Physics · Rung 7 of 23

Energy

Energy is not used up: it changes place and form, and you can always follow it. An old light bulb turned less than a twentieth of what went in into light, and the rest left as heat — it did not vanish, it warmed the room. Following that trail is what explains why a phone gets hot while charging, why braking wears the pads down, and why a power station needs a river next to it. The question here is where it came from and where it went, never how much.

Where this sits

Comes after
Quantities
Opens up
The experiment, Writing the formula, Heat and temperature, Work and power

Exercises from this rung

Generated by the app, from easiest to hardest

  1. a solar panel at noon

    • motion → heat
    • light → electrical
    • chemical → heat
    • electrical → light

    Why From light into electrical. It uses about a fifth of what reaches it; the rest heats it up. Nothing is lost: what is not used ends up as heat.

    easier

  2. a microwave with a glass of water

    • electrical → heat
    • electrical → light
    • height → electrical
    • light → electrical

    Why From electrical into heat. It is one of the most direct there is: it shakes the water molecules without heating the plate. Nothing is lost: what is not used ends up as heat.

    in between

  3. a bike braking downhill

    • height → electrical
    • electrical → light
    • motion → heat
    • light → electrical

    Why From motion into heat. The pads heat up: that is why they wear down and why they smell after a long descent. Nothing is lost: what is not used ends up as heat.

    harder