- Define and explain Conservation and Efficiency in your own words
- Use key terms such as efficiency accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Conservation and Efficiency: using the conservation of energy and calculating the efficiency of devices.
Using the conservation of energy and calculating the efficiency of devices.
Key ideas
No device is 100% efficient
Efficiency = useful energy out ÷ total energy in × 100%. A filament bulb is only about 10% efficient — most energy leaves as unwanted heat. Real processes always waste some energy, usually as thermal energy spreading to the surroundings.
Energy is stored in different ways
A moving car holds kinetic energy, a raised weight holds gravitational potential energy, a stretched spring holds elastic potential energy, and food and fuels hold chemical energy. The unit is always the joule, and energy can move from one store to another — a battery's chemical store becomes electrical, then light.
Key term — efficiency: The fraction of input energy usefully transferred, usually given as a percentage.
A motor takes in 500 J of electrical energy and outputs 400 J of kinetic energy. What is its efficiency?
(400 ÷ 500) × 100% = 80%.
Answer: (400 ÷ 500) × 100% = 80%.
- Forgetting to square the velocity in kinetic energy KE = ½mv² — doubling the speed quadruples the energy, so the square matters enormously.
- Saying energy is 'used up' Energy is never used up; it transfers to other stores, often dissipating as heat.
Practice
½ × 1000 × 20² = ½ × 1000 × 400 = 200,000 J (200 kJ).
Chemical energy (battery) → electrical energy → light energy, plus some thermal energy in the bulb.
None is lost — some transfers to thermal energy and sound in the ball and ground on each bounce.
Kinetic energy depends on v², so doubling v multiplies the energy by 2² = 4.
Quick check
Which of these best defines "efficiency"?
A 3 kg book is lifted 2 m (g = 10 N/kg). Find its gain in GPE.
- Conservation and Efficiency: using the conservation of energy and calculating the efficiency of devices.
- No device is 100% efficient: Efficiency = useful energy out ÷ total energy in × 100%.
- joule: The unit of energy, symbol J.
- Watch out for: forgetting to square the velocity in kinetic energy