What a learner can do afterwards
- States the law of conservation of energy
- Explains that an isolated system's total energy stays constant because no energy enters or leaves, even when its stores change
- Explains dissipation and identifies causes such as friction, air resistance and electrical resistance
- Gives an example showing why 'wasted' energy is not lost but transferred to the surroundings
1 · Read
Energy cannot be created or destroyed. Energy can remain stored without changing stores or form. An isolated system exchanges no energy with its surroundings, so its total stays constant. Energy can leave an object without being destroyed. In an energy account, input plus decreases in stores equals output plus increases in stores. A discharging battery can supply output by reducing its chemical store. Include the surroundings when energy leaves the object.
Dissipation means energy spreads into thermal stores and becomes less available for the intended job. It is harder to collect and reuse, not destroyed. Friction, air resistance and electrical resistance can cause this spreading.
Electrical resistance warms parts in a charger or motor. A motor can also warm through friction between moving parts. The warmer parts then transfer energy to cooler surroundings. A charger does not need moving parts to warm up.
A fuel-burning engine heats its parts and exhaust gases. Friction also warms moving parts. These transfers leave less energy for useful motion. Air resistance acts on the moving vehicle, not on the engine alone. None of this energy is destroyed.
A bouncing ball transfers energy to thermal stores in the ball and floor, and sound carries energy away. Less remains in its motion and height stores, so later bounces are lower. The ball can lose energy while the total including surroundings is conserved.
Bike brake pads rub against a wheel or brake disc. Friction reduces the bike's kinetic store and increases thermal stores in the brakes. The brakes then heat their surroundings. The kinetic energy has been transferred, not destroyed.
Energy is conserved when all stores and transfers are counted. An isolated system exchanges no energy. Dissipation spreads energy into thermal stores, making it less available for the intended job.
Original readings written for this collection. The linked conservation video is from Khan Academy.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.