The fraction or percentage of the total energy input that is transferred as useful energy output.

Write the equation for efficiency.

Efficiency = useful energy output / total energy input (x 100% for a percentage).

What is the unit of efficiency?

It has no unit. It is a ratio, or a percentage.

Can a device be 100% efficient?

No. Some energy is always wasted.

Where does wasted energy usually end up?

In the thermal store of the surroundings, where it dissipates.

The rate of transferring energy, in joules per second.

Write the equation for power.

Power (W) = energy transferred (J) / time (s).

One joule of energy transferred each second.

Give two ways to reduce wasted energy.

Lubricate moving parts, insulate hot objects, or use a more efficient design.

Why is an LED lamp more efficient than a filament lamp?

It transfers a smaller fraction of its input energy as thermal energy and a larger fraction as light.

Efficiency and powerIB MYP Sciences: Flashcards

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What is efficiency?

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What is efficiency?: The fraction or percentage of the total energy input that is transferred as useful energy output.
Write the equation for efficiency.: Efficiency = useful energy output / total energy input (x 100% for a percentage).
What is the unit of efficiency?: It has no unit. It is a ratio, or a percentage.
Can a device be 100% efficient?: No. Some energy is always wasted.
Where does wasted energy usually end up?: In the thermal store of the surroundings, where it dissipates.
Define power.: The rate of transferring energy, in joules per second.
Write the equation for power.: Power (W) = energy transferred (J) / time (s).
What is one watt?: One joule of energy transferred each second.
A device takes in 200 J and wastes 50 J. What is its efficiency?: 150 / 200 × 100% = 75%.
A heater transfers 6000 J in 30 s. What is its power?: 6000 / 30 = 200 W.
Give two ways to reduce wasted energy.: Lubricate moving parts, insulate hot objects, or use a more efficient design.
Why is an LED lamp more efficient than a filament lamp?: It transfers a smaller fraction of its input energy as thermal energy and a larger fraction as light.

Exam questions on Efficiency and power

A student in Lagos compares a filament lamp with an LED lamp. Both are connected to the mains. After a few minutes the filament lamp is too hot to touch, but the LED lamp is only warm.
Explain why the filament lamp is less efficient than the LED lamp.2 marks
A construction company in Mumbai uses a crane powered by an electric motor to lift steel beams to the top of a tall building.
In one lift the motor takes in 6000 J of electrical energy and transfers 4200 J as gravitational potential energy of the beam. Calculate the efficiency of the motor.2 marks
Students in Cape Town compare two electric kettles, P and Q. Each kettle heats 1.0 kg of water through 20 °C. Kettle P has a thick plastic body and kettle Q has a thin plastic body. Kettle P took in 100 kJ of electrical energy and kettle Q took in 120 kJ. In both cases the water gained 84 kJ of energy.
State a hypothesis that the students could test about the two kettles, giving a scientific reason.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).