A step up transformer increases p.d. and decreases current

Electricity is then transmitted at high p.d. and low current

Low current means low power losses in the transmission cables

...because P = I²R

... so the system is more efficient: more power gets to consumers and less is lost from the lines as thermal energy

An alternating current in the primary coil

... causes a changing magnetic field throughout the iron core

This links to the secondary coil, which will cut the field lines due to the field changing

A conductor which is cutting lines of magnetic flux will have a p.d. induced across it

So a changing p.d. is induced across the secondary

DC current will cause the primary coil to behave as an electromagnet

So a constant field will be set up in the iron core

This field links to the secondary coil, but it is not changing

So the secondary coil does not cut any lines of magnetic flux

So no p.d. is induced across the secondary coil

Sound waves cause a diaphragm to vibrate.

The diaphragm is attached to a coil, which also vibrates

The coil is in the field of a permanent magnet

So when the coil moves, a p.d. is induced across it

This causes a current to flow in the circuit that the microphone is connected to.

A coil of wire with a current passing through it has a magnetic field around it

This interacts with a permanent magnetic field from a magnet near the coil

This causes a force on the coil of wire

The coil turns

Finally, a commutator allows current to continue to flow into the coil as it spins