Specific heat capacity (c): heat needed to change the temperature of 1 kg of a substance by 1 K (or 1 °C). SI unit: J kg⁻¹ K⁻¹. Water 4200, dry soil 800, aluminium 900.
Heat capacity: heat needed to raise the temperature of a whole object by 1 K. Unit J K⁻¹ (= mc).
Conduction: transfer of heat in solids by collisions of atoms/molecules and by motion of free electrons, from the hot part to the cold part.
Convection: transfer of heat by bulk movement of molecules in fluids (liquids and gases) because of density differences. It cannot occur in solids.
Radiation: transfer of heat as electromagnetic (infrared) waves. No medium needed. It is the fastest method (speed of light).
Thermal equilibrium: object absorbs and radiates heat at the same rate, so its temperature stays constant.
Greenhouse effect: greenhouse gases (CO₂, methane, water vapour) absorb heat radiated by Earth and send part of it back, keeping Earth warm. Global warming: gradual rise in Earth's average temperature due to extra greenhouse gases from human activity.
Formulas
Q = m c ΔT (ΔT in °C equals ΔT in K).
Method of mixtures: heat lost by hot solid = heat gained by water + calorimeter, so ms cs (T2 − T3) = mw cw (T3 − T1) + mc cc (T3 − T1).
Electrical method: V I t = m c ΔT.
Must-know points
Uses of the large specific heat of water: moderate climate near the sea; lakes and oceans change temperature slowly so aquatic life survives; helps the body (about 60 % water) keep 37 °C; coolant in car radiators and power plants; good for cooking and keeps tea hot.
Why coastal areas are moderate: land has small c (about 5 times smaller than water), so it heats and cools fast. Sea heats and cools slowly, so coast temperature range is small (Karachi 19 to 32 °C).
Metals conduct well because of many free electrons, which carry energy quickly in addition to vibrating atoms. Non-metals only pass energy by vibrating atoms, so they conduct slowly. Experiment: wax and drawing pins on rods of copper, aluminium and iron; the pin on the best conductor drops first.
Convection experiment: potassium permanganate crystals at the bottom of a beaker; the coloured stream rises when heated.
Convection and marine life: warm water rises, cold dense water sinks. Upwelling brings cold, nutrient-rich water up (food for phytoplankton and fish); downwelling carries oxygen down; currents spread heat and stabilise temperature.
Sea breeze (day): land heats faster, hot air above land rises, cool air from sea moves in. Land breeze (night): land cools faster, so air above the warmer sea rises and cool air from land moves out to sea. Together they keep coastal weather moderate.
Birds (eagles, hawks) ride thermals (columns of rising warm air) with wide wings, circling inside them, so they rise and glide without flapping. Cyclones: warm ocean water evaporates, moist air rises, cools, condenses and releases latent heat, pressure falls and Earth's rotation makes it spin.
Radiation and surfaces: black/dull/rough = best absorber and best emitter, worst reflector. Shiny silver/white/smooth = best reflector, worst absorber and emitter. Larger surface area and higher temperature mean more radiation. Leslie cube compares emission from four faces using an infrared thermometer.
Applications: thermos flask (vacuum stops conduction and convection, silvering reflects radiation, plastic lid and steel inside reduce conduction); insulation of buildings (double glazing, false ceilings); gas heater warms a room by convection; household geyser (hot water rises to the top); IR thermometer (distance to spot ratio).
Global warming effects: glacier melt, sea level rise, flooding; stronger hurricanes; heavier rain; droughts; wildfires; winter storms. Pakistan: 2022 floods and heatwave above 51 °C. Remedies: stop deforestation, plant trees, use clean energy.
Geothermal activity: Earth has core (about 6000 °C, iron and nickel), mantle (thickest, plastic), crust. Magma moves by convection, makes volcanoes, and drags tectonic plates (7 major plates), causing earthquakes. Core stays hot because of residual heat from formation, radioactive decay (U-238, Th-232, K-40), gravitational compression, and the crust acting as insulator.
Past-paper questions with model answers
Heat to raise 2 kg from 40 to 45 °C, c = 420: Q = 2 × 420 × 5 = 4200 J.
Heat to warm 25 kg of water by 50 °C: Q = 25 × 4200 × 50 = 5.25 × 10⁶ J (5.25 MJ).
Why coastal areas have moderate temperatures: high specific heat of water plus sea and land breezes (explain both as above).
How convection in seawater supports marine life: upwelling brings nutrients, convection carries oxygen to deeper water, spreads heat.
Sea and land breezes help moderate coastal climate (3 marks): explain day and night cases with direction of air.
Explain greenhouse effect, is it good or bad, consequences (Section C, 2+1+2): effect needed for life (without it Earth would be extremely cold at night); becomes harmful when extra gases from fossil fuels and deforestation trap more heat (global warming); consequences: floods, droughts, hurricanes, heatwaves, melting glaciers.
Heat conduction in solids through lattice vibrations and in metals through free electrons; compare metals and non-metals (Section C).