Quantity of Heat

In Physics, heat is defined as a form of energy that is transferred from one body to another as a result of a temperature difference. It is measured in Joules (J).

1. Heat Capacity (C)

Heat Capacity is the quantity of heat required to raise the temperature of the entire mass of a body by one Kelvin (1 K) or one degree Celsius (1°C).

Formula: C = Q / Δθ

Unit: Joules per Kelvin (J/K) or Joules per degree Celsius (J/°C).

2. Specific Heat Capacity (c)

Specific Heat Capacity is the quantity of heat required to raise the temperature of a unit mass (1 kg) of a substance by one Kelvin (1 K).

Formula: c = Q / (m × Δθ)

Relationship: C = mc

Unit: Joules per kilogram per Kelvin (J/(kg·K)).

3. Determination of Heat Capacity

Two common methods are used to determine specific heat capacity:

• Method of Mixtures: Based on the principle of conservation of energy. If no heat is lost to the surroundings, Heat Lost by Hot Body = Heat Gained by Cold Body.
• Electrical Method: Heat energy supplied by an electrical heater (VIt or I²Rt) is equated to the heat gained by the substance (mcΔθ).

4. Newton's Law of Cooling

Newton's Law of Cooling states that the rate of loss of heat of a body is directly proportional to the temperature difference between the body and its surroundings, provided the temperature difference is small.

5. Solved Example

Example: Calculate the heat required to raise the temperature of 2kg of water from 20°C to 70°C. (Specific heat capacity of water = 4200 J/kgK)

Solution: Q = mcΔθ = 2 × 4200 × (70 - 20) = 2 × 4200 × 50 = 420,000 Joules (420 kJ).