Energy and Mains Electricity

How do you work out current using power and voltage ?

current = power over voltage

I = P over V

How do you work out energy transferred from current, voltage and time ?

energy = current x voltage x time

E = I x V x t

How do you work out power using energy and time ?

power = energy over time

P = E over t

A long piece of wire is made into a coil. The coil is connected to a low voltage power supply. How could this coil be used instead of the Bunsen burner ?

• you can put the coil in the water

• there’s electric current in the wire/coil

• thermal energy can be transferred in the wire

6 MARKER:

Look at the image below.

Explain in terms of the movement of charged particles, how energy is transferred from the battery, through the lamp, to the surroundings ?

• the batteries store energy as chemical energy

• the energy is transferred to electrons to make them flow

• the current is a flow of electrons

• the electrons flow through the metal/filament

• the electrons collide with the ions in the lattice

• the collisions make the ions vibrate more

• the increased vibrations make the lattice/filament hotter

• the heat energy is dissipated to the surroundings

• the ions give out light

6 MARKER:

Look at the image below.

A fault occurs in the kettle causing the love wire to touch the metal case of the kettle.

Explain how the safety features of the plug operate when this fault occurs ?

Earth:

• earth wire connected to metal case

• metal case is a conductor

• when live touches case, resistance between live and earth is very low

• (very) large current to earth though (low resistance) earth wire

• case is kept at same potential as earth

• so can’t get a shock if (earthed) person touches metal case

Fuse:

• made of thin wire

• fuse connected between live pin and wire to kettle

• temp of wire depends on current in it

• when current is very large, the temp of wire increases beyond melting point of wire

• fuse (wire) breaks

• disconnects mains supply to kettle

• prevents damage to house wiring

• (Now)no possibility of live wire in kettle being at mains voltage