E5: Efficiency — D level

Energy efficiency is a way to judge the work done by a device. There is a formula for it:

Efficiency = Work done ÷ Energy used x 100%

Here is an example:

A certain car uses 500 kJ of chemical energy in the fuel to achieve kinetic energy of 100 kJ. What is its efficiency?

Efficiency = 100 kJ ÷ 500 kJ x 100% = 20%

Remember: J is the unit for measuring energy. Its full name is “Joule”.

Part 1:

Each of these light bulbs makes the same amount of light.

Calculate the efficiency of each light bulb.

1. LED: Uses 7 J of electricity to make 5J of light.
2. Incandescent: Uses 100 J of electricity to make 5 J of light.
3. CFL: Uses 17 J of electricity to make 5J of light.
4. Halogen: Uses 70 J of electricity to make 5J of light.

Part 2:

1. Which light bulb is the most efficient?
2. Which light bulb is the least efficient?
3. Which light bulb will be the cheapest to use in your home?