Electromagnetism

• when a wire carrying a current is in a magnetic field, a force may be exerted on the wire = the motor effect
• no force when wire parallel to magnetic fields, biggest force when wire perpendicular to magnetic field (shown in diagram)
• increase by: stronger magnetic fields or increasing size of current

• coil is connected to battery by two metal brushes
• brushes are fixed to a split-ring commutator

the coil spins when a current is passed through it because:

• a force acts on either side due to the motor effect
• the force on one side is opposite to the force on the other side (one goes up and the other down)

Split-ring commutator

• split-ring commutator reverses the current (changes the direction) every half turn of the coil
• since it is swapped every half turn, it is pushed continuously in the same direction
• i.e. the force is up on one side, and down on the other - when the current is reversed, the up force switches to down and the down force switches to up so it moves continuously
• imagine it turning clockwise - as it approaches down, that down force is changed to up so it turns up - it rotates in a clockwise direction and keeps going due to the split-ring commutator

• coils of wire spin (move) in a magnetic field
• a potential difference (voltage) is induced (made) when the wire cuts across magnetic field lines
• this is why more voltage is induced when the wire is perpendicular to the magnetic field lines
• if you connect the wire to an ammeter and move it between the two poles of a U-shaped magnet, the ammeter pointer will deflect as a current is generated when the wire cuts the magnetic field lines
• this is known as the dynamo effect

to increase the voltage induced:

• increase the speed of the movement
• increase the strength of the magnet
• increase the number of coils on the wire
• increase the area of the coil

• brushes attached to slip rings
• alternating current

• faster the coil rotates, larger the peak value of the alternating current and greater the frequency

• they prevent the wires in the generator getting tangled
• one slip ring connects to the end of the wire carrying the current, and the other is connected to the other end
• the brushes connect the slip rings to the external circuit 
• (which connects to something else e.g. a lamp and it will light up as the circuit supplies it with voltage induced in the generator)