Heat Is Not Temperature

Put a metal spoon in a cup of hot tea and leave it. Come back a minute later and the handle — the end nowhere near the tea — is warm.

Nothing moved it. Nobody touched it. So something travelled up that spoon, and it travelled from the hot end towards the cold end, never the other way.

That something is heat, and it is the last form of energy from the previous lesson, now studied on its own.

But first there's a word problem to clear up, because two words get mixed together constantly and they mean genuinely different things. Heat and temperature are not the same. A cup of tea is much hotter than a bathtub of warm water, yet the bathtub holds far more heat. Once you can say why, most of this topic falls into place.

By the end of this lesson you'll be able to:

• explain the difference between heat and temperature
• describe the three ways heat travels, with an example of each
• name the six changes of state and say which need heat put in
• explain the strange thing that happens to the temperature of melting ice

Two different words

Temperature is how hot something is. You read it off a thermometer, in degrees Celsius. It doesn't care how much stuff there is — one spoonful of boiling water and a whole pot of it are both at 100 °C.

Heat is energy that moves from a hotter thing to a colder thing. It does care how much stuff there is, because it's a total amount, not a level.

That's the tea and the bathtub. The tea is at 90 °C and the bath is at 40 °C, so the tea wins on temperature. But the bath holds maybe fifty times as much water, so it holds far more heat in total. Tip the tea into the bath and the bath barely notices.

Now the rule that governs everything heat does:

Heat always flows from hotter to colder. Never the other way, on its own.

Your tea goes cold because heat leaves it for the cooler room. Ice in your glass doesn't send cold into the drink — the drink sends heat into the ice. There's no such thing as cold travelling anywhere. Cold is simply the absence of heat, the way dark is the absence of light.

And the flow keeps going until both things are at the same temperature. Then it stops. That's why a spoon left in tea long enough ends up exactly as warm as the tea, and no warmer.