Joule is the unit of which physical quantity?

• A. Unit of energy
• B. Unit of electric current
• C. Unit of power
• D. Unit of resistance

Answer: (A)

Joule is the unit of energy. In the SI system, energy, work, and heat are measured in joules, with one joule defined as the work done when a force of one newton moves an object one meter in the direction of the force. Power is the rate of energy transfer and is measured in watts, which equals one joule per second. Electric current uses amperes, and resistance uses ohms, so those are not joules. In practice, energy use is often shown in kilowatt-hours, where 1 kWh equals 3.6 million joules. For example, lifting a 1-newton weight by 1 meter requires 1 joule of energy, and a 100-watt light bulb running for 10 seconds uses about 1000 joules. This shows why joule is the unit of energy, not current, resistance, or power.