Magnetism and Electricity

ELECTRICITY AND MAGNETISM


Magnetism

 When 2 magnets are brought closer together, there is a force between them
o 2 like magnetic poles repel one another
o 2 unlike magnetic poles attract each other
 Always point away from north pole towards south poles
 Magnet can attract magnetic materials (iron, cobalt, nickel)

Magnetic materials Non-magnetic materials
 Respond to a magnetic field (attract)  Not be attracted to a magnet
 Have free electrons  Cannot be magnetised by a magnet
 Can be magnetised by a magnet


Type Description Examples Uses
Hard Retains magnetism well Hard steel Permanent magnets,
but difficult to compass needles,
magnetise loudspeaker magnets
Soft Easy to magnetise but Soft iron Cores for electromagnets,
readily loses its transformers, radio aerials
magnetism
Methods of magnetisation

 Stroke with a permanent magnet consistently from end to
end repeatedly
 Connect to a d.c. power source
 Hammer the material placed in a magnetic field in south-
north direction

Methods of demagnetisation

 Hammer the material while it is placed in east-west direction
 Place in a.c.
 Heat the magnet

Induced magnetism

 When pole of a bar magnet is brought close to an magnetic object, the end of
the nearest to the pole becomes temporarily magnetic south/ north pole of
the object automatically
 After the magnet is withdrawn, the object’s magnetic south pole disappears,
return to unmagnetised state or retain small amount of magnetism

Experiment: identify the pattern of magnetic field lines, including the direction

1. Place a bar magnet in the centre of a sheet of paper and draw around it
2. Place a plotting compass near one of the poles, it turns to indicate the
direction of field. Mark dots 1 and 2 to indicate 2 ends of the compass needle

, ELECTRICITY AND MAGNETISM


3. Move compass away from magnet, position it so 1 end is in dot 2 position,
mark dot 3
4. Repeat until you have moved round to other pole
5. Remove compass, connect the line then repeat the process for another field
line

Permanent magnet Electromagnet
Design  Permanently magnetic  Temporarily magnetic
 Made from hard steel  Made from a coil of copper wire
(solenoid)
Uses Compasses, generators Relays, speakers, motors


Electrical quantities

 There are positive and negative charges, measured in coulombs (C)
 Unlike charges attract and like charges repel
 Direction of an electric field at a point is the
direction of the force on a positive charge at
that point

Experiments: show the production and
detection of electrostatic charges

1. Rub a rod using a cloth and suspend it by
hanging
2. Rub another rod
3. Place the free rod near the suspended
one, if they repel -> they have same
charge
4. Place the cloth near the suspended rod, if
the attract -> they have opposite
charges
 Charging a body involves the addition or
removal of electrons

Electric field: a region in which an electric charge experiences a force

Charging by induction

a, A has a large negative charge -> electrons in B are repelled away

b, B is touched by a wire connected to earth so electrons can escape

c, Connection is removed. B has positive charge and must be disconnected before A
is moved away

d, A is taken away so B can have a uniformly distributed positive charge