Science 10 · Energy and work
Energy is conserved and degraded
Two statements that sound contradictory and are both true: the total amount of energy never changes, and the useful amount always falls. Everything in this unit is one of those two.
- 1. The words, first
- 2. Work, and what it is not
- 3. Following the energy
- 4. Efficiency and the second law
- 5. What costs marks
The words, first
The idea: Work, power and energy are used loosely in ordinary speech and precisely here.
| Word | What it means |
|---|---|
| Energy | The capacity to do work. Measured in joules (J). |
| Work | A transfer of energy by a force moving something: W = Fd. Holding a heavy box still is no work at all in this sense. |
| Kinetic energy | Energy of motion: Ek = ½mv². Note the square — doubling the speed quadruples it. |
| Potential energy | Energy of position or arrangement. Gravitational: Ep = mgh. |
| Power | The rate of energy transfer: P = W/t, in watts. A watt is a joule per second. |
| Efficiency | Useful output ÷ total input × 100%. Never above 100%. |
| Energy conversion | Changing energy from one form to another. Every conversion degrades some of it into heat. |
| Law of conservation of energy | Energy cannot be created or destroyed, only converted. |
Work, and what it is not
The idea: Force alone is not work. The thing has to move, and it has to move in the direction of the force.
W = F × d (joules = newtons × metres)
Worked. A 20 N force moves a box 4 m: W = 80 J.
Zero work. Holding a heavy box motionless is exhausting and does no work, because d = 0. Carrying it horizontally at a steady speed also does no work against gravity, because the upward force and the horizontal motion are perpendicular. Both feel wrong and both are right.
Work and energy are the same currency. Doing 80 J of work on something gives it 80 J more energy. That is why both are measured in joules, and it is why energy problems can often be solved by tracking work instead of motion.
Following the energy
The idea: Name where it started and where it ended. The totals must match, and the ones that seem not to are hiding heat.
Worked: a falling ball. A 2 kg ball dropped from 5 m has Ep = mgh = 2 × 9.8 × 5 = 98 J at the top. Just before landing all of it is kinetic, so Ek ≈ 98 J. No motion equations were needed — conservation did the work.
Worked: a bouncing ball. It comes back lower each time. Nothing was destroyed: the missing energy became heat and sound in the ball and the floor. Saying energy was lost is the habit to break; saying where it went is the habit to build.
Power. A machine doing 600 J in 20 s has P = 30 W. Turning it round, a 60 W bulb running for 5 minutes uses E = Pt = 60 × 300 = 18 000 J — and the time has to be in seconds.
Efficiency and the second law
The idea: The first law says the total is conserved. The second says its usefulness is not, and that is why efficiency is always below 100%.
Efficiency = (useful energy out ÷ total energy in) × 100%
Worked. A motor takes 500 J and delivers 150 J of useful work: efficiency = 30%. The other 350 J became heat and sound — so 70% is what was wasted, not the efficiency.
Typical values worth knowing. An incandescent bulb is about 5% efficient at making light; an LED is around 25%. A car engine is roughly 25%. An electric motor can exceed 90%.
Why over 100% is impossible. It would mean more energy came out than went in, which breaks conservation. This is not a limit better engineering could beat — it is an accounting identity. A device claiming more is either measuring wrongly or omitting an input.
What costs marks
The idea: Units and definitions.
- Answering a work question in watts, or a power question in joules.
- Leaving time in minutes in E = Pt.
- Saying energy is lost. Say where it went.
- Calling the wasted fraction the efficiency.