Slides in this set
A magnetic field is a region where magnetic materials
or wires carrying currents experience a force acting
A magnetic field is represented by a field
diagram. The arrows on the field line
always point from the north pole of the
magnet to the south pole.
When a current flows through a wire, a
magnetic field is created around the
wire. The field is made up of
concentrated circles with the wire at
the centre.…read more
The magnetic field inside a coil of wire is strong and uniform.
Outside the magnetic field is just like one round a bar magnet.
The strength of the magnetic can be increased by adding a
magnetically "soft" iron core through the middle of the coil. It is
then an electromagnet.
A magnetically soft material magnetises and demagnetises very easily. So
as soon as you turn off the current through the solenoid, the magnetic field
disappears the iron doesn't stay magnetised. This is useful for switching
a magnet on and off.
Electromagnets are used in cranes in scrap yards because they can attract
and pick things up, then drop things as soon as the current is switched off. If
an ordinary magnet was used it would attract the object but never drop it
which isn't very useful.…read more
Take a look at the diagram below. When the circuit is switched
on a current flows through it.
The points you must make:
current flows through the coil
magnetic field produced
(short side of) iron bar
attracted to electromagnet
contacts pushed together
(by iron bar)
starter motor circuit
current flows through starter
The Motor Effect
A force experienced by a current-carrying wire in a magnetic field
is known as the motor effect.
The force on the wire is at 90º to
both the wire and to the
magnetic field. If the direction of
the current or the magnetic field
is reversed then the force is also
reversed (so it would go up).
To experience the full force the wire
has to be at 90º to the magnetic
field. If it is parallel it won't
experience any force.
Fleming's left hand Rule
There is a way to work out what direction the force will be in.
(F) First finger is the force (F), the first
field is the direction of the
90º (B) magnetic field (the unit for
N S magnetic field strength is B)
and the second field is current (i
+ 90º is for intensity of current).
- So all you need to remember is FBI