AC and DC generation

Where does alternating current come from ?

mains

Where does direct current come from?

cell / battery

current

flow of electricity around a circuit

How do we change the flow of the current?

• electrons are negative so are repelled by negative side of the power supply and are attracted to the positive side

• so if we swapped over the positive & negative (reversed the polarity) the current would flow on the opposite direction

potential difference

• a measure of how much energy is transferred by each coulomb of charge

• measured in volts using voltmeter

How do we change the flow of the current : potential difference ?

what pushes the current so that the electrons move around a circuit faster → if we swap the direction of potential difference the current will flow in the other direction

direct current

• potential difference doesn’t change so electrons travel in one direction

• electricity from cells in a battery (dynamo) have a direct current

alternating current

• rapidly changing potential difference means that the direction the electrons flow changes so that the electrons move backwards and forwards (they alternate)

• electricity from the mains and from an alternator is an alternating current

symbol for an A.C power supply eg the mains

A.C frequency

• with alternating current the frequency is how many times the potential difference / current changfes per second

• number of complete cycles an alternating current goes through every second

• UK mains has a frequency of 50 Hz

alternating currents : alternators

• slip ring (connection)

• side a will always be attached to the same side of the circuit

direct current : dynamos

• split ring commutator (connection)

• side A of the coil will attach to a different side of the circuit each half turn

slip rings

• the coil is rotated in the magnetic field and so a current is induced in the coil

• brushes ensure the continuous contact between the circuit and the slip rings

• key idea - the slip rings mean that side A of the coil is always attached to the same wire in the circuit whether it is on the left (moving up) or on the right (moving down)

A dynamo is used to generate an electric current explain why a direct current is induced in the coil when rotated. (5)

• the coil rotates in a magnetic field (cuts through mag filed lines)

• which induces a potential difference

• in a complete circuit so current flows

• the split ring commutator switches brushes (which reverses the connection every half turn)

• the current only flows in one direction → direct current

A lamp is disconnected from the dynamo. Explain why the dynamo becomes much easier to turn (3)

• breaks circuit so no current flowing

• no magnetic force

• no opposition to spin so it spins faster

The handle of the alternator is turned, causing the coil to rotate. Explain why an alternating current is induced the coil. (5)

• the coil rotates within a magnet so the amount of magnetism changes

• this induces a potential difference in the coil

• every half turn potential difference reverses direction

• current changes direction

• brushes touching slip ring allow alternating current

Changing alternating current

the faster the coil rotates:

• the more cycles per second → the higher the frequency

• the bigger the peak value of the alternating current

Ways to increase the peak value of AC

• using a stronger magnet

• using a bigger area / with more turns on it

direct current from a dynamo (DC) from a dynamo

• the induced potential difference does not reverse its direction, but goes from zero to maximum twice in each cycle

• it never reverse polarity unlike AC