Calculating efficiency (in terms of energy and power) | Starter Quiz | Oak National Academy

Which of the following energy stores does friction transfer energy into?

• chemical store

• gravitational store

• kinetic store

• thermal store

Answer explanation

When two surfaces rub together, the particles in the surfaces will vibrate with more energy. This increases the energy in the thermal store, causing the surfaces to heat up.

Why does a hot cup of tea cool down?

• All of its energy is used up.

• All of its energy is transferred to the surroundings.

• Some of its energy is used up.

• Some of its energy is transferred to the surroundings.

Answer explanation

Energy is not used up, but some of it can be transferred to the surroundings. At room temperature, a cup of tea will still have enough energy to melt an ice cube!

Why is the rebound height of a ball not the same as the height that it was dropped from?

• Earth's gravitational force is pulling the ball downwards.

• The normal reaction force from the ground is less than the gravitational force.

• Pushing air out of the way uses up some of the ball's energy.

• Pushing air out of the way transfers some energy to the air particles.

Answer explanation

When the ball falls, it pushes air particles out of the way, which makes them move faster in that direction. The particles push back with an equal-sized force, acting in the opposite direction to the ball's motion, causing it to slow down.

The law of conservation of energy states that energy cannot be created or ___.

Answer explanation

Energy cannot be created or destroyed, but it can be transferred when objects (or systems) interact with each other.

Match the values on the left to the equivalent values on the right.

• 0.34 kJ

• 3.4 kJ

• 15 W

• 0.015 kW

• 1.5 kW

• 340 J

• 3400 J

• 1500 W

Answer explanation

You have correctly paired up the equivalent values.

Which of the following statements explains why an electric heater can be considered to be 100% efficient?

• An electric heater transfers no energy to the surroundings.

• An electric heater transfers all energy usefully into thermal stores.

• An electric heater has a low power input.

• An electric heater has a high power output.

Answer explanation

Efficiency is a measure of how well an appliance transfers the input energy/power into useful output energy/power. In the case of the heater, if all the energy supplied is transferred usefully into a thermal store, it is 100% efficient.