Joule (J)

The joule is the SI unit of energy, work, and heat. It is defined as the energy transferred when a force of 1 newton moves an object 1 meter in the direction of the force. The unit is named after physicist James Prescott Joule.

SI Base Unit Representation

As a derived unit, the joule is expressed in SI base units as:

1 J = 1 kg·m²/s²

This reflects energy as a function of mass, distance, and time.

Energy in Different Contexts

The joule appears in various physical equations:

• Mechanical work:
  W = F × d
  (force × distance)

• Electrical energy:
  W = V × I × t
  (voltage × current × time)

• Thermal energy:
  Q = m × c × ΔT
  (mass × specific heat capacity × temperature difference)

Practical Examples

• A falling apple (~100 g, 1 m height) releases about 1 J of gravitational potential energy

• A AA battery stores around 9,000 J

• A lightning strike may release up to 5 billion J

Applications

• Physics and engineering: Universal unit for energy analysis

• Electrical systems: Power consumption and energy billing (e.g., 1 kWh = 3.6 million J)

• Thermodynamics: Quantifying heat and internal energy

• Nutrition science: 1 calorie = 4.184 J, used in food energy conversion

• Industrial processes: Measuring input and output energy in mechanical and thermal systems