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    1. Describe the pattern of the magnetic field due to currents in straight wires and in solenoids



When an electric current passes along a wire, a magnetic field is set up around the wire. These are at right angles to the direction in which the electric current flows. The field is strongest near the wire. The direction of the field is reversed if the direction of the current is reversed.

You can use your right-hand (as shown here) to determine the direction of the magnetic field with respect to the direction of the current. If current is upwards, the magnetic field runs in anti-clockwise direction and if the current is downwards, the field is in clockwise direction.

A solenoid is a long coil of wire. When current is passed through a solenoid, a magnetic field develops in and around the solenoid. Reversing the direction of the current reverses the current of the magnetic field.

Outside the solenoid, the magnetic field pattern is that of a bar magnet. When wrapping the right hand around the solenoid with the fingers in the direction of the  current, the thumb points in the direction of the magnetic north pole (the solenoid rule). And the other side will be the south pole. Inside the solenoid, the magnetic field is uniform because the field has the same strength except at the ends where the field gets weaker.

  1. Describe the effect on the magnetic field of changing the magnitude and direction of the current.

The strength of the current is proportional to the strength of the magnetic field. If one increases, so does the other.
When direction of the current is changed, he polarity of the magnetic field changes as well. So the north and south pole of the magnetic field is reversed when the direction of the current is reversed.

  1. Describe applications of the magnetic effect of current, including the action of a relay.

Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off. When a current flows through the coil it produces a magnetic field.  This field is temporary and is lost when the current is switched off.

Electromagnets are used in relays. A relay is a device which uses a low current circuit to switch a high current circuit on or off. In relays, an electromagnet is used to move the contact from one circuit to another. For example, a very big relay is used in cars for switching the starter motor, because it draws a very big current

  • When the switch in the low current circuit is closed, it turns the electromagnet on which attracts the iron armature.
  • The armature pivots and closes the switch contacts in the high current circuit.
  • When the low current switch is opened the electromagnet stops pulling the armature and the high current circuit is broken again.


Other applications of electromagnets include electric bell circuits. When the bell is pressed the circuit is complete and current flows. The soft iron core of the electromagnet becomes magnetised and attracts the iron armature. When the armature moves, the hammer strikes the gong and at the same time a gap is created at the contact screw. The circuit is incomplete and current stops flowing.



Notes submitted by Lintha

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