Energy
8th Grade Science · 6 lessons
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Lessons
- 1Energy Is Transferred Between Stores, Never CreatedEnergy is conserved: every joule leaving one store arrives in another. A 0.5 kg ball dropped from 2 m transfers about 10 J from the gravitational store (E = mgh) into the kinetic store, and on landing that 10 J ends up in thermal stores and sound. Saying energy was 'lost' always means it was dissipated — spread into thermal stores so dilute that no useful work can be drawn from them. The total never changed; only its usefulness did.Experiment
- 2Kinetic Energy Rises with the Square of SpeedKinetic energy is Ek = ½mv², so doubling the speed quadruples the energy that braking has to remove. A car at 60 km/h carries four times the kinetic energy it had at 30 km/h and needs roughly four times the braking distance on the same surface. Momentum, by contrast, only doubles. That difference between a linear and a squared relationship is the whole quantitative argument behind speed limits in built-up areas.Experiment
- 3Power Is Energy Divided by TimePower P = E/t is measured in watts, one watt being one joule per second. A 500 N student climbing 3 m of stairs does 1,500 J of work whether they stroll or sprint; taking 10 s means 150 W, taking 5 s means 300 W. Work measures the job done and power measures how quickly it was done — the same distinction that lets a small motor lift a heavy load eventually, but only a large one lift it fast.Experiment
- 4Efficiency and the Sankey PictureEfficiency is useful output energy divided by total input energy, and it can never exceed one, because the output cannot contain joules the input did not supply. An LED converting 45 J of every 50 J into light is 90% efficient; a filament lamp managing 5 J of the same 50 J is 10%, with the other 45 J dissipated as heat. A Sankey diagram draws every branch with a width proportional to its energy, so the wasted branch is seen rather than merely admitted.Experiment
- 5Specific Heat Capacity: Why Water Warms SlowlyRaising one kilogram of a substance by one degree Celsius takes a fixed amount of energy, its specific heat capacity, and the energy needed is E = mcΔθ. Water's value of about 4,200 J/kg°C is unusually high — roughly four times that of most metals — so the same heater warms a kilogram of copper far faster than a kilogram of water. That single number explains why coastal air swings less than inland air and why water is the working fluid in heating systems.Experiment
- 6Elastic Stores and the Square Law in a SpringUp to its limit of proportionality, a spring's extension is proportional to the force applied, F = kx, and the energy stored is the area under that force-extension graph, ½kx². Because area grows with the square of extension, stretching a spring twice as far stores four times the energy, not twice. That is why a bow drawn to full length delivers so much more than one drawn halfway, and why a car bumper is designed to crumple over a long distance rather than a short one.Experiment
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