Free Energy Losses and Efficiency MCQs with Answers

1. A machine takes in 500 J and delivers 350 J of useful output. Its efficiency is

• A. 30%
• B. 43%
• C. 70%
• D. 143%

Explanation: Efficiency is useful output divided by total input times 100, which is 350 over 500 times 100, giving 70%. The remaining 150 J is lost, mostly as heat through friction. An efficiency above 100% is impossible, so the last option can be discarded on principle alone.

2. The efficiency of a machine is

• A. always greater than 100 per cent
• B. the ratio of useful output energy to total input energy, expressed as a percentage
• C. the ratio of input to output
• D. equal to the power output

Explanation: Efficiency can never exceed 100 per cent, because energy is conserved and some is always lost to friction, sound and heat. An electric motor may reach 90 per cent while a petrol engine manages only about 25 per cent. The wasted energy is not destroyed, merely converted into forms that are not useful.

3. A machine takes in 500 J and delivers 400 J of useful output. Its efficiency is

• A. 80 per cent
• B. 125 per cent
• C. 100 per cent
• D. 20 per cent

Explanation: Dividing 400 by 500 and multiplying by 100 gives 80 per cent, with the remaining 100 J lost mainly as heat. Dividing the wrong way round gives 125 per cent, which should be recognised at once as impossible. The 20 per cent figure is the fraction wasted rather than the efficiency.

4. In a real pendulum swinging back and forth, the total mechanical energy slowly decreases because

• A. energy is destroyed
• B. energy is transferred to the surroundings as heat and sound by air resistance and friction at the pivot
• C. gravity weakens
• D. the mass of the bob decreases

Explanation: Energy is conserved overall, but the mechanical energy of the pendulum is gradually converted into thermal energy of the air and the pivot, so the amplitude decays. In an ideal frictionless case the exchange between kinetic and potential energy would continue indefinitely. Nothing is ever destroyed, which is what the conservation principle asserts.

5. Which of the following is NOT a renewable source of energy?

• A. Solar
• B. Wind
• C. Coal
• D. Hydroelectric

Explanation: Coal is a fossil fuel formed over hundreds of millions of years, so it is consumed far faster than it can be replaced and its combustion releases carbon dioxide. Solar, wind and hydroelectric are all replenished continuously by natural processes. Pakistan's electricity mix relies heavily on hydroelectric and thermal generation.

6. Earth receives large amount of energy directly from:

• A. Wind
• B. Water
• C. Sun
• D. Moon

Explanation: Solar radiation is the primary energy input to the Earth, and wind and flowing water are themselves driven by the uneven solar heating of the atmosphere and oceans, which makes them secondary rather than direct sources. The Sun's energy comes from the fusion of hydrogen into helium in its core. Tidal energy is the one significant contribution that traces back to the Moon instead.