Heat & Thermodynamics

Heat vs Temperature

• Heat: energy that flows from a hotter body to a colder one because of a temperature difference; always from high to low temperature, until both are equal.
• Temperature measures the average kinetic energy of the particles, not the total energy stored. Heat is the total thermal energy; temperature is the average per particle.
• Thermal equilibrium: no net heat flows between two bodies.

• Heat vs Temperature

Units of Heat

• SI unit of heat: the joule (J).
• 1 calorie: the heat that raises 1 g of water from 14.5°C to 15.5°C; 1 calorie = 4.186 J (the mechanical equivalent of heat). 1 kilocalorie = 1000 cal, the unit of food energy.
• James Prescott Joule measured the mechanical equivalent of heat, proving heat is energy.

Laws of Thermodynamics

• Thermodynamics studies heat, work, temperature and energy conversions.
• Zeroth law: if A and B are each in equilibrium with C, then A and B are in equilibrium; this defines temperature and makes thermometers work.
• First law: energy is conserved; heat supplied raises internal energy or does work: Q = ΔU + W. Internal energy is the total kinetic + potential energy of a body's particles.
• Second law: heat cannot flow on its own from a colder to a hotter body; external work is needed.

• Laws of Thermodynamics

Thermometers & Temperature Scales

• A thermometer uses mercury (or alcohol), which expands on heating. Clinical: a narrow 35–42°C; laboratory: about −10°C to 110°C.
• Normal body temperature: about 37°C (98.6°F).
• A pyrometer measures very high temperatures (furnaces, molten metal) without contact.
• Absolute zero: 0 K = −273.15°C, the lowest possible temperature, where particle motion is minimum.
• Temperature Scales

Converting Between Scales

• °F = (°C × 9/5) + 32; °C = (°F − 32) × 5/9; K = °C + 273.15 (273 in exams). Example: 50°C = 122°F.
• A Celsius degree and a kelvin are the same size; only the starting point differs.

• Scale Conversion Formulae

Specific Heat Capacity & Calorimetry

• Specific heat capacity: heat to raise 1 kg of a substance by 1°C (1 K), SI unit J kg⁻¹ K⁻¹. Heat needed: Q = m × c × ΔT.
• Water's specific heat is unusually high, 4186 J kg⁻¹ K⁻¹: it heats and cools slowly, which makes coastal climates mild and water a good radiator coolant.
• Calorimetry: heat lost by the hot body = heat gained by the cold body, measured in a calorimeter.

Change of State & Latent Heat

• A change of state (melting, freezing, boiling, condensation) moves matter between solid, liquid and gas. Ice melts at 0°C; water boils at 100°C.
• Latent heat: the hidden heat absorbed or released in a change of state at constant temperature. Fusion (ice → water) ≈ 336 kJ/kg; vaporisation (water → steam) ≈ 2260 kJ/kg.

• Changes of State

Evaporation & Cooling

• Evaporation: the slow change of a liquid to vapour from its surface, at any temperature. It absorbs heat from the surroundings, so it cools.
• Sweat cools the body by taking latent heat from the skin; water in an earthen pot stays cool as water seeping through the pores evaporates.
• Evaporation increases with temperature, surface area, wind and lower humidity.

Modes of Heat Transfer

• Three modes: conduction (through a solid without the solid moving, as a metal spoon gets hot), convection (by the movement of heated fluid, as in boiling water or a sea breeze), radiation (needs no medium, crosses a vacuum, as the Sun's heat does).
• Metals conduct heat well; wood, plastic, air and water are poor conductors (insulators).
• Albedo: the fraction of solar radiation a surface reflects; bright, white surfaces have high albedo.

• Modes of Heat Transfer

Thermal Expansion & Applications

• Most substances expand on heating and contract on cooling; gases expand most and solids least (solids show linear, area and volume expansion).
• Gaps are left in railway tracks and bridges for summer expansion.
• A bimetallic strip bends on heating because its two metals expand by different amounts; it is used in fire alarms and thermostats to make or break a circuit.

Boiling, Pressure & Pressure Cooker

• The boiling point rises with pressure and falls when pressure is reduced.
• A pressure cooker traps steam, raising the pressure and the boiling point above 100°C (nearly 120°C), so food cooks faster and fuel is saved.
• Ponds freeze from the top down because of the anomalous expansion of water; the ice floats and fish survive below.

Insulation, Thermos Flask & Heat Engines

• A thermos (vacuum) flask stops all three modes of heat transfer: double walls with a vacuum block conduction and convection, silvered surfaces block radiation, and the cork reduces loss at the top.
• A heat engine turns heat into mechanical work. Internal combustion engines (petrol and diesel) burn fuel inside the cylinder.
• Relative humidity: the water vapour in air compared with the most it can hold; it decreases as the air warms.