Heat and Thermal Phenomena
Heat and Thermal Phenomena tests temperature, thermal expansion, specific heat, latent heat, calorimetry and modes of heat transfer. The exam expects you to distinguish temperature from heat and to use Q = mcΔT and Q = mL correctly.
Core rules
- Heat is energy transferred because of temperature difference; temperature is a measure of average molecular kinetic energy. Celsius to kelvin: K = C + 273 approximately.
- Specific heat capacity c is heat needed to raise 1 kg by 1 K: Q = mcΔT. Water has a high specific heat, about 4200 J kg^-1 K^-1.
- Latent heat L is heat needed per kg for a change of state at constant temperature: Q = mL. For water, latent heat of fusion is about 3.34 x 10^5 J/kg and of vaporisation about 2.26 x 10^6 J/kg.
- Solids, liquids and gases expand on heating. Water shows anomalous expansion between 0 C and 4 C, so ice floats and lakes freeze from the top.
- Conduction needs a medium and is mainly in solids; convection is bulk movement in fluids; radiation needs no medium and travels at the speed of light.
Worked example
How much heat raises 2 kg of water from 25 C to 75 C? Use c = 4200 J kg^-1 K^-1. ΔT = 75 - 25 = 50 K. Q = mcΔT = 2 x 4200 x 50 = 420000 J = 4.2 x 10^5 J. The reasoning is to use the temperature change, not the final temperature alone. No phase change occurs, so latent heat is not added.
Traps
Do not use final temperature instead of change in temperature. During melting or boiling the temperature stays constant while latent heat is absorbed. Specific heat is per kilogram and per kelvin. Radiation is not blocked by a vacuum. Do not add latent heat and specific heat for the same stage unless the question includes both a temperature change and a phase change.