What is wasted energy?

Energy transferred to a store that is not useful.

Where does wasted energy usually end up?

In the thermal store of the surroundings.

What does 'dissipated' mean?

Spread out into the surroundings so it cannot easily be used again.

What is the equation for efficiency?

Efficiency = useful energy output ÷ total energy input.

A device takes in 200 J and gives out 50 J usefully. What is its efficiency?

50 ÷ 200 = 0.25, which is 25%.

Why can efficiency never be more than 100%?

The useful output cannot be greater than the total input.

How does lubrication improve efficiency?

It reduces friction, so less energy is wasted as thermal energy.

How does insulation improve efficiency?

It reduces the thermal energy transferred to the surroundings.

Why can wasted energy not be used again?

It is spread out, so it is too spread out to be collected.

Conservation of energy and efficiencyIB MYP Physics: Flashcards

Card 1 of 120 of 12 known

Question

State the law of conservation of energy.

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

State the law of conservation of energy.: Energy cannot be created or destroyed, only transferred from one store to another.
What is wasted energy?: Energy transferred to a store that is not useful.
Where does wasted energy usually end up?: In the thermal store of the surroundings.
What does 'dissipated' mean?: Spread out into the surroundings so it cannot easily be used again.
What is the equation for efficiency?: Efficiency = useful energy output ÷ total energy input.
How do you turn an efficiency into a percentage?: Multiply by 100.
A device takes in 200 J and gives out 50 J usefully. What is its efficiency?: 50 ÷ 200 = 0.25, which is 25%.
Why can efficiency never be more than 100%?: The useful output cannot be greater than the total input.
How does lubrication improve efficiency?: It reduces friction, so less energy is wasted as thermal energy.
How does insulation improve efficiency?: It reduces the thermal energy transferred to the surroundings.
A machine wastes 30 J from an input of 100 J. How much is useful?: 100 − 30 = 70 J.
Why can wasted energy not be used again?: It is spread out, so it is too spread out to be collected.

Exam questions on Conservation of energy and efficiency

A school in Seoul is replacing its filament lamps with LED lamps. Each second a filament lamp takes in 60 J of electrical energy and gives out 3 J as light. An LED lamp takes in 8 J each second and gives out 6 J as light.
Show that the LED lamp is more efficient than the filament lamp.2 marks
A motor in a warehouse in Lagos lifts boxes. Each second the motor takes in 500 J of electrical energy and the boxes gain 350 J of gravitational potential energy. The rest of the energy is wasted because of friction in the bearings.
Suggest one way to improve the efficiency of the motor and explain how it works.2 marks
A student investigates which material is the best insulator for a beaker of hot water. For each test she starts with water at 80 °C, wraps the beaker in a different material and measures the water temperature after 10 minutes. The results are: no insulation 62 °C, bubble wrap 71 °C, cotton wool 73 °C and aluminium foil 66 °C.
Identify the independent variable and the dependent variable, and state one control variable.3 marks

See the full worksheet

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).