6.3 Efficiency Calculations

Which two equations can be used to calculate efficiency?

• equation A

• equation B

• equation C

• equation D

What unit is power measured in?

• joules (J)

• watts (W)

• seconds (s)

What unit is energy measured in?

• Joule

• Meter

• Kilogram

• Second

Efficiency can be expressed .................... or ..................

• as a percentage
• in joules
• as a decimal

• in watts

One watt is equal to one ..............

• joule per second (J/s)
• kilojoule (kJ)

• kilojoule per second (kJ/s)

A radio supplied with 500 J of energy has a useful energy output of 400 J. Calculate the efficiency of the radio. Give your answer as a decimal.

• 0.8

• 0.75

• 0.85

• 0.9

The total power input to a hedge trimmer is 600 W. The useful power output from the hedge trimmer is 150 W. The efficiency of the hedge trimmer is ___%.

• The efficiency of the hedge trimmer is 25%.

• The efficiency of the hedge trimmer is 50%.

• The efficiency of the hedge trimmer is 75%.

• The efficiency of the hedge trimmer is 10%.

Calculate the efficiency of the solar cell, and give your answer as a decimal.

• Efficiency = Useful Power Output / Total Power Input = 1.5 W / 3.0 W = 0.5

• Efficiency = Useful Power Output / Total Power Input = 1.0 W / 3.0 W = 0.33

• Efficiency = Useful Power Output / Total Power Input = 2.0 W / 3.0 W = 0.67

• Efficiency = Useful Power Output / Total Power Input = 1.5 W / 4.0 W = 0.375

Calculate the efficiency of the vacuum cleaner and give your answer as a decimal.

• 0.48

• 0.57

• 0.52

• 0.36

When a kettle is used to boil some water, the total input energy is 80 J. 25 J of the total input energy is wasted by the kettle. The efficiency of the kettle is ____%

• 69%

• 50%

• 75%

• 80%