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Heat flows from hot to cold

Heat and matter · level 3, lesson 1 of 6

A question to start

You pour a jug of hot water and a jug of cold water into one bowl. What temperature do you end up with?

The idea

Heat is energy moving because of a difference in temperature. It always flows from the hotter thing to the colder one, never the other way by itself.

It keeps flowing until both are at the same temperature. Then the flow stops. Scientists call this thermal equilibrium.

60 °C20 °C40 °C, 40 °C heat flowslater

Worked examples

1. Equal jugs. One litre at 60 °C and one litre at 20 °C, mixed: the hot water cools and the cold water warms. You end with 2 litres at about 40 °C, halfway.

2. Unequal jugs. One litre at 60 °C and three litres at 20 °C: the cold water has more to warm up, so you end closer to 20 °C. The answer is 30 °C: \(\dfrac{1\times60 + 3\times20}{4} = 30\).

3. A rover at night. Inside, its electronics box is warm; outside, Mars is far below freezing. All night, heat flows out of the box into the cold. Heaters must keep putting heat back in.

Try it

  1. Which way does heat flow between a hot drink and the air?
  2. When does the flow stop?
  3. Mix 2 litres at 50 °C with 2 litres at 10 °C. What temperature do you get?
  4. Why can an ice cube never make a warm drink hotter?
Answers
  1. From the hot drink to the cooler air.
  2. When both are at the same temperature.
  3. Halfway: 30 °C.
  4. Heat flows from the warm drink into the colder ice, so the drink cools.

Common mistake

"Cold flows into things." Only heat flows. A fridge feels cold because heat flows out of your hand into it.

Where it leads