Grade 8

Energy and Heat

Six lessons from naming the forms of energy to the reason no machine ever returns everything it is given, with heat treated throughout as energy on the move rather than as a substance stored in things.

6 lessons

  1. 1Which forms of energy are present in a kitchen while a meal is cooked?Almost all of them at once. The gas holds chemical energy that turns into heat; the boiling pot holds thermal energy; the spinning mixer has kinetic energy and runs on electrical energy; the lamp overhead gives out light; and the rattling lid makes sound. Energy is not a material — it is the capacity to cause change, and it is measured in joules.30m
  2. 2Where does the energy go when a bouncing ball rises lower each time?Nowhere — it is all still there. On every impact some energy goes into deforming the ball, into warming the rubber and the floor, and into the sound of the bump. The ball rises less because it has less mechanical energy left, but the total has not changed at all. It has simply spread into forms that are hard to see.35m
  3. 3How does a cup of boiling water differ from a bath at forty degrees?In temperature the cup wins easily, but in thermal energy the bath holds far more. Temperature measures the average speed of the particles; thermal energy depends on how many particles there are as well. The bath contains thousands of times more molecules, so it can deliver much more heat despite being merely warm.35m
  4. 4Why does a metal spoon in soup get hot while a wooden one does not?Because of conduction. In a metal, free electrons carry the particle motion quickly along the spoon, so the heat reaches your fingers within seconds. Wood has no such carriers, the motion passes slowly from particle to particle, and the handle stays cool. It is a difference in thermal conductivity, not in the temperature of the soup.40m
  5. 5Why does boiling water stay at one hundred degrees while the hob is still on?Because the incoming energy is not speeding the particles up — it is pulling them apart. At the boil, the bonds holding the molecules together in the liquid are being broken, and every joule goes into that job. Temperature measures average speed, and while the speed is not rising, neither is the reading, even after an hour.35m
  6. 6Why does no machine give back everything you put into it?Because at every conversion some of the energy leaves in a form you did not want, and almost always that form is heat. Conservation still holds perfectly — not one joule was destroyed — but heat that has spread into the air is no longer useful. Efficiency is the ratio of useful output to total input, and it is always below one hundred percent.30m