P3.3 Keeping Things Moving

There is a link between electricity, magnetism and movement. This principle is very important for everyday devices, e.g. hair dryers, motors, transformers, electric vehicles

Electromagnets

When an electric current flows through a wire a magnetic field is generated. The direction of the field can be determined by the right hand grip rule.

If we make a coil of wire, the magnetic field around each wire creates a larger field.

A current flowing through a wire creates a magnetic field. If the current-carrying wire is placed in a magnetic field, the two magnetic fields interact and always acts perpendicular to both the direction of the current and the magnetic field the wire has been places in.

We can predict the direction of the current using the left hand rule.

Using this principle, we can put a loop of wire in a magnetic field.

Each of the sides of the loop that are perpendicular to the field will experience a force, one down and one up, which will cause the loop to rotate.
However, once the loop is vertical, the forces do not create a resultant moment and the loop stops rotating.

To keep the loop rotating we must reverse the direction of the current, to do this we use a split-ring commutator. To increase the strength of a motor we can:

• increase the munber of coil
• increase the current
• increase the strength of the field we put the coil in

If we move a conductor in or through a magnetic field we induce a current in the conductor. If that conductor forms part of a circuit, a current will flow around the 'circuit'.

This means that if we can continuously move a conductor perpendicular to a magnetic field we can generate a current, i.e. generate electricity.

A generator is a device that converts mechanical energy into electrical energy. Power stations use generators to produce electricity on a large scale.

This current is generated because of an induced electromotive force (EMF) on the electrons in the conductor. We can increase the induced EMF, and therefore that induced current by:

• moving the conductor through the field faster
• increase the strength of the magnetic field
• increase the number of coils/loops

Note: The movement must be perpendicular to the field for the maximum induced current.

Right Hand Generator Rule

A transformer is a adevice that uses the principle of electromagnetic induction (the generator effect) to allow us to step up or step down voltages e.g. the National Grid, and many household devices.

A transformer only functions when driven by an alternating current. This is because we need a changing magnetic field to induce an EMF (Electromotive Force) in the other side of the transformer.

Transformers can either step up a voltage or step down the voltage. This is governed by the ratio of the number of coils in the primary and secondary coils.

The transformer equation: