A bar magnet is a permanent magnet as it is always magnetic. A magnet can always attract magnetic materials, like iron, nickel and cobalt. The space around the magnet is called the magnetic field, where magnetic materials experience a force. Opposite magnetic poles attract while like poles repel. Induced magnets are magnetic materials in a magnetic field which become magnetic themselves. Magnets are used in electric motors, generators, loudspeakers and other electric devices, as well as simple things like door latches and knife holders. Magnetic field lines show how an isolated north pole would move from north the south.
The shape of a magnetic field can be found using plotting compasses. The needle of a compass is a small magnet which points to the Earth’s magnetic south pole, or the Earth’s geographical north pole. The behaviour of compasses shows that the Earth has a magnetic field, which is thought to be caused by electric currents in the molten outer core.
A current flowing through a wire causes a magnetic field. The strength of the field depends on the size of the current and the distance from the wire.
The direction of the magnetic field can be worked out using Fleming’s Right Hand Rule. If the current is coming towards you (in a diagram, the wire is depicted using a dot) then point your thumb at yourself. The direction of your curled fingers (anticlockwise) is the direction of the magnetic field. If the current is going away (wire is shown as a cross), the field lines are clockwise.
When a wire is made into a coil, called a solenoid, the magnetic fields of different parts interact to form an overall magnetic field similar to that of a bar magnet. There is a strong field inside the coil, but outside the fields of individual coils cancel out to make a weaker field. A coil of wire carrying a current is called an electromagnet. The magnetic field can be strengthened by adding a soft iron core which becomes a temporary magnet
A current carrying conductor placed near a magnet experiences a force and an equal and opposite force acts on the magnet. A magnetic force is caused by interacting magnetic fields. The force is greatest when the wire is at right angles to the magnetic field produced by the magnets. The direction of the force depends on the directions of the magnetic field and the current. Fleming’s Left-Hand Rule shows how the directions are related. The hand rotates to match the information given in the question and the direction of movement is shown by the thumb’s direction
Use the equation where F = Force on Conductor acting at Right Angles to the Magnetic Field (N), B = Magnetic Field Strength (T), I = Current (A) and l = Length (m)
- The force on a conductor in a magnetic field is used to cause rotation in electric motors
- A motor has a coil of wire in a magnetic field. The coil is attached to a split-ring commutator which has electrical contact with a circuit through carbon brushes
- There is a force on the sides of the coil because the magnetic forces from the magnet and the wire interact causing a force on the wire
- The magnetic field lines interact and cause movement
- The two sides of the coil move in the same direction, so the coil begins to turn
- The coil continues to turn as the split ring commutator reverses the direction of the current every half-turn to ensure that the coil continues to turn in the same direction