Electromagnetism and solenoids

In a nutshell

Electrical currents produce magnetic fields. A solenoid is created by running a current through a coil of wire. Electromagnetic are magnets that can be switched on or off.

Currents

A current is a flow of charges, or charges moving in the same direction. Moving charges create a magnetic field. This means that a current produces a magnetic field.

The magnetic field created by a current in a long, straight wire is made up of concentric circles around the wire. Concentric circles are circles of different sizes with the same central point.

Physics; Magnetic fields; KS4 Year 10; Electromagnetism and solenoids

Right-hand rule

The magnetic field created by a moving current can be found by using the right-hand rule.

To do this, point the thumb of your right hand in the direction of the current. The direction your fingers curl is the direction of the magnetic field.

Physics; Magnetic fields; KS4 Year 10; Electromagnetism and solenoids

1.	Direction of current
2.	Magnetic field lines

The strength of the magnetic field depends on the size of the current. A higher current will give a stronger magnetic field.

The strength also changes with the distance from the current. The magnetic field will be stronger the closer to the current. The concentric circle field lines are closest together near the wire. This is because the stronger the magnetic field, the closer the field lines are to each other.

Solenoids

A solenoid consists of a coil of wire. The field lines of the current in each loop of the wire line up and the overall strength of the magnetic field increases.

Physics; Magnetic fields; KS4 Year 10; Electromagnetism and solenoids

The shape of the magnetic field also changes. A solenoid has strong, uniform magnetic field lines going straight through the inside of the coil. It has outer magnetic field lines which are weaker and curve round, like those on bar magnets.

To increase the field strength of a solenoid further, an iron block can be placed inside. The iron becomes an induced magnet. It is magnetised by the flowing current. If the current were to be turned off, the iron would lose its magnetism.

Electromagnets

Definition

Magnets that can be switched on or off.

Example

A solenoid with an iron core is an example of an electromagnet. Turning the current off causes the magnetism of the iron the quickly disappear.

Uses of electromagnets

*Cranes in scrap yards*	Can attract and pick up magnetic materials. They can be switched off when materials need to be dropped.
*Electric starters of motor*	When switched on, the electromagnet attracts iron on a rocker (part that pivots). The rocker pivots and closes contact for another circuit. This turns on the motor.