Topic 3 - Conservation of energy

Question 1

What is a closed system?

Answer 1

A physical system that doesn’t allow certain types of transfers in or out of the system

Question 2

What equation is used to calculate the change in gravitational potential energy?

Answer 2

Change in GPE = Mass(kg) x Gravitational field strength(N/kg) x Change in vertical height(metres)

Question 3

What equation is used to calculate the amounts of energy associated with a moving object (kinetic)?

Answer 3

Kinetic energy(J) = 1/2 x Mass(kg) x Velocity squared(m/s sqaured)

Question 4

Describe Sankey diagrams.

Answer 4

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.

Question 5

Describe Energy Transfer diagrams

Answer 5

These diagrams ONLY show the useful transfers so they don’t show unnecessary ones such as heat from a bulb.

Question 6

Describe the conservation of energy

Answer 6

Energy is always conserved – the total amount of energy present stays the same before and after any changes.

Question 7

Analyse the changes involved in the way energy is stored when an object is projected upwards or up a slope
b a moving object hitting an obstacle
c an object being accelerated by a constant force
d a vehicle slowing down
e bringing water to a boiling an electric kettle

Answer 7

Kinetic energy is transferred to gravitational potential energy: the velocity of the object decreases as its height increases.

Question 8

Analyse the changes involved in the way energy is stored when a moving object hitting an obstacle.

Answer 8

Kinetic energy in the obstacle, heat energy & sound energy (which eventually is converted into heat energy) is created. If the object is immovable, then no kinetic energy is introduced into it.

Question 9

Analyse the changes involved in the way energy is stored when an object is accelerated by a constant force.

Answer 9

The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object.

Question 10

Analyse the changes involved in the way energy is stored when a vehicle slows down.

Answer 10

The source of energy providing the acceleration (usually chemical, heat or electrical) is transferred to kinetic energy in the object

Question 11

Analyse the changes involved in the way energy is stored when water is boiled in an electric kettle

Answer 11

Electrical energy input is converted to heat energy in the kettle. Some will be lost, not all the electrical energy is converted to heat energy within the water. The water will boil and the electrical energy will then be used in turning the water to steam, that is assuming that the kettle does not turn itself off then.

Question 12

What happens when there are energy transfers in a closed systems?

Answer 12

When there are energy transfers in a closed system, there is no net change in the total energy in the system.

Question 13

When and why do mechanical processes become wasteful?

Answer 13

Mechanical processes become wasteful when the useful energy transfers into unnecessary energy such as heat. As this energy is then lost to the surroundings so the process has less energy.

Question 14

Why is energy dissipated in all system changes?

Answer 14

Any energy that is not transferred to useful energy stores is said to be wasted because it is lost to the surroundings. Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air.

Question 15

Explain ways of reducing unwanted energy transfer including through lubrication, thermal insulation.

Answer 15

In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces - so the internal (thermal) energy of the surfaces increases. Adding lubricant between the surfaces reduces this friction and so less heat is wasted, like on a conveyor belt for example.

Also, if you want your coffee to remain hot it can be kept insulated in a flask so that heat isn’t lost to the surroundings.

Question 16

Describe the effects of the thickness and thermal conductivity of the walls of a building on its rate of cooling qualitatively

Answer 16

To reduce thermal energy transferred from a warm house, the walls can be built thicker, so the energy must travel further before it is transferred to the outside.

Thermal energy transfers can be reduced further if there are two walls with an air gap between them, as air has a lower thermal conductivity than brick. This is known as a ‘cavity wall’.

Question 17

How do you work out efficiency?

Answer 17

Efficiency = Useful energy transferred by the energy ÷ total energy supplied to the device.

Question 18

How can efficiency be increased?

Answer 18

For systems that are designed to transfer thermal energy, the efficiency can be improved by reducing the wasteful dissipation of thermal energy to the surroundings, for example by using insulation.

Question 19

Describe the energy source - fossil fuels. (5)

Answer 19

Chemical energy store
Non-renewable
Transport, heating, electricity generation (uses)
High power
Releases CO2 (causes global warming)

Question 20

Describe the energy source - nuclear. (5)

Answer 20

Nuclear energy store
Non-renewable
Electricity generation (uses)
Very high power
Radioactive waste (needs to be disposed of safely)

Question 21

Describe the energy source - biofuel. (4)

Answer 21

Chemical
Renewable
Transport, heating, electricity generation Medium power
Carbon-neutral so low impact

Question 22

Describe the energy source - wind. (5)

Answer 22

Kinetic energy store
Renewable 
Electricity generation
Very low power 
Take up large areas that could be used for farming, people say windmills spoil their view

Question 23

Describe the energy source - hydroelectricity. (5)

Answer 23

Gravitational potential energy store
Renewable
Electricity generation
Medium power 
Local habitats are affected by the large areas that need to be flooded to build dams

Question 24

Describe the energy source - geothermal. (5)

Answer 24

Internal energy store
Renewable 
Electricity generation, heating 
Medium power
Very low impact on the environment

Question 25

Describe the energy source - tides. (5)

Answer 25

Kinetic energy store
Renewable
Electricity generation
Potentially very high power but hard to harness
Tidal barrages can block sewage which needs to go out to sea

Question 26

Describe the energy source - sun. (5)

Answer 26

Nuclear energy store
Renewable
Electricity generation
Potentially very high, but hard to harness 
Very little impact

Question 27

Describe the energy source - water waves. (5)

Answer 27

Kinetic energy store
Renewable 
Electricity generation
Low power
Very low impact on environment

Question 28

Describe patterns and trends of energy sources. (5)

Answer 28

During the Industrial Revolution, advances in automation and transport caused a significant increase in the amount of fossil fuels extracted and burnt.

In the 20th century, electricity became a convenient way of distributing energy that can be used for a wide range of devices and applications such as lighting, heating, computing technologies and operating machinery.

Demand for energy varies with the time of year and the time of day. During early evenings a lot of energy is needed for heating, lighting and cooking but overnight there is very little needed while people sleep. During winter there is more heating and lighting required than in summertime.