An engine takes in 500 J from its fuel. It turns 150 J of that into motion.
What share of the 500 J becomes useful output?
Count it: 150 ÷ 500 = 0.30. The useful output is 0.30 of the input.
efficiency = useful energy out ÷ total energy in
The engine's efficiency is 0.30.
Efficiency is often expressed as a percentage: 0.30 × 100 = 30. The engine is 30% efficient.
The percentage reads naturally: 30% efficient means 30 J of every 100 J of input becomes useful output.
Efficiency is a share, so it has no unit.
The rest of the input is the wasted output: 500 - 150 = 350. 350 J leaves as waste, mostly heat.
Q: A microwave takes in 1200 J and delivers 660 J to the food. Calculate its efficiency, as a percentage.
Write down the values:
useful energy out = 660 J, total energy in = 1200 J, efficiency = ?
Write down the equation:
efficiency = useful energy out ÷ total energy in
Substitute in the values, and calculate:
efficiency = 660 ÷ 1200 = 0.55
As a percentage: 0.55 × 100 = 55
The microwave's efficiency is 55%.
Q: A phone charger takes in 80 J from the outlet and puts 68 J into the battery. Calculate its efficiency, as a percentage.
Write down the values:
useful energy out = 68 J, total energy in = 80 J, efficiency = ?
Write down the equation:
efficiency = useful energy out ÷ total energy in
Substitute in the values, and calculate:
efficiency = 68 ÷ 80 = 0.85
As a percentage: 0.85 × 100 = 85
The charger's efficiency is 85%.
An electric scooter's motor takes in 400 J and delivers 300 J of useful work. Enter its efficiency, as a percentage.
A blender takes in 250 J; 100 J of that becomes the blades' useful work. Lumi says: 'The blender's efficiency is 40%.' Is she correct?
A gas water heater takes in 1600 J from burning gas, and 1040 J of that ends up in the water. Enter the heater's efficiency, as a percentage.
A dishwasher is 60% efficient. Which statement says what that means?
An air compressor takes in 900 J of electrical energy; 324 J of that becomes useful work. Enter the compressor's efficiency, as a percentage.
A device turns a share of its total energy input into useful output. What is that share called?