An electromagnet is a device which is magnetized when current flows through it and loses its magnetism when the current is switched off. An electromagnet basically consists of solenoid with a core made from a soft magnetic material like iron. Figure below shows a simple electromagnet.

When the current is switched on, the soft iron is magnetized by the magnetic field created in the solenoid. The soft iron also concentrates the field lines, producing a field which is far stronger than that from the coil alone.

As the core is made from a soft magnetic material, it immediately demagnetizes when the current is switched off. This enables the magnetism of the electromagnet to be switched on and off.

The strength of the electromagnet can be increased by:

1. Increasing the magnitude of the current.

2. Increasing the number of turns per unit length of the coil.

Uses of the electromagnet

The electromagnet has a variety of uses, some of which include;

• Use in the electric bell; Figure below shows a simple construction for the electric bell, showing how an electromagnet is put to use.
• When the switch is closed, current flows through the electromagnet and it becomes magnetized. The iron armature is pulled towards the electromagnet, making the hammer to hit the gong. At the same time, the contact at S is broken and current immediately stops
• The electromagnet becomes demagnetized and the iron armature is released; the contact at S is made again and the process repeats.

• Therefore the bell can continue to ring until the switch button is released.

   

• Use in the magnetic lift to separate magnetic materials like iron and steel from non-magnetic materials like copper, aluminium, etc.

•  

  Electromagnets are also widely used in the telephone earpiece, the relay switch, circuit breakers, moving-coil current meters, etc.

   

  Electromagnets have the following advantages over permanent magnets

• Their magnetism can be switched on and oR unlike that of permanent magnets.
• The strength of the electromagnet can also be varied by varying either the magnitude of the current or the number of turns per unit length of the wire.
• The motor effect

• When a current-carrying conductor is placed in a magnetic field different from that due to itself, it experiences a mechanical force. This is referred to as the motor effect and is illustrated in figure below.

When the switch is closed, the wire is seen to be pulled inwards.

EXPLAINING THE MOTOR EFFECT

The motor effect can be explained in terms of the interaction between the magnetic field due to the current-carrying conductor and the field in which the conductor is placed.

The following figures below shows the resultant magnetic field created when a current-carrying conductor is lying between two magnetic poles.

Magnetic field lines tend to repel one another sideways.

• In the field pattern in the figures below, the field lines are closer together on one side of the wire than the other exerting a resultant sideways push or force on the wire.
• The direction of the force can be reversed by either reversing the direction of the current or reversing the direction of the field in which the conductor is placed. Changing both parameters does not affect the direction of the force.

When the switch is closed, the wire is seen to be pulled inwards.

EXPLAINING THE MOTOR EFFECT

The motor effect can be explained in terms of the interaction between the magnetic field due to the current-carrying conductor and the field in which the conductor is placed.

The following figures below shows the resultant magnetic field created when a current-carrying conductor is lying between two magnetic poles.

Magnetic field lines tend to repel one another sideways.

• In the field pattern in the figures below, the field lines are closer together on one side of the wire than the other exerting a resultant sideways push or force on the wire.
• The direction of the force can be reversed by either reversing the direction of the current or reversing the direction of the field in which the conductor is placed. Changing both parameters does not affect the direction of the force.

Generally for a wire to experience a force when lying in a magnetic field:

1. It must be lying across the field lines. The force is greatest when the conductor makes an angle of 90° with the field lines. If the conductor is lying parallel to the field lines, it will not experience any force.
2. A current must be flowing through the conductor.

Direction of the force

If the current and the field directions are at right angles, the direction of the force can be determined using Fleming's left hand rule. By this rule;

If the thump and the first two fingers of the left hand are held at right angles to one another, then

• The Thumb gives the direction of the force (thrust) if
• The First finger is in the direction of the field and
• The seCond finger in the direction of the conventional current.

Factors affecting the magnitude of the force

When a current-carrying conductor is placed in a magnetic field, the magnitude of the force on it depends on the following factors: the;

1. Magnitude of the current; the larger the current in the conductor, the greater the force it experiences.
2. The strength of the magnetic field; the stronger the magnetic field in which it is placed, the greater the force.
3. The length of the conductor in the field; the longer the section of the conductor in the field, the greater the force. The length of the conductor in the field can be increased by winding the wire into a coil of many turns. The greater the number of turns of wire, the greater the force.
4. The angle between the conductor and the field; if θ is the angle between the field and the conductor, then the force is greatest when θ = 90^o (perpendicular) and zero when θ = 0° (parallel).

Force on a charge moving through a magnetic field

Electric current is the flow of charges in a wire. Therefore if a stream of charges enters into a magnetic field, it experiences a force; compared to the force experienced by a current-carrying conductor. The magnitude of this force can be increased by;

• Increasing the velocity of the moving charges
• Increasing the quantity of charge
• Increasing the strength of the magnetic field

Note that for a charge to experience a force in a magnetic field, it must be moving across the field and not parallel to the field.