Power and Efficiency

Power and Efficiency

By Mr Math n Physics

​Energy transferred per unit time

​Work done per unit time

POWER

​P = Power measured in watt or W
W = Work or Energy measured in joule or J
t = time measured in second or s

Worked Example

Calculate the energy transferred when an iron with a power rating of 2000 W is used for 5 minutes.

​P = 2000 W
t = 5 x 60 s = 300 s

W = P x t
W = 2000 x 300
W = 60000 J = 600 kJ

Multiple Choice

Identify the incorrect definition of power.

• A force acting over a distance

• Work done per unit time

• Energy transferred per second

• The rate of work done

Multiple Choice

Four different electric bicycles are driven along a road. The work done by their motor, and the time taken to do that work is shown in the table below. Which of the electric bicycles produces the most power?

• Bicycle A
• Bicycle B
• Bicycle C
• Bicycle D

Multiple Choice

A crane lifts four pallets of bricks, each of pallets which weigh 5000 N. The crane lifts each pallet a height of 30m. The crane takes 4 minutes to do this. How much useful power did the crane produce to lift the bricks?

• 625 W

• 2500 W

• 3000 W
• 4000 W

Multiple Choice

Three boxes each weigh 100 N. A man lifts all the boxes together from the ground on to a shelf that is 1.5m above the ground. The man takes 2.0s to do this. How much useful power does the man produce to lift the boxes?

• 180 W
• 300 W
• 225 W
• 75 W

Multiple Choice

A car of 1000 kg of mass moved from rest to 108 km/h. if time taken is 10 s, the power of cars engine is ....

• 45 kW

• 75 kW

• 30 kW

• 60 kW

Efficiency

• The ratio of the useful power or energy output from a system to its total power or energy input

• percentage of the useful power or energy output from a system to its total power or energy input

Worked Example

An electric motor take 100 J of energy on batteray for 8.0 s. If it produce 10 W of power, the efficiency of electric motor is ...

Multiple Choice

The diagram shows an LED light bulb. Which expression gives the efficiency of the bulb?

• $$\frac{8.0}{0.5}\times100\%$$

• $$\frac{7.5}{0.5}\times100\%$$

• $$\frac{0.5}{8.0}\times100\%$$

• $$\frac{7.5}{8.0}\times100\%$$

Multiple Choice

Which of the following energy transfers is most efficient?

• Useful energy out = 70%

• Wasted energy out = 45%

• Useful energy out = 35%

• Wasted energy out = 25%

Multiple Choice

The diagram shows the energy used by a modern lamp. The efficiency of the lamp is ....

• 10%
• 30%
• 50%
• 20%

Multiple Choice

An electric motor has an efficiency of 35%. It lifts a 7.2 kg load through a height of 5 m in 3 s. Calculate the input power of the motor. take gravitational acceleration 9.8 m/s²

• 256 W

• 566 W

• 446 W

• 336 W

Multiple Choice

Look at the diagram of the Sankey diagrams for three different lights (traditional filament lamp, LED, CFL). Which lamp has the highest efficiency?

• Filament lamp
• CFL lamp
• Incandescent bulb
• LED light