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4.2.5 Electrical energy and electrical power

Syllabus
0625–2026–2027
Topic
4.2.5
Level
—

Learning objectives

4.2.5.1Electric circuits transfer energy from• Understand: electric circuits transfer energy from a source of electrical energy, such as an electrical cell or mains supply, to the circuit components and then into the surroundings4.2.5.2Recall/use: for electrical power P =• Recall/use: for electrical power P = IV4.2.5.3Recall/use: for electrical energy E =• Recall/use: for electrical energy E = IVt4.2.5.4Kilowatt-hour (kWh) and calculate the• Define the kilowatt-hour (kWh) and calculate the cost of using electrical appliances where the energy unit is the kWh

Trace energy through an electric circuit

An electric circuit transfers energy from a source, through electrical working, to circuit components and then into the surroundings. Energy is transferred and conserved; it is not used up or destroyed.

Stage Example Energy change
source cell or battery energy from its chemical store is transferred electrically
component lamp electrical transfer leads to light and heating
component motor electrical transfer leads to kinetic energy and heating
surroundings air and nearby objects transferred energy eventually spreads mainly by heating

A mains supply is also an electrical energy source for the circuit. The appliance does not store all the received energy permanently: its useful output and any heating are eventually transferred to the surroundings.

Current is the movement of charge, not a flow of energy that disappears inside a component. Describe both the source and the receiving component when tracing an energy pathway.

Calculate electrical power with P = IV

Electrical power is the rate at which a component transfers electrical energy. One watt means one joule transferred each second.

P = IV

Quantity Symbol Unit
electrical power PP watt, W
current through the component II ampere, A
p.d. across the component VV volt, V

A 12 V supply delivers 0.35 A to a circuit: P=0.35×12=4.2P = 0.35 \times 12 = 4.2 W. Rearrange the same equation as I=P/VI=P/V or V=P/IV=P/I when current or p.d. is required.

Use the current through and p.d. across the same component. Convert prefixes first: 1.0 kW = 1000 W, 0.10 mA = 0.00010 A and 400 kV = 400 000 V.

Calculate electrical energy with E = IVt

The electrical energy transferred depends on the power and on how long the transfer continues. Since P=IVP=IV, energy transferred is E=Pt=IVtE=Pt=IVt.

E = IVt

Quantity Symbol Unit for a joule answer
energy transferred EE joule, J
current II ampere, A
potential difference VV volt, V
time tt second, s

A 3.0 V torch lamp carries 20 mA for 5.0 minutes. Convert first: I=0.020I=0.020 A and t=300t=300 s. Then E=3.0×0.020×300=18E=3.0\times0.020\times300=18 J.

For an answer in joules, use volts, amperes and seconds. Minutes are not seconds, and milliamperes are not amperes. The symbol EE here is energy, so use the wording and unit to distinguish it from e.m.f.

Use kilowatt-hours to calculate electricity cost

One kilowatt-hour (kWh) is the energy transferred when a power of 1 kW operates for 1 hour. It is a unit of energy, not power.

1\text{ kWh}=1000\text{ W}\times3600\text{ s}=3.6\times10^6\text{ J}

  1. Convert appliance power from W to kW.
  2. Convert operating time to hours.
  3. Calculate energy: E(kWh)=P(kW)×t(h)E(\text{kWh})=P(\text{kW})\times t(\text{h}).
  4. Calculate cost: energy in kWh × price per kWh.

A 2200 W heater runs for 48 minutes at 0.25perkWh.0.25 per kWh.P=2.2kWandkW andt=0.80h,soh, soE=2.2\times0.80=1.76kWh.CostkWh. Cost=1.76\times0.25=$0.44$.

A utility 'unit' means 1 kWh. Do not multiply watts directly by hours and label the result kWh; divide watts by 1000 first. A tariff is a price per kWh, so cost is not found from power alone.