Topic 3 - Conservation of energy COPY

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 and 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. [Vacuum reduces the amount of energy transferred by conduction.]

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, cheap to extract
Transport, heating, electricity generation (uses)
High energy - and still plenty of fuel to meet current demand, so can respond quickly to changes in demand [but slowly running out]
Releases CO2 (causes global warming) - adds to greenhouse effect.
Burning coal and oil can also release sulfur dioxide, which causes acid rain.

Question 20

Describe the energy source - nuclear. (5)

Answer 20

Nuclear energy store
Non-renewable
Electricity generation (uses)
Very high power and clean
Radioactive waste (needs to be disposed of safely and decommission)
Could lead to disasters like Chernobyl [nuclear waste is highly dangerous]

Question 21

Describe the energy source - biofuel. (4)

Answer 21

Chemical
Renewable
Transport, heating, electricity generation [uses]
Medium power
Carbon-neutral so low impact [if you’re growing plants at the rate you’re burning things.]
In some regions large areas have to be cleared to make room to grow bio-fuels resulting in loss of species, natural habitats.
Decay of burning cleared vegetation increases methane and CO2 emissions.

Question 22

Describe the energy source - wind. (5)

Answer 22

Kinetic energy store
Renewable
Electricity generation - wind rotates the blades which turn the generator and produce electricity.
Very low power - initial costs high but running costs minimal.
Take up large areas that could be used for farming, people say windmills spoil their view.
Only works when its windy, so can’t always supply electricity, or respond to high demand.

Question 23

Describe the energy source - hydroelectricity. (5)

Answer 23

Gravitational potential energy store - involves flooding a valley by building a big dam - rainwater is caught and allowed through turbines [no pollution]
Renewable
Electricity generation - Can immediately respond to increased electricity demand - more water can be let out through the turbines to generate more electricity.
Medium power - initial costs high but minimal running costs [generally a reliable resource]
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 - big dams built across river estuaries with turbines in them.
Renewable - as tides come in it fills up the estuary - water is then let out through turbines at a controlled speed to generate electricity.
Electricity generation - there is no pollution but affect boat access, can spoil the view and alter habitats for wildlife, eg. wading birds.
Potentially very high power but hard to harness
Tidal barrages can block sewage which needs to go out to sea.
They are pretty reliable but heights of the tide is variable and barrages don’t work when water level is the same on each side.
Moderately high initial costs,but no fuel costs and running costs.

Question 26

Describe the energy source - solar cells. (5)

Answer 26

Thermal energy store - solar cells are made from materials that use energy transferred by light to create an electric current.
Renewable - often used in remote places where there’s not much choice [eg. Australian outback] and to power road signs and satellites.
Electricity generation - no pollution [although lots of energy to make]
Potentially very high power, but hard to harness - used to generate electricity on a small scale eg, homes.
Very little impact - solar power is most suitable for sunny countries[although can be used in cloudy countries like Britain]
You can’t make solar power at night or increase production when there’s extra demand.

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.

Question 29

What are the different types of energy stores?

Answer 29

• Kinetic
• Thermal
• Chemical
• Gravitational potential
• Elastic potential
• Electrostatic
• Magnetic
• Nuclear

Question 30

What are the four main ways energy can be transferred between stores?

Answer 30

• Mechanically [a force acting on an object eg. pushing]
• Electrically [a charge doing work against resistance [eg. charges moving round a circuit]
• By heating [energy transferred from a hotter object to a colder object eg. heating a pan on a hob]
• By radiation [energy transferred by waves eg. energy from the sun reaching Earth by light].