Energy - Forces Doing Work

8.1 What happens to energy stores when a system changes?

Energy is transferred between energy stores when a system changes. The total amount of energy is conserved.

8.2 What should energy transfer diagrams show?

They should show the energy stores involved and the pathways through which energy is transferred between them.

8.3 What happens to the total energy in a closed system?

There is no net change to the total energy in a closed system because energy cannot be created or destroyed.

8.4 How can the energy of a system be changed?

Energy can be transferred by work done by forces, electrically in electrical equipment, or by heating.

8.5 What is work done?

Work done is the energy transferred when a force causes an object to move through a distance in the direction of the force.

8.5 What is the relationship between work done and energy transferred?

Energy transferred is equal to the work done. Both are measured in joules (J).

8.6 What is the equation for work done?

E = F × d

8.6 What do the symbols in E = F × d represent?

E = energy transferred/work done (J), F = force (N), d = distance moved in the direction of the force (m).

8.6 What are the units in the work done equation?

Energy/work done: joules (J); force: newtons (N); distance: metres (m).

8.7 How does work done by forces change the energy of a system?

Work done transfers energy between stores. For example, work done to lift an object transfers energy to its gravitational potential energy store.

8.8 What is the equation for change in gravitational potential energy?

ΔGPE = m × g × Δh

8.8 What does m represent in ΔGPE = m × g × Δh?

Mass, measured in kilograms (kg).

8.8 What does g represent in ΔGPE = m × g × Δh?

Gravitational field strength, measured in newtons per kilogram (N/kg).

8.8 What does Δh represent in ΔGPE = m × g × Δh?

Change in vertical height, measured in metres (m).

8.8 What is the unit of gravitational potential energy?

Joules (J).

8.9 What is the equation for kinetic energy?

KE = ½ × m × v²

8.9 What does m represent in KE = ½ × m × v²?

Mass, measured in kilograms (kg).

8.9 What does v represent in KE = ½ × m × v²?

Speed, measured in metres per second (m/s).

8.9 What is the unit of kinetic energy?

Joules (J).

8.10 What is energy dissipation?

Energy dissipation is the transfer of energy to less useful energy stores, usually thermal energy stores in the surroundings.

8.10 Give an example of energy dissipation.

When friction acts on a moving object, some of its kinetic energy is transferred to thermal energy stores of the object and surroundings.

8.10 Why is dissipated energy described as less useful?

It is transferred to stores such as the thermal energy of the surroundings, where it is difficult to use for useful purposes.

8.11 Why do mechanical processes become wasteful?

Mechanical processes become wasteful when they cause a rise in temperature, dissipating energy as heating to the surroundings.

8.11 How does friction cause unwanted energy transfer?

Friction transfers some useful mechanical energy into thermal energy, heating the object and its surroundings.

8.12 What is power?

Power is the rate at which energy is transferred.

8.12 What does a high power mean?

A high power means energy is transferred quickly.

8.12 What does a low power mean?

A low power means energy is transferred more slowly.

8.13 What is the equation for power?

P = E ÷ t

8.13 What does P represent in P = E ÷ t?

Power, measured in watts (W).

8.13 What does E represent in P = E ÷ t?

Energy transferred or work done, measured in joules (J).

8.13 What does t represent in P = E ÷ t?

Time taken, measured in seconds (s).

8.14 What is one watt equal to?

One watt is equal to one joule per second: 1 W = 1 J/s.

8.15 What is the equation for efficiency?

Efficiency = useful energy transferred ÷ total energy supplied.

8.15 How do you calculate percentage efficiency?

Percentage efficiency = (useful energy transferred ÷ total energy supplied) × 100.

8.15 What does an efficiency of 1 mean?

It means 100% of the input energy is transferred usefully and none is dissipated.

8.15 Can efficiency be greater than 1?

No. Efficiency cannot be greater than 1, or 100%.

What happens to energy when an object is raised?

Work is done against gravity, transferring energy to the gravitational potential energy store of the object.

What happens to the energy store of an object when it accelerates?

Work is done by a force, transferring energy to the object's kinetic energy store.

What happens to kinetic energy when a moving object slows down?

Kinetic energy decreases and is transferred to other energy stores, often thermal energy stores due to friction or air resistance.

What happens to gravitational potential energy when an object falls?

Gravitational potential energy decreases and is transferred mainly to kinetic energy, with some energy potentially dissipated by air resistance.

How can unwanted energy transfers be reduced?

Unwanted energy transfers can be reduced by methods such as lubrication to reduce friction and thermal insulation to reduce heating.

What happens when a force moves an object in the direction of the force?

Energy is transferred by mechanical work.

What happens if a force acts but the object does not move?

No work is done because there is no distance moved in the direction of the force.

What happens if an object moves perpendicular to a force?

That force does no work because there is no movement in the direction of the force.

What practical can be used to investigate power?

Moving up stairs, doing step-ups onto a low platform, or lifting objects of different weights.

How can power be investigated by climbing stairs?

Measure the person's mass and the vertical height climbed to calculate the energy transferred using ΔGPE = m × g × Δh, then measure the time taken and calculate power using P = E ÷ t.

What measurements are needed when investigating power by climbing stairs?

Mass, vertical height climbed and time taken.

How can work done be calculated from force and distance?

Multiply the force by the distance moved in the direction of the force: E = F × d.

How can gravitational potential energy change be calculated?

Use ΔGPE = m × g × Δh.

How can kinetic energy be calculated?

Use KE = ½ × m × v².

How can power be calculated?

Divide the energy transferred or work done by the time taken: P = E ÷ t.

How can efficiency be calculated?

Divide useful energy transferred by total energy supplied.

What happens to efficiency when more energy is dissipated?

Efficiency decreases because a smaller proportion of the input energy is transferred usefully.

What happens to efficiency when less energy is dissipated?

Efficiency increases because a greater proportion of the input energy is transferred usefully.

What does the area under an energy transfer diagram represent?

The amount of energy transferred to each energy store/pathway.

What mathematical skills are required for Topic 8?

Calculating energy changes, selecting and using equations, converting between newton-metres and joules, using ratios and proportional reasoning, and calculating rates such as power.

What is the relationship between a newton-metre and a joule?

1 newton-metre (N m) = 1 joule (J).