Electromagnet

Electromagnet

An electromagnet is a type of magnet in which the magnetic field is produced by a flow of electric current. The magnetic field disappears when the current ceases.

Magnetic Field of a Current-carrying Straight Wire

A magnetic field will be generated when a current flows through a wire.

The magnetic field forms by straight wire are concentric circles around the wire as shown in the figure above.

If the direction of the current is inversed, the direction of the magnetic field line is also inversed.

Plan View of the Magnetic Field

SIn plan view, a dot in the wire shows the current coming out from the plane whereas a cross in the wire shows the current moving into the plane.

Right Hand Grip Rule

Grip the wire with the right hand, with the thumb pointing along the direction of the current. The other fingers give the direction of the magnetic field around the wire.

Maxwell's Screw Rule

Also known as the Maxwell's Corkscrew Rule.

Imagine a right handed screw being turn so that it bores its way in the direction of the current in the wire. The direction of rotation gives the direction of the magnetic field.

Strength of the Magnetic Field

The strength of the magnetic field form by a current carrying conductor depends on:

• the magnitude of the current. A stronger current will produce a stronger magnetic field around the wire.
• the distance from the wire. The strength of the field decreases as you move further out.

Field Pattern of a Flat Coil

The direction of the field can be determined by the Right Hand Grip Rule. Grip the wire at one side of the coil with your right hand, with thumb pointing along the direction of the current. Your other fingers will be pointing in the direction of the field.

Plan View of the Field Pattern

Factors affecting the strength

There are 2 ways to increase the strength of the magnetic field:

• increase the current and
• increase the number of turns of the coil.

Solenoid

A solenoid is a long coil made up of a numbers of turns of wire.

The magnetic field of a solenoid resembles that of the long bar magnet, and it behaves as if it has a North Pole at one end and a South Pole at the other.

Determining the Pole of the Magnetic Field - Right Hand Grip Rule

Imagine your right-hand gripping the coil of the solenoid such that your fingers point the same way as the current.

Your thumb then points in the direction of the field. Since the magnetic field line is always coming out from the North Pole, therefore the thumb points towards the North Pole.

Strength of the Magnetic Field

The strength of the magnetic field can be increased by

• Increasing the current,
• Increasing the number of turns per unit length of the solenoid,
• Using a soft-iron core within the solenoid.