This is all about the connection between
electricity and magnetism.
How do you make a magnet using
electricity?
When a current flows through a wire
(or any conductor) a magnetic field is created around the wire.
The field (of the magnetic flux) can be
imagined as a series of concentric circles at right-angles (perpendicular)
to the wire - which is at the centre of the magnetic field (see the diagrams
below).
You can show the magnetic field patterns by sprinkling fine iron filings
on a sheet of paper at 90o to the copper wire carrying a d.c. electric
current.
You can demonstrate the presence of the
magnetic field with iron filings and thickish wire carrying a relatively
high current.
For the above diagram - imagine the
current flowing through a straight wire and the magnetic field can be
envisaged as a series of concentric rings about the axis of the conducting
wire.
The direction of the magnetic field can
be predicted from the 'right-hand thumb' rule.
If the current is flowing 'up' through
the wire, the magnetic field runs anticlockwise and perpendicular to the
wire.
You can show the direction of the
field with a small plotting compass - two shown on the diagram above -
and you can trace out the circular pattern of the magnetic field.
A few simple rules (apart from the
right-hand thumb rule)
(i) If you reverse the direction
of current flow, you also reverse the direction of the magnetic field.
(ii) The strength of the magnetic
field is increased overall by increasing the current.
(Don't say by 'increasing the
p.d.' without saying to increase the current!)
(iii)
For any current carrying wire, the closer you are to the wire, the
greater the strength of the magnetic field.
The
magnetic field flux lines get closer and closer together the nearer
you are to the wire - meaning the magnetic field strength increases
the nearer you are to the wire.
The strength of the magnetic
field falls away quite rapidly at first as you get further
from the wire, then the reduction rate slows down with increasing
distance.
Its a non-linear graph.
Know that electromagnetism is when a magnetic field is
created around a
wire carrying an electric current.
Know that from the direction of convention current, you can
predict the magnetic field direction from north to south
with the right-hand thumb rule.
Be able to describe how to show or plot the magnetic field
around an electromagnet using iron filings or a plotting
compass.
