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Energy transfers for OCR GCSE Physics

Energy transfers

This page covers the following topics:

1. Work
2. Energy changes
3. Conservation of energy

Work is said to be done whenever a force causes an object to move through a distance. Work done is calculated by multiplying the force exerted by the distance moved by the object. Since work done describes energy transfer, its unit is the Joule (J).

Work

When the state of a system changes, the energy it possessed initially is transferred to other types of energy, thus energy changes occur. In systems where there are frictional forces, work must be done against these frictional forces. This causes the temperature of the system to rise, and thus there is wasted energy as it is transferred to heat energy.

Energy changes

The principle of conservation of energy states that in a closed system, the total energy before an energy change is equal to the total energy after. In other words, the total energy in a closed system is constant. This means that energy cannot be destroyed nor created.

Conservation of energy

1

Serkan is pushing a box horizontally for 25 m across rough ground. Given that the frictional force exerted by the ground is 50 N, calculate the work done against friction by Serkan.

W = Fd
W = 25 m ร— 50 N = 1250 J

1250 J

Serkan is pushing a box horizontally for 25 m across rough ground. Given that the frictional force exerted by the ground is 50 N, calculate the work done against friction by Serkan.

2

A mechanical lift is being used to lift up an object of 4.5 kg. Given that the work done by the lift is 600 J and that there are no frictional forces, calculate the height through which the box is lifted.

By the principle of conservation of energy, the total energy remains constant.
GPE = W
mgh = Fd
4.5 kg ร— h ร— 9.8 = 600
h = 13.6 m (to 3 sf)

13.6 m

A mechanical lift is being used to lift up an object of 4.5 kg. Given that the work done by the lift is 600 J and that there are no frictional forces, calculate the height through which the box is lifted.

3

A light bulb is connected in a circuit to a battery. Describe the energy changes that occur in the circuit when it is closed.

When the circuit is closed, the chemical energy of the battery will be transferred into electrical energy, as the electrons will flow through the circuit, and light energy, as the bulb lights up. Some of the chemical energy will also be transferred into heat energy, as the temperature of the light bulb rises.

chemical energy โ†’ electrical energy โ†’ light energy + heat energy

A light bulb is connected in a circuit to a battery. Describe the energy changes that occur in the circuit when it is closed.

4

A vertical lift is lifting up a box of 700 g along a distance of 3 m. Calculate the work done by the lift to life the box up.

The force needed to lift the box will be equal to the weight of the box.

weight = mg
weight = 9.8 N/kg ร— 0.7 kg = 6.86 N

w = Fd
w = 6.86 N ร— 3 m = 20.58 J

21 J

A vertical lift is lifting up a box of 700 g along a distance of 3 m. Calculate the work done by the lift to life the box up.

5

A football is kicked across the ground. Explain the energy changes that occur until the ball becomes stationary again.

When the ball is kicked, work is done on the ball. This energy is transferred into kinetic energy, as the ball starts moving. This kinetic energy will slowly be transferred into heat energy, as work is being done against the frictional forces on the ball from the ground. When the ball becomes stationary, all of its kinetic energy will be transferred into heat energy.

work done โ†’ kinetic energy โ†’ heat energy

A football is kicked across the ground. Explain the energy changes that occur until the ball becomes stationary again.

End of page

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