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Temperature, Heat and Thermal Energy
Thermodynamics
Temperature, heat and thermal energy sound similar, but they are different ideas. Temperature measures how hot something is, heat is energy moving because of a temperature difference, and thermal energy is the energy stored inside a substance.
Three different ideas
Temperature is a measure of the average kinetic energy of the particles in a substance: hotter means the particles move faster on average. It is measured in degrees Celsius in everyday life and in kelvin in physics, where 0 K is absolute zero, the lowest possible temperature at which particles have minimum motion. Thermal energy is the total kinetic and potential energy of all the particles in a substance, so it depends on mass and temperature together. A large cool lake holds far more thermal energy than a small hot cup of tea. Heat is not a substance inside an object at all: it is energy transferred from a hotter place to a colder place because of a temperature difference.
Heating, cooling and changing state
The specific heat capacity of a substance is the energy needed to raise one kilogram of it by one kelvin. Water has an unusually high value, about 4180 J/kg K, which is why oceans moderate climates and water is used as a coolant. Metals have low specific heat capacities, so a metal pan heats up quickly. When a substance changes state, for example melting or boiling, its temperature stays constant while energy goes into breaking bonds between particles. That energy is called latent heat. This is why steam burns are severe: condensing steam releases a large amount of latent heat onto the skin.
Where you see it
Heat flows from hot to cold until temperatures equalise, a process called thermal equilibrium. A thermometer works by reaching equilibrium with what it measures, and its reading then matches that temperature. Hot drinks cool, ice melts in a warm room, and a house loses heat to cold air outside, all because of this one-way flow. Insulation slows the flow but cannot stop it entirely. Understanding the difference between temperature and energy also explains why a small flame can light a match but cannot warm a room: temperature alone does not tell you how much energy is available.
📝 Worked example: heating water
Heat 0.50 kg of water from 20 degrees C to 40 degrees C. Mass m = 0.50 kg, specific heat capacity c = 4180 J/kg K, temperature change delta T = 40 - 20 = 20 K. Use Q = m c delta T. Substitute: Q = 0.50 x 4180 x 20. 0.50 x 4180 = 2090. 2090 x 20 = 41,800 J. So 41.8 kJ of heat is needed. A temperature change in degrees Celsius is the same size as in kelvin, so delta T = 20 K.
Common mistakes
- Saying an object contains heat: objects contain thermal energy; heat is the transfer of energy, not a stored quantity.
- Confusing temperature and energy: a spark has a high temperature but little thermal energy, while a warm bath has a lower temperature but far more energy.
- Using Celsius for absolute temperature: temperature differences can use Celsius or kelvin, but absolute temperature in formulas such as the ideal gas law must be in kelvin.
Practice questions
- How much heat raises 2.0 kg of aluminium (c = 900 J/kg K) by 50 K?
Show answer
Q = 2.0 x 900 x 50 = 90,000 J = 90 kJ.
- What is the lowest possible temperature called?
Show answer
Absolute zero, 0 K or -273.15 degrees C, the lowest possible temperature.
- Can heat flow from a cold object to a hot one on its own?
Show answer
No. Heat flows from the hotter body to the colder one until they reach the same temperature.
Frequently asked questions
How do you convert Celsius to kelvin?
It rises by 273.15; for example 0 degrees C is 273 K and 100 degrees C is 373 K. Changes in degrees C and kelvin are the same size.
Is heat the same as thermal energy?
No. Heat is energy in transit because of a temperature difference; once it arrives it becomes internal energy.