Physics - Currents and magnetism
Currents and magnetism
View the conventional current flow video.
Whenever a current flows in a wire, a magnetic field is automatically
produced around the wire. In this manner, magnetic fields can be turned on
or off at the flick of a switch. The magnetic field only exists while the
current is flowing. Unlike the fields of bar magnets, these fields are not
dipolar. Rather they are circular. The direction of the field is determined
by the direction of current flow in the wire. The right-hand-grip-rule is
the easiest way to determine the field direction. If the thumb of the right
hand is aligned with the direction of conventional current, then the curved
fingers indicate the direction of the circular field as shown below.
If the current were travelling perpendicularly into or out of the page,
the diagrams below would be used.
The magnetic field produced around an individual length of wire is quite
weak and it reduces considerably in strength as the distance from the wire
increases. The most effective way to create a strong magnetic field is to
wind the current-carrying wire into loops. This is called a solenoid or
The direction of current flow determines the direction of the field
formed. The magnetic field of a solenoid is similar to that of a bar magnet
as shown below, and so it is convenient to think of the ends of the
solenoid as equivalent to the north or south pole of a bar magnet. A quick
way of remembering which pole is produced by a given current direction is
shown below. If the observer is 'seeing' current flow anti-clockwise they
would be viewing the north end of the solenoid. If the observer is 'seeing'
clockwise current then they would be viewing the south end of the solenoid.
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