Magnetism Magnets have 2 poles north pole and

Magnetism Magnets have 2 poles – north pole and south pole. Like poles repel and unlike attract. Magnets occur naturally (loadstone) and can be created from iron nickel or cobalt. A current carrying wire has a magnetic field. A magnetic field is the area around a magnet where a magnetic force is felt. The direction of a magnetic field is that in which a north pole would move.

Plotting Magnetic Fields • Magnetic fields are plotted using compasses. Bar magnet Straight wire Right hand grip rule Solenoid Loop

The horseshoe magnet

The Earths Magnetic Field The angle between the geographic and magnetic north is called the magnetic declination or magnetic variation.

The Earths Magnetic Field • The earths magnetic field is used in navigation. • It also protect the earth from cosmic radiation.

Current in a magnetic field A current carrying wire will experience a force when placed in a magnetic field.

The Left Hand Rule • Index finger field • Middle finger current • Thumb force

Current in a magnetic field The force experienced, the current direction and the magnetic field direction are all perpendicular to each other. The left hand rule will predict direction. This is the principle behind loud speakers moving coil metres the electric motor,

The speaker Theloud Loudspeaker A current carrying wire experiences a force when placed in a magnetic field.

The Motor A current carrying wire experiences a force when placed in a magnetic field.

The moving coil meter A current carrying wire experiences a force when placed in a magnetic field.

To demonstrate the force experienced by a conductor when placed in a magnetic field. Place a piece of aluminium foil between the poles of a strong magnet. Pass current through the foil and it will move under the force experienced. Reverse the direction of the current and note that the foil moves in the opposite direction. Why is aluminium used?

The size of the force depends on: 1. The current (I) 2. The length of the conductor (L) 3. The magnetic flux density (B) F = BIL The magnetic flux density (B) is the force per unit current per unit length on a conductor at right angles to a magnetic field. The unit is the TESLA.

When the conductor makes a non right angle with the field. It is the component of B perpendicular to the conductor that causes the force. i. e. BSinϑ

The Ampere 1 Amp 1 m 2 x 10 -7 N/m

The Ampere is the current in 2 straight parallel conductors one metre apart in a vacuum that causes each to exert a force of 2 x 10 -7 N per metre length on the other. Tandem Felix

The Ampere Set up the circuit as shown. The foil strips are seen to move away from each other. This is the experiment on which the definition of the Ampere is based.

Force on a charge in a magnetic field • The force is perpendicular to the direction that the charge moves in and so the force causes the charge to move in a circular path. • This force is a centripetal force. • Circular motion equations apply.

Force on a charge in a magnetic field