A magent has a north pole at one end and a south pole at the other end. The region around a magnet where metals are attracted is called the magnetic field.
When a current flows through a wire, a magnetic field is produced around it. An electromagnet is made by wrapping insulated wire around a piece of iron called the core. When current flows through the wire the iron becomes strongly magnetised and when current stops flowing it loses its magnetism, this is called temporary magnetism. They can be used for circuit breakers, electric bells and relays.
If an electrical conductor cuts through magentic field lines, potential difference is induced across the ends of the conductor. This is called electromagnetic induction. If the direction of movement of the wire is reversed the direction of the induced pd is also reversed. The size of the induced pd is increased by increasing:
- the speed of movement, strength of magnetic field or number of turns on the coil
The Motor Effect:
The motor effect is when we place a wire carry electrical current in a magnetic field and it experiences a force. The force is a maximum if the wire is at a 90* angle and at 0 if the wire is parallel to the magnetic field. Flemings left hand rule is used to determine the direction of the force:
- the first finger represents magentic field
- the second finger represents the current
- the thumb represents the direction of force
The size of the force can be increased by increasing the strength of the magnetic field or the size of the current.
The direction of the force on the wire is reversed if either the direction of the current or the the direction of the magnetic field is reversed.
The Electric Motor:
The speed of the electric motor is imcreased by increasing the current and the direction of the motor is reversed by reversing the direction of the current.
When current passes through the coil it spins because a force acts on either side due to the motor effect and these forces are opposite to each other. The split ring communicator reverses the direction of the current around the coild every half turn.
A transformer consists of two coils of insulated wire called the primary and secondary coil. Both coils are wound round the same iron core. When alternating current passes through the coil it produces and alternating magnetic field in the core. The alternating magentic field lines pass through the secondary coil which induces an alternating pd across its ends.
The coils of wire are insulated so current flows around the whole coil. The core is made of iron so it is easily magnetised. Transformers are used in the national grid:
- step up transformers make the pd across the secondary coil greater than the pd across the primary coil
- step down transformers make the pd across the secondary coil less than the pd across the primary coil
Switchmode transformers are lighter, smaller, use less power and operate at a higher frequency than traditional transformers.
Transformers in Action:
The national grid use transformers to step up the pd from power stations because the higher the pd at which electricity is transmitted the smaller the energy wasted. Step down transformers are used to reduce the pd so that its safe to use for consumers. To find the number of turns and the pd we can use this equation:
Pd across the primary coil (v) = Number of turns on the primary coil
Pd across the secondary coil (v) Number of turns on the secondary coil
For 100% efficiency:
Pd across the primary coil (V) x the current in the primary coil (A)
Pd across the secondary coil (V) x the current in the secondary coil (A)