Thermodynamics Fundamentals: Heat, Temperature and Laws
This section covers Thermodynamics Fundamentals: Heat, Temperature and Laws. The exam tests the difference between heat and temperature, specific and latent heat, heat transfer, and the zeroth, first and second laws.
Core concepts
- Heat is energy transferred because of temperature difference; temperature measures average molecular kinetic energy. SI heat energy is the joule; 1 cal = 4.18 J. Kelvin = Celsius + 273.
- Specific heat capacity c is energy needed to raise 1 kg by 1 K. Water is about 4.18 kJ/kgK. Latent heat changes phase without temperature change: latent heat of fusion for melting, latent heat of vaporisation for boiling.
- Conduction transfers heat through solids; convection uses fluid motion; radiation needs no medium.
- Zeroth law supports temperature measurement. First law is energy conservation for a closed system: Q − W = ΔU. Second law states spontaneous heat flow is hot to cold and cyclic engines cannot be 100% efficient.
Worked example
Heat 2 kg of water from 20°C to 70°C. Temperature rise ΔT = 50 K. Q = mcΔT = 2 × 4.18 × 50 = 418 kJ. The reasoning uses the temperature interval, not the absolute Celsius values. If the water then boiled at 100°C, extra latent heat would be needed without a further temperature rise.
Traps that cost marks
Do not use 20°C and 70°C directly as absolute temperatures in Q = mcΔT; use the 50 K rise. Do not confuse heat capacity with specific heat capacity. Latent heat does not change temperature. Watch the sign convention for work in the first law.