Brush Up On Magnetism

S N

Lines Of Force

• Lines of Force have direction. We Say that their direction is from north to south.

• The strength of the field is represented by how close the lines of force are together. (the stronger the field the closer the lines)

S N

The field is stronger at the poles

S N S N

The field between oppite poles on adjacent bar magnets.

N S S NX

Neutral point

Neutral points occur where the sum of the fields is zero

With like poles the lines of force are pushed apart.

lines of force never cross.

THE MAGNETIC FIELD OF THE EARTH

The north pole of a free swinging magnet points north

Earth’s local magnetic field appears uniform locally. This is how a compass behaves as it is moved across a lab bench

This is only the horizontal component of the field

There is also a dip angle θ is around 680 in the north of England. So a compass on its side points down at this angle.

θ

Soft iron in the Earth field

The Earth’s magnetic field is deviated by “ferromagnetic” materials like iron, nickel or cobalt

Current Carrying Wires Have An Associated Magnetic Field

Placing a compass at points around a long straight wire reveals the field

The concentric field lines around a straight current carrying wire.

X

The convention for current into the paper (left) and out of the paper (right)

The direction of the conventional current is found by the corkscrew rule

Current into the paper (i.e. positive terminal at the top)

An alternative way of remembering is the handgrip rule

The strength of the magnetic field is a vector quantity. In a coil the “sense” (direction) of the field lines are the same and add and the result is a field in which the lines are almost parallel in the centre.

COILS

Around a helical coil (solenoid) the field is startlingly similar to the field around a bar magnet.

The field inside a solenoid

An easy way to remember the poles of a solenoid given the direction of the current looking at each end of the coil.