Energy Conversion and Conservation
FoundationEnergy never disappears — it only changes form (law of conservation of energy). Battery: chemical → electrical; bulb: electrical → light + heat; motor: electrical → kinetic. Every conversion leaks some 'useless' heat, so efficiency is always below 100%.
- Conservation: energy only converts, never vanishes
- Every device: trace in → out
- Efficiency <100%; losses are mostly heat
§Text Passage
1The law of conservation of energy states that energy is never created out of nothing nor destroyed, only changed from one form into another. A torch is a simple example: the battery stores chemical energy, which becomes electrical energy when the torch is switched on, and the bulb then turns the electrical energy into light and heat. To analyse any device, ask what form of energy goes in and what forms come out, then trace the flow.
2Every conversion carries losses: touch a torch's cover and you feel the heat — energy with no useful purpose. Efficiency is therefore always below 100%, calculated as useful output energy divided by total input energy, multiplied by 100%. With 100 J in and 60 J useful, the efficiency is 60%; the remaining 40 J has not vanished but escaped as heat and sound.
3The commonest losses come from friction and resistance: rubbing machine parts turn kinetic energy into heat, which lubricants reduce; wires have resistance, so some electrical energy becomes heat during transmission — one reason long-distance transmission uses high voltage. Devices also differ widely: LED bulbs convert about 40% of electrical energy into light against roughly 5% for filament bulbs, while a car engine manages only about 25-30%, most of its chemical energy leaving directly as heat.
4Engineers work to cut losses and raise efficiency: switching to LED bulbs saves power at once; electric cars turn battery energy into motion at the wheels with up to about 90% efficiency, clearly above petrol engines; oiling a bicycle chain and shaping a high-speed train's nose into a streamline both reduce losses. Trace where energy enters and where it goes, and you can explain why devices warm up — and why saving energy really means wasting less.
1Summary 重點摘要
Energy never disappears — it only changes form (law of conservation of energy). Battery: chemical → electrical; bulb: electrical → light + heat; motor: electrical → kinetic. Every conversion leaks some 'useless' heat, so efficiency is always below 100%.
2Vocabulary cards Vocabulary
Energy can't be created or destroyed — only converted between forms.
The energy of a moving object.
Stored energy due to position or state — e.g. water held high.
Useful output ÷ total input × 100%. Always <100% (losses).
Energy stored in substances — batteries, food and fuels.
Energy carried by electric current — easy to transmit and convert.
The energy form given out by bulbs and screens.
The commonest loss — it warms devices up, like a torch heating in your hand.
Energy given out as noise by motors and fans — usually a loss.
3Step by step Steps
- Trace the energy chainTo analyse any device's energy conversions, ask two questions first: what form of energy goes in, and what forms come out? An electric fan takes in electrical energy and gives out kinetic energy, with sound and heat tagging along — the extras are usually the losses. A device warming up as it runs is exactly this wasted energy showing itself; get into the habit of listing inputs and outputs before discussing efficiency, and your answers gain a clear structure.
- Common chainsA few common chains are worth memorising: a battery stores chemical energy; a torch converts chemical energy to electrical, then to light and heat; an electric fan turns electrical energy into kinetic and sound; hydroelectric generation converts water's potential energy into kinetic, then electrical. With these memorised, exam questions can be answered straight away — and in addition, solar panels convert light energy into electricity.
- ConservationThe law of conservation of energy states that energy is never created out of nothing nor destroyed, only changed from one form into another. Total input energy therefore equals the useful output plus the energy lost, the losses being mostly heat and sound; the total before and after conversion never changes. This law is the foundation for analysing every energy problem, and it applies without exception.
- EfficiencyEfficiency is calculated as useful output energy divided by total input energy, multiplied by 100%, and the result is always below 100%. For example, with 100 J in and 60 J useful, the efficiency is 60%; the remaining 40 J has not vanished — it has become heat and sound that escape. LED bulbs reach about 40% efficiency, far above the roughly 5% of filament bulbs, a very striking gap.
- Where losses come fromThe commonest losses come from friction and resistance: rubbing machine parts turn kinetic energy into heat, which lubricants reduce; wires have resistance, so some electrical energy becomes heat during transmission — one reason long-distance transmission uses high voltage to cut the loss. A car engine manages only about 25-30% efficiency, most of its chemical energy leaving directly as heat, leaving plenty of room for improvement.
- Raising efficiencyEngineers keep finding ways to cut losses: LED bulbs convert about 40% of electrical energy into light against roughly 5% for filament bulbs, so switching saves power at once; electric cars turn battery energy into motion at the wheels with up to about 90% efficiency, clearly above petrol engines; oiling a bicycle chain and shaping a high-speed train's nose into a streamline to reduce air resistance are examples of loss-reduction from daily life to engineering.
4Diagrams Diagrams
5Worked example Example
Charge a phone, then play a game — trace the energy chain.
- Charging: mains electrical → chemical (stored in battery)
- Playing: chemical → electrical
- Screen + chip: electrical → light + heat
- The phone warms up: that's the 'lost' heat
- Answer: While charging, the mains' electrical energy is stored as chemical energy in the battery; when playing, chemical energy turns back into electrical energy, and the screen and chip convert it into light and heat — the warm body is the leaked heat. The chain runs electrical → chemical → electrical → light + heat, and every step loses some energy as heat, so efficiency is always below 100%; write each step as 'form in → forms out' in order to score. Also state why efficiency is low: part of the energy escapes as heat.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the energy chain of an electric fan?
Q2A machine takes 100J and gives 60J of useful output. Efficiency = ?
Q3Where does the 'lost' energy go?
Q4What is the energy conversion in a filament bulb, and why is it so inefficient?
Q5Compare the energy conversion efficiency of electric and petrol cars.
8Recap Recap
- Conservation: energy only converts, never vanishes
- Every device: trace in → out
- Efficiency <100%; losses are mostly heat
- Total input = useful output + losses (heat, sound)
- Examples: battery chemical → electrical; motor electrical → kinetic + heat + sound