Energy resources and efficiency
Physics · Key Stage 3 · The School
Renewable and not
NON-RENEWABLE resources — coal, oil, gas, nuclear fuel — exist in a fixed amount and are not replaced on any human timescale. RENEWABLE resources — wind, solar, hydroelectric, tidal, geothermal, biomass — are replenished. Each has real trade-offs in cost, reliability, land use and emissions, and an honest comparison names them rather than presenting one as simply best.
Efficiency
EFFICIENCY is the useful energy out divided by the total energy in, usually given as a percentage. A filament lamp is about 5% efficient because most of the energy leaves as heat; an LED is around 90%. No device is 100% efficient, and the energy that is not useful is nearly always dissipated as heat to the surroundings.
Energy is conserved
Energy is never created or destroyed, only TRANSFERRED between stores. So "using energy" really means transferring it into a less useful form — usually spread-out heat, which is very hard to gather back. That is why energy bills are real even though no energy is destroyed, and why efficiency matters more than the phrase "saving energy" suggests.
An old filament lamp takes in 100 J of electrical energy every second and emits about 5 J as light. Efficiency = 5 ÷ 100 = 5%. The other 95 J is transferred to the surroundings by heating — the lamp is mostly a heater that glows. An LED emits about 40 J of the same 100 J: 40% efficient. In both cases all 100 J is still in the universe. Nothing was used up; it was spread out where it can no longer do a useful job.