Last reviewed 30 Sept 2026 · 8 min read
Heat and temperature
Heat is the form of energy that flows from a hotter body to a colder body because of a temperature difference. Its SI unit is the joule (J); the older unit is the calorie (1 cal ≈ 4.18 J). The kilocalorie (Cal) is used for food energy.
Temperature is the degree of hotness or coldness of a body; it tells the direction in which heat will flow. It is a measure of the average kinetic energy of the molecules. Its SI unit is the kelvin (K).
| Heat | Temperature | |
|---|---|---|
| Nature | energy in transit | measure of hotness |
| Depends on | mass, temperature difference, material | only the state of hotness |
| Unit | joule / calorie | kelvin / °C / °F |
| Instrument | calorimeter | thermometer |
Temperature scales
| Scale | Freezing point of water | Boiling point of water |
|---|---|---|
| Celsius (°C) | 0 | 100 |
| Fahrenheit (°F) | 32 | 212 |
| Kelvin (K) | 273.15 | 373.15 |
Conversions:
Absolute zero is 0 K = −273.15 °C, the lowest possible temperature, at which molecular motion is (classically) minimal. −40 °C equals −40 °F. The normal human body temperature is about 37 °C = 98.6 °F.
Convert 104 °F to °C and K.
Solution. ; .
Thermometers
- Mercury-in-glass: mercury expands uniformly, does not wet glass, is opaque and a good conductor; range about −39 °C (freezing) to 357 °C.
- Alcohol thermometer: for very low temperatures (freezes at −115 °C).
- Clinical thermometer: reads 35–42 °C; has a constriction near the bulb so that the mercury does not fall back; must be shaken down.
- Maximum–minimum (Six's) thermometer: records the highest and lowest temperature of a day.
- Pyrometer: measures very high temperatures (furnaces, the Sun) without contact.
- Platinum resistance thermometer: wide range; used in laboratories.
- Thermocouple: two different metals joined; uses the thermoelectric effect.
Thermal expansion
Most substances expand when heated and contract when cooled.
- Linear expansion: , where is the coefficient of linear expansion.
- Area expansion: ; volume expansion: .
- Gases expand most, liquids less, solids least for the same rise in temperature.
- Applications and precautions: gaps between rails and expansion joints in bridges; iron rims fitted on cartwheels by heating; bimetallic strips in thermostats and fire alarms (two metals with different bend on heating); overhead wires are kept slack; a glass tumbler cracks when hot water is poured suddenly into it (uneven expansion); Pyrex glass has a low expansion coefficient.
Anomalous expansion of water
Water contracts when heated from 0 °C to 4 °C and expands above 4 °C, so it has its maximum density at 4 °C. Therefore in winter lakes freeze from the top; the layer at the bottom remains at 4 °C, which lets aquatic life survive. Ice is less dense than water and floats.