P7.2.5 - Efficiency

Question 1

What is efficiency?

Answer 1

A measure of how much of the input energy of a device, is transferred to useful energy.

Question 2

How is efficiency measured?

Answer 2

As a ratio or a percentage

Question 3

What is the equation for efficiency?

Answer 3

Efficiency = (useful energy/total energy) x 100

Question 4

What do Sankey diagrams?

Answer 4

It shows energy transfer through a device by separating out the useful energy and wasted energy

Question 5

How do you increase efficiency.

Answer 5

You can increase efficiency by reducing the wasted energy
You can:
- use insulation to reduce heating of the surroundings
- make devices from materials that reduce unwanted energy transfer
- use technology to produce devices that are better at their job, for e.g. LEDs

Question 6

Suggest how to improve the efficiency of an oven.

Answer 6

Insulate the oven to ensure minimise wasted energy transferred to the surroundings as heat

Question 7

Explain why using LEDs might not reduce your electricity bill. (Hint: think what it will feel like to be in a room with LEDs rather than incandescent bulbs)

Answer 7

• LEDs are more efficient
• so waste less energy by transferring less to energy in the surroundings
• a room lit by LEDs will feel colder than a room lit by incandescent bulbs
• any electricity that may have been saved will now just be used instead in heating the house