2 - ELECTRICITY 

Energy and power in circuits

Resistors -

• Produce heat when electric current passes through them 
• Energy transfer which heats the resistor 
• Heat increases the resistors resistance - less current will flow/greater voltage needed to produce same current
• Can cause components in circuit to melt - breaks circuit 
• Fuses use melting to protect circuits - melt and break the circuit if the current gets too high 
• Toasters contain coil of wire with very high resistance = heat
• Current passes through coil - temperature increases - glows and gives off infrared (heat) radiation 

Electrical power and fuse ratings -

• Electrical power - rate at which an appliance transfers energy 
• An appliance with a high power rating transfers a lot of energy in a short period of time
• Energy comes from current flowing through it 
• High power rating appliance - draws a large current from the supply 

Formula for electrical power =
ELECTRICAL POWER = CURRENT X VOLTAGE 

• Most appliances show their power rating and voltage rating 
• Fuses should be rated near but slightly higher than the normal operating current 
• Working out fuse needed - 

eg: a hair dryer is rated at 230V, 1kW - find the fuse needed

1kW = 1000W
I = P/V
  = 1000/230
  = 4.3A
Slightly higher = 5 amp fuse

Appliances transfer electrical energy -

• Current flows through component, energy is transferred 
• eg: lightbulb - electrical energy transferred into light energy and (waste) heat energy
• Energy transferred depends on the current through it, the voltage supplied and how lond (s) it is on for 

ENERGY TRANSFERRED = CURRENT X VOLTAGE X TIME