Electrical Energy

Electrical energy is the energy carried by moving charges in a circuit. A cell or the mains supply provides energy, which is transferred to components (like lamps, heaters, motors) and then to the surroundings as light, sound, or thermal energy.

Energy Transfer in Circuits

• Battery (chemical store) or mains (electrical supply) gives energy to charges.
• Charges move through components and do work on them.
• Components convert electrical energy into useful forms and waste thermal energy.

Key Formulas

Electrical energy transferred:

$$E = IVt$$

• E is energy in joules (J)
• I is current in amperes (A)
• V is voltage in volts (V)
• t is time in seconds (s)

Power is the rate of energy transfer:

$$P = \frac{E}{t} = IV$$

Units and the Kilowatt-hour (kWh)

• Power in watts: 1 W = 1 J/s. 1 kW = 1000 W.
• Electricity bills use energy in kilowatt-hours.

$$1\,\text{kWh} = 1000\,\text{W} \times 3600\,\text{s} = 3.6\times10^6\,\text{J} = 3.6\,\text{MJ}$$

Cost of using an appliance: cost = energy used (in kWh) × price per kWh.

Worked Examples

Common Misconceptions

• Power (kW) is not the same as energy (kWh). Power is how fast; energy is how much.
• Do not use minutes in E = IVt unless you convert to seconds.
• A higher voltage does not always mean more energy; time and current matter too.

Key Points to Remember

• $E = IVt$ links energy to current, voltage, and time.
• $P = IV$ and $E = Pt$.
• 1 kWh = 3.6 MJ; bills use kWh and price per kWh.
• Energy is transferred from the source, through components, to the surroundings.