Physics P3 - The Motor Effect

The Motor Effect

A Magnetic Field is a region where magnetic materials (e.g. iron and steel) and also wires carrying currents experience force acting on them. Magnetic fields can be represented by field diagrams. The arrows on the field lines always point from the North Pole of the magnet to the South Pole of the magnet.

A current-carrying wire creates a magnetic field. The motor effect is the term used when a current carrying wire in the presence of a magnetic field experiences a force.

The size of the force can be increased by:

· Increasing the strength of the magnetic field

· Increasing the size of the current.

· Increasing the number of turns on the coil

· Adding a soft iron core in the coil

Magnetically soft material (like iron) can magnetise and demagnetise very easily. So, as soon as you turn off the current through a solenoid, the magnetic field disappears – the iron doesn’t stay magnetised.

A Solenoid is a coil of wire, whereas when you increase the strength by introducing an iron core, the solenoid is an electromagnet.

When a current carrying wire is placed between two magnetic poles, the two magnetic fields affect one and other. The result is a force on the wire.

To experience the full force, the wire has to be perpendicular (at 90°) to the ma gnetic field. If the wire is parallel (runs along the wire), it won’t experience any force at all. If the current or the magnetic field is made bigger, the force increases.

The direction of the force acts is dependant on the direction of the magnetic field and the direction of the current. This is shown by Flemings Left Hand Rule.

ThuMb = Motion

First finger = Field

seCond finger = Current

The Simple Electric Motor

Look at the diagram above. A rectangular loop of wire is sitting inside a magnetic field. We can consider the current in the four sections of the loop and work out which way the force acts.

• On the left hand side of the loop the current is flowing into the page or screen. The magnetic field will be going from left to right so from Fleming’s left…