The force between magnets: like poles repel; unlike poles attract.
An electromagnet consists of a coil of insulated wire wrapped round an iron core. The electromagnet turns off when the current is turned off.
- Scrapyard crane
- Electric bell
- Circuit breaker
Electromagnetic induction is the process of creating a potential difference using a magnetic field. When a conductor cuts the lines of a magnetic field, a potential difference is induces across the ends of the conductor. When an electromagnet is used, it needs to be switched on or off to induce a pd.
The motor effect
In the motor effect, the force:
- is incresed if the current or the strength of the magnetic field is increased.
- is at right angles to the direction of the magnetic field and to the wire
- is reversed if the direction of the current or the magnetic field is reversed.
An electric motor has a coil which turns when a current is passed through it and it changes direction when the current changes direction.
- A transformer is used to change the size of the alternating pd from generator to homes.
- A transformer only works on ac because a changing magnetic field is necessary to induce ac in the secondary coil.
- A transformer has an iron core unless it's a switch mode transformer with an a ferrite core.
- A switch mode transformer is lighter and smaller than an ordinary transformer. It operates at high frequency
A step up transformer makes the pd across the secondary coil greater than the pd across the primary coil. It's secondary coil has more turns than it's primary coil.
A step-down transformer makes the pd across the secondary coil less than the pd across the primary coil. It's secondary coil has less turns than it's primary coil.
- In a transformer, an alternating current is passed through the primary coil
- This coil creates an alternating magnetic field that passes through the secondary coil
- As a result, an alternating pd is induced in the coil
Primary pd, Vp/Secondary pd, Vs = np/ns (np/ns, number of primary/secondary turns)
For a 100% transformer, Vp x Ip = Vs x Is (I=current, V=potential difference)