1. Thermal Equilibrium
14.1
Which way energy flows
When two objects are in thermal contact, thermal energy is transferred from the region of higher temperature to the region of lower temperature. The transfer continues until the temperatures are equal. Objects at the same temperature are in thermal equilibrium: energy still passes both ways at the molecular level, but there is no net transfer.
Temperature is therefore not the same as thermal energy. A bath of warm water holds far more internal energy than a red-hot pin, yet energy flows from the pin into the water.
Practice Question
A thermometer at 20 °C is placed in tea at 70 °C. Explain why the reading is the tea’s temperature only after a short wait, and why the tea cools very slightly.
2. Temperature Scales
14.2
Thermometric properties and kelvin
Any physical property that varies with temperature can be used to measure it: the density of a liquid (liquid-in-glass), the volume of a gas at constant pressure, the resistance of a metal, the e.m.f. of a thermocouple. A scale is made by calibrating at fixed points and assuming the property varies linearly between them. Different properties do not vary in exactly the same way, so two thermometers can disagree between fixed points.
The thermodynamic (kelvin) scale does not depend on the property of any particular substance. Its zero, absolute zero (0 K), is the lowest possible temperature, where a system has minimum internal energy. A temperature difference of 1 K equals 1 °C.
Conversion
T/K = θ/°C + 273.15 Property scale: θ = 100 × (Xθ − X0)/(X100 − X0)
Worked Example
A platinum resistance thermometer reads 100.0 Ω at 0 °C and 138.5 Ω at 100 °C. Its resistance in a liquid is 119.0 Ω. Find the temperature on this scale, and in kelvin.
Practice Question
Convert −196 °C (boiling liquid nitrogen) to kelvin.
3. Specific Heat Capacity and Specific Latent Heat