hand grip rule: Wrap fingers around solenoid in
direction of current; thumb indicates N pole.
Note the similarity
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.
N
B I
L
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
Catapult effect
Fleming’s
left hand
rule
Force on a current-carrying wire in a B-field. Compare AC to DC.
Simple DC motor
Motors & loudspeakers
Westminster kit motor
http://www.nuffieldfoundation.org/practical-physics/electric-motor
Model loudspeaker
http://www.nuffieldfoundation.org/practical-physics/modelloudspeaker
homopolar motor
Parallel currents
parallel - attract
anti-parallel - repel
Force per unit length, at spacing r,
F=
mo I1I 2
2πr
The ampere defined
1 ampere: the steady current
which, when flowing in two
straight parallel wires of infinite
length and negligible crosssection, 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
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
Lenz’s law illustrated
AC generator
Motor/generator
SEP unit
Transformer
the ‘turns-ratio equation’
Vp N p
=
Vs N s
Ideal transformer
power in primary coil = power in secondary coil
I pVp = IsVs
Is Vp
=
I p Vs
How a transformer works:
micro.magnet.fsu.edu/electromag/java/transformer/index.html
High voltage transmission
Heating loss in a transmission cable:
P  IV  I ( IR)  I R
2
Keep current small by making voltage large.
Grid voltages: 275 kV, 400 kV
Model power line
www.electrosound.co.uk
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)
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
Electromagnetism: a summary
• The force, F, acting on charge q
F  qE  v  B 
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.
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.
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
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