Temperature is not heat

Word	What it means
Temperature	The average kinetic energy of the particles. Measured in °C or K.
Thermal energy	The total energy of all the particles. Depends on how much stuff there is as well as how hot it is.
Heat	Thermal energy in transit, always from hotter to colder. An object does not contain heat; it contains thermal energy.
Specific heat capacity (c)	The energy needed to raise 1 kg of a substance by 1 °C. Water's is unusually high at about 4200 J/(kg·°C).
Conduction	Transfer by direct particle collisions. Best in solids, especially metals.
Convection	Transfer by a moving fluid: warm parts rise, cool parts sink.
Radiation	Transfer by electromagnetic waves. The only one that works in a vacuum.
Thermal expansion	Most materials get bigger when warmed, because the particles move further apart.

• Conduction needs touching particles. Metals are good at it because their free electrons carry energy through quickly, which is why a metal spoon in soup heats far faster than a wooden one.
• Convection needs a fluid that can move. Warming makes it less dense, so it rises and cooler fluid takes its place. It drives weather, ocean currents and the water in a kettle.
• Radiation needs nothing at all. It is how the Sun's energy crosses 150 million kilometres of vacuum.

A vacuum flask attacks all three separately. The vacuum between the walls stops conduction and convection; the silvered surfaces reflect radiation back. That is why it works so much better than a thick mug, and it is a good test of whether the three are understood.

Q = mcΔT  (energy = mass × specific heat capacity × temperature change)

Worked. Heating 2.0 kg of water by 10 °C: Q = 2.0 × 4200 × 10 = 84 000 J. All three factors multiply, so dropping any one changes the answer by a large factor.

Comparing two metals. Give two blocks the same energy. The one with the lower c warms more, because ΔT = Q/(mc). That is why a metal spoon becomes uncomfortable long before the soup cools.

Why water's high value matters. Coastal climates are milder than inland ones because the sea absorbs and releases enormous energy for a small temperature change. It is also why water is used as a coolant and why the oceans are absorbing much of the extra energy trapped by greenhouse gases.

First law. Energy cannot be created or destroyed, only converted. Friction does not destroy energy — it turns organised motion into scattered thermal energy.

Second law. In every conversion, some energy becomes less useful, and heat spreads out rather than concentrating. Heat will not flow from cold to hot on its own; a fridge only does it by spending energy.

Why this matters. The first law alone would permit a perfectly efficient engine. The second is why there is no such thing, and why every real machine warms its surroundings.

• Using heat and temperature as synonyms.
• Saying an object contains heat. It contains thermal energy.
• Saying cold moves into a warm object. Energy moves out of the warm one.
• Dropping a factor in Q = mcΔT.