Where Heat Goes

What this book is about

A cup of tea goes cold. The back of the fridge stays warm. Gaps are left in railway lines. And on a hot afternoon the breeze comes in off the sea. Behind all of it lie two sentences: the energy ledger must balance, and heat travels in one direction only.

Textbooks teach heat as a list of formulas. Memorise Q = ms∆θ, memorise the latent heat values, memorise the efficiency formula. What heat actually *is*, and why it always moves from hot to cold, are usually left blank. Here the answer comes first. Heat is not a substance; it is the energy of the random jiggling of particles, handed on from one place to another. Everything else in the book follows from that one sentence.

You need multiplication and division, percentages, and a rough sense of the word "energy". Knowing how a lever or a balance stays level helps on page 17, and that much is recalled there anyway.

Stop at page 17. Mixing two buckets of water, the final temperature is worked out twice by completely separate routes — once by balancing the energy ledger, once by drawing nothing more than a balance beam. Both give exactly 40. That page is the spine of the book. Keep two mugs, a little hot water and a thermometer to hand; the experiment on page 74 takes ten minutes and makes the whole thing stick.

What is inside

• Heat and temperature are not the same: a drop against a bucket, what each word means, and

water's stubbornness

• How heat moves — conduction and the spoon handle, why iron feels colder than wood,

convection, radiation, and what a quilt, a flask and a white roof have in common

• What heat does: everything expands, water's strange exception, hidden heat, why

evaporation cools, and why the sea is the country's air conditioner

• Heat into work — the first law, squeeze it and it warms, the engine, the fridge and the AC
• Why it only goes one way: tea does not reheat itself, the second law, entropy as something

countable, Carnot's ceiling, and why we need the sun at all

• Who worked it out — caloric, a fluid that does not exist; Rumford's cannon; Joule and his

apparatus; Carnot deriving a law from an engine; Clausius and Boltzmann

• Two experiments, the formulas and numbers on one sheet, exercises with answers, a glossary

and a quiz