Focus 5Magnetism and Electricity

Behaviour of Magnets

Like poles repel Unlike poles attract

A Magnetic Field is defined as the direction a small north pole would move if placed at that point. The symbol for magnetic field is B.

The strength of a magnetic field is measured in Teslas (T)

Note that you can’t have a north pole without a south pole, but the definition ignores this!

A Magnetic Field

Magnetic fields around pairs of magnetic poles

Ørsted’s Experiment Ørsted was the first scientist to record evidence that

magnetism and electricity were connected. While setting up a demonstration for a lecture he noticed that when he brought a magnet close to an electric current the magnet changed direction.

We now know that any moving charged particle produces a magnetic field.

Since an electric current is the movement of electrons, which have a negative charge, a wire with an electric current through it will have a magnetic field around it.

The shape of the magnetic field produced depends on the shape of the wire carrying the current.

Long straight wires carrying an electric current have circular magnetic fields around them.

These are shown by concentric circles with arrows to indicate the direction of the field.

Long Straight Wires

Use the links below to investigate the magnetic fields surrounding long straight current carrying wires. The links are interactive “virtual experiments”.

http://www.walter-fendt.de/ph11e/mfwire.htm

http://www.magnet.fsu.edu/education/tutorials/java/magwire/index.html

http://www.magnet.fsu.edu/education/tutorials/java/fieldlines/index.html

Your right hand is a very useful tool in working out directions for electric currents and magnetic fields. › Your thumb is used to show the direction of

anything that is straight› Your fingers are used to show the direction

of anything that is curving.

Note that you can’t use your left hand for this!!!

Right-hand Grip Rule

The right hand grip rule can be used to determine the direction of the field lines around a wire.

“Grip the wire with your right hand so that your thumb is pointing in the direction of the conventional current. The coil of your fingers shows the direction of the magnetic field”.

Your fingernails are like the arrow heads.

Drawing Cross-sections Often wires and magnetic fields are drawn in cross

section.

When drawing these cross sections the following symbols are used to represent the directions into and out of the page:

- means the current or field line is moving out of the page.

- means the current or field line is moving into the page.

Bar Magnet Imagine that the bar magnet below was viewed

from each end.

From this end the magnetic field lines would appear to come out of the page (or towards you).

While from this end the magnetic field lines would appear to be going into the page (or away from you).

Drawing wires in cross-section Let’s look at an example of a wire. Imagine we took the wire

below cut it along the red line and viewed it in cross section.

If you looked at the cross section from the direction shown would the current be moving towards you or away from you?

Conventional current

Answer: The current appears to be moving towards you.

The cross section of the wire would be drawn like so….

We can use the right hand grip rule to add the magnetic field to the cross section.

According to the rule the magnetic field will be anti-clockwise around the wire.

Conventional current out of the page

Drawing magnetic fields in cross-section

Going back to the straight wire.

If we drew a cross section of the magnetic field around the wire it would look like this.

Solenoids A solenoid is basically a long wire that

has been looped around many, many times to form a tightly packed helix, as shown in the diagram below.

The diagram shows the magnetic field around one of the loops in the solenoid. Use the right hand rule to check the direction of the magnetic field around the loop.

When many loops are packed tightly together the following magnetic field is generated.

Compare the magnetic fields surrounding a solenoid and a bar magnet.

Solenoids have a north and south pole similar to a bar magnet.

The north pole is the end of the solenoid from which the field lines emerge.

The south pole is the end of the solenoid where the field lines enter.

Properties of Magnetic Fields around Solenoids

The magnetic field inside a solenoid is uniform.

The field surrounding a solenoid is identical to that around a bar magnet.

The field within a solenoid is proportional to the current flowing and to the number of turns of wire per metre.

Finding the poles of a solenoid The location of the north and south poles

of a solenoid can be found using a variation on the “Right hand grip rule”.

If you imagine gripping the solenoid in your right hand with your fingers pointing in the direction of conventional current, your thumb points to the north pole.