Download Magnetic Field Patterns


When an electric current passes through a wire, a
magnetic field is created around the wire.
The lines of force
are in circles around
the wire.
 The field is
strongest near the
wire.
 The direction of the
field is reversed if
the direction of the
current is reversed.


We can use the right hand rule.
 Thumb points for current, fingers show the
magnetic field direction

Draw the current lines for:
Solenoid: A long coil of wire wound around a tube
that may contain an iron core or be filled with air.
 There is a magnetic field in and around the solenoid.
 Concentrated at the ends
 Parallel inside
 Spread out beyond the ends

Reversing current, reverses the field
lines
 Increasing current increases the
strength


The magnetic field inside a solenoid is used in
cathode ray TV tubes & ‘magnetic resonance’ (MR)
brain scanner.

The magnetic field outside a solenoid is used
in scrap yards machines and in electrical
relays.

Motor Effect: A force is created on a wire
when it is in a magnetic field.


Electric Motors work due to the motor effect.
The wire and the magnet exert equal and
opposite forces on each other.

The force can be increased by:
 Increasing the current
 Using a stronger magnet

The direction of the force is reversed if the
direction of the current or the magnetic field
is reversed.

The force depends on the angle between the
wire and the magnetic field lines. The force is:
 Greatest at 90°
 Zero then they are parallel
 Fleming’s left hand Rule:
▪ Thumb = Force
▪ Index Finger = Magnetic Field
▪ Middle Finger = Current
The magnetic field of the magnet and the wire cancel out
on one side and reinforce each other on the opposite side.
 This results in a force on the wire sideways towards the
weakest part of the field.

1.
2.
Define ‘motor effect’
What is Flemming’s left hand rule?
A coil in a magnetic field
can be forced to turn by
passing a current through
it.
 When a current goes
through the coil, a force
acts on each of the two
long sides of the coil.
 As these forces are in
opposite directions,
they act to make the coil
turn.


A simple electric motor has a rectangular coil of wire
that spins in a magnetic field when a current passes
through the coil.
In a practical motor, the rotating part of the motor
or ‘armature’ consists of several evenly-shaped coils
wound on an iron core.
 The iron core makes the field much stronger so the
turning effect is much greater.


Electromagnetic
Induction: the creation of
an e.m.f. when a wire
moves across a magnetic
field.
 Induced e.m.f. (makes an
emf)
 How a generator works - a
generator contains coils of
wire that spin in a magnetic
field.
Basically the opposite of the motor effect. Instead
of using an e.m.f. to move the wire, the wire is
moved to make an e.m.f.
 If the wire is part of a circuit, the induced e.m.f.
makes an electric current pass around the circuit.


You can test a coil of insulated wire
connected to an ammeter to see if a current is
induced.