Joule’s Law

1. Definition

Joule’s Law states that the heat produced in a conductor due to an electric current is proportional to the square of the current, the resistance of the conductor, and the time for which the current flows.

Joule's Law Expirement

Expirement

An electric current passes through a heating coil immersed in water inside a calorimeter. The digital thermometer measures the rise in water temperature over time. By recording the current, resistance, and time, the experiment confirms that the heat generated matches Q=I2Rt, demonstrating Joule’s Law in action.

1.1. Formula

Q = I² × R × t where:

  • Q = Heat energy produced (in joules)
  • I = Current (in amperes)
  • R = Resistance (in ohms)
  • t = Time (in seconds)

2.1. Explanation

When an electric current flows through a conductor, the moving electrons collide with the atoms of the material.
These collisions convert electrical energy into heat energy. Joule’s Law quantifies the relationship between current, resistance, and heat produced.

2. Usage

  1. Electric heating devices (e.g., toasters, electric kettles, irons)
  2. Incandescent light bulbs
  3. Fuses and circuit breakers (heat causes them to operate)
    • Soldering irons

4. Important Notes

  • The heat generated increases with the square of the current ; doubling the current produces four times the heat.
  • High resistance materials generate more heat for the same current.
  • Excessive heat can damage components if not managed properly.

5. Limitations

  1. Assumes resistance is constant (not true if temperature changes).
  2. Applies mainly to purely resistive conductors (ohmic materials).
  3. Heat losses to surroundings are ignored.
  4. Valid for steady DC currents (AC requires RMS values).

5. Video Explanation

video coming soon