Heat and Thermal Phenomena
Heat and Thermal Phenomena covers thermal expansion, calorimetry and the three modes of heat transfer. The exam expects correct temperature intervals, correct units and a clear distinction between heat and temperature.
Temperature and expansion
- Convert Celsius to kelvin by adding 273; a temperature difference of 1 degree C equals a difference of 1 K.
- Linear expansion: change in length = original length x coefficient of linear expansion x temperature rise. Area and volume expansion use their own coefficients.
- A bimetallic strip bends because the two metals expand by different amounts.
Calorimetry
- Heat gained or lost: Q = mc(delta T), where c is specific heat capacity.
- During a change of state, temperature stays constant: Q = mL, where L is latent heat.
- Specific heat of water is about 1 cal/g degree C or 4200 J/kg K; latent heat of fusion of ice is about 80 cal/g.
Heat transfer
- Conduction needs a medium and is best in solids; convection transfers heat by fluid movement; radiation needs no medium.
- Dark, rough surfaces absorb and emit radiation better than shiny surfaces.
Worked example
Find the heat needed to warm 100 g of water from 20 degree C to 70 degree C. Take c = 1 cal/g degree C. Q = mc(delta T) = 100 x 1 x (70 - 20) = 5000 cal. Since 1 cal = 4.2 J, Q = 5000 x 4.2 = 21000 J.
Traps
- Using absolute temperatures in Q = mc(delta T) instead of the interval.
- Adding latent heat while also changing temperature during melting or boiling.
- Mixing calories and joules within one calculation.
- Ignoring heat loss to the container in calorimetry problems.