Solenoid

A solenoid is a long coil of wire. When current flows through it, it produces a magnetic field pattern similar to that of a bar magnet, with a north pole at one end and a south pole at the other.

Magnetic field of a solenoid

Inside the solenoid, the field lines are parallel, evenly spaced, and run from the south end to the north end (inside the coil). This means the field inside the solenoid is strong and approximately uniform.

Outside the solenoid, the field pattern looks like that of a bar magnet: field lines emerge from the north pole, curve around, and re-enter at the south pole.

Determining the poles

Use the right-hand grip rule: curl the fingers of your right hand in the direction of current flow around the coil. Your thumb points towards the north pole.

Alternatively, look at one end of the solenoid. If the current flows anticlockwise, that end is the north pole. If the current flows clockwise, that end is the south pole.

Making the field stronger

The magnetic field of a solenoid can be increased by:

• Increasing the current
• Increasing the number of turns (more turns per unit length)
• Adding a soft iron core inside the coil (this concentrates the field lines)

A solenoid with a soft iron core is called an electromagnet. The soft iron core magnetises and demagnetises quickly, which is essential for applications like relays, circuit breakers, and electric bells.

Applications

Solenoids are used in electromagnets, relays, electric bells, and MRI scanners. The uniform field inside a solenoid is also useful in physics experiments that require a known, steady magnetic field.