Power is a measure of how quickly energy is transferred. Efficiency is a measure of how much useful energy or power is transferred. By considering where energy is ‘wasted’ we can improve efficiency.

What is the main energy transformation in a cooker?

Electrical to sound energy

Electrical to heat energy

Electrical to light energy

What is the main energy transformation in a toaster?

Which of the following is the correct equation for calculating electrical power?

\[\text{Power} = \text{energy} \times \text{time}\]

\[\text{Power} = \text{time} \div \text{energy}\]

\[\text{Power} = \text{energy} \div \text{time}\]

What power rating should a kettle that uses \(20\, 000\, J\) of energy in 10 seconds have?

\[0.0005\, W\]

\[2000\, W\]

\[200\,000\, W\]

What is the typical power rating of a hair-dryer?

\[135\, W\]

\[1200\, W\]

\[9000\, W\]

What is the typical power rating of a fridge?

\[1000\, W\]

What happens to energy during energy transformations?

Energy is made from nothing

Energy cannot be made or destroyed, it can only be transformed from one type to another

Energy is destroyed and lost forever

In what form is energy generally lost to the surroundings?

Heat or sound

Electricity

Kinetic

How do we calculate efficiency?

\[\text{Efficiency} = (\frac{{useful\,output}}{{input}}) \times 100\]

\[\text{Efficiency} = (\frac{{input}}{{useful\,output}}) \times 100\]

\[\text{Efficiency} = \left( {\frac{{useful\,output}}{{input}}} \right) \div 100\]

What is the percentage efficiency of an electric drill that uses \(300\, J\) of electrical energy each second but only does \(100\, J\) of useful work?

\[0.33\%\]

\[300\%\]

\[33\%\]