Electromagnets

• Electric current through a wire produces a magnetic field These are arranged in concentric rings around the wire.

Bat te ry+-

Recall the direction of a magnetic field

S N

• so place a compass on a cardboard card with the current traveling as shown.

Bat te ry+-

e -

Use the left hand rule

– given that the electron flow is from – to + (non – conventional current)

– grab the wire with your left hand with the thumb in the direction of the electron flow

– your fingers will point in the direction of the magnetic field.

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This is also known as Ampere's Law.

Solenoids

• A solenoid is a collection of wire loops. One loop when looked at may look like this.

Draw in the lines of magnetic flux

These may be simplified like this

Note that the lines inside the loop travel in the same direction. These will add together to a more dense or stronger field. The lines on the outside travel in opposite directions, so they will cancel out.

Suppose we have a card with 4 loops in it.

Draw in the magnetic field lines for each loop

Replace the individual lines with a larger field drawing showing fields lines added together.

Rotate card to look at it from above.

In effect a bar magnet has been created. Which end is North?

Left Hand rule for a solenoid– Grasp solenoid with your left hand so that

fingers go in direction of electron flow.– Thumb points towards North pole of solenoid.

e le c tro n f lo w

N o r th