Electromagnets

Magnetic Field Lines We can represent the magnetic field around a wire or magnet using field lines.

Magnetic Field Lines The arrows show the direction a compass needle would point at that point in the field.

Field around a magnet

Moving charges (currents) Moving charges (electric currents) also produce a magnetic field Conventional current – electrons flow in the opposite direction

Magnetic field around a straight conductor Current OUT of page Current IN to page You can use the right hand screw rule to decide which direction the field goes

Electromagnets

Field around a solenoid

Field around a solenoid

Electromagnets

To make an electromagnet stronger The 3 C’s More Coils More Current Use an iron Core

Electric Bells When the switch is on, the circuit is completed and current flows. The electromagnet becomes magnetised and hence attracts the soft-iron armature and at the same time pull the hammer to strike the gong. As soon as the hammer moves towards the gong, the circuit is broken. The current stops flowing and the electromagnet loses its magnetism. This causes the spring to pull back the armature and reconnect the circuit again. When the circuit is connected, the electromagnet regain its magnetism and pull the armature and hence the hammer to strike the gong again. This cycle repeats and the bell rings continuously.

Circuit Breaker The figure shows the structure of a circuit breaker. A circuit breaker is an automatic switch that cut off current in a circuit when the current become too large. When the current in a circuit increases, the strength of the electromagnet will increase in accordance; this will pull the soft iron armature towards the electromagnet. As a result, the spring pulls apart the contact and disconnects the circuit immediately, and the current stop to flow. We can reconnect the circuit by using the reset button. The reset button can be pushed to bring the contact back to its original position to reconnect the circuit.

Relay A relay is an electrical switch that opens and closes under the control of another electrical circuit. A relay has at least two circuits. One circuit can be used to control another circuit. The 1st circuit (input circuit) supplies current to the electromagnet. The electromagnet is magnetised and attracts one end of the iron armature. The armature is then closes the contacts (2nd switch) and allows current flows in the second circuit. When the 1st switch is open again, the current to the electromagnet is cut, the electromagnet loses its magnetism and the 2nd switch is opened. Thus current stop to flow in the 2nd circuit.