Energy is never lost or used up. It is transferred to other stores. A good explanation names where the energy started, how it moved, and where it ended up, then says that the total is unchanged but the energy is now spread out and less useful.
This is the writing skill that goes with calculating efficiency. It also supports practical and data questions in power and energy resources.
How do I build the explanation sentence by sentence?
Use this frame.
- Start. “Energy was stored as…” (for example gravitational potential energy).
- Transfer. “It was transferred by…” (mechanical work, heating, electrical work or radiation).
- Destination. “Some of it ended up in the thermal store of…” and name what heated up.
- Conservation. “The total energy is unchanged, but the thermal energy is spread out, so it is not useful.”
Words to use: transferred, dissipated, wasted, less useful, spread to the surroundings. Words to avoid: lost, used up, disappears, destroyed, gone.
Worked example
A rubber ball of mass 0.10 kg is dropped from 1.0 m and rebounds to 0.64 m. Take g = 10 N/kg. (Invented example data.)
Step 1, starting store. GPE at 1.0 m = 0.10 × 10 × 1.0 = 1.0 J.
Step 2, end store. GPE at 0.64 m = 0.10 × 10 × 0.64 = 0.64 J.
Step 3, difference. 1.0 − 0.64 = 0.36 J.
Step 4, explanation. “The ball starts with 1.0 J of gravitational potential energy. As it hits the floor, the ball and floor deform and the ball is briefly compressed, transferring 0.36 J to the thermal store of the ball and floor and to sound. The total energy is still 1.0 J, but the 0.36 J is spread into the surroundings and is not available to raise the ball.”
Check: the ball keeps 0.64 ÷ 1.0 = 64% of its energy for the bounce.
What mistake should I watch for?
The usual slip is wording, not calculation.
Mistaken answer: “The ball loses 0.36 J because the energy is used up when it hits the floor.”
This says energy disappears, which contradicts conservation.
The correction is to replace “used up” with a transfer: “0.36 J is transferred to the thermal store of the ball and floor and to sound.” Another partial answer is “heat is lost” with no store named. Name both the store and the place.
Check yourself
Try these, then open each answer.
1. Rewrite this sentence correctly: “Some of the energy in the motor is lost.”
Show answer
“Some of the electrical energy supplied to the motor is transferred to the thermal store of the motor and surroundings, because of friction and electrical resistance, and is wasted.” The total energy is unchanged.
2. A cyclist brakes and stops. Describe where the kinetic energy goes.
Show answer
Friction between the brake pads and wheel rims transfers the kinetic energy to the thermal store of the pads, rims and air. Some energy is also transferred as sound. The total energy stays the same.
3. A machine takes in 100 J, and 70 J is useful. State the wasted energy and where it goes.
Show answer
Wasted energy = 100 − 70 = 30 J. It is transferred to the thermal store of the surroundings and to sound, and the total output is still 100 J.
Where this leads next
You have finished the route of lessons in work, energy and efficiency. Try the mixed practice set to see how all five skills work together, and then move on to power and energy resources.
Good explanations come from practice with feedback on the wording itself. That is something a teacher can do in online one-to-one Physics tuition, using your own answers as the starting point.