Topic 3 - Conservation of energy

Question 1

change in gravitational potential energy = ?

Answer 1

change in gravitational potential energy (J) = mass (kg) * gravitational field strength (N/kg) * change in vertical height (m)

∆GPE = mg∆h

Question 2

kinetic energy = ?

Answer 2

kinetic energy (J) = 1/2 * mass (kg) * (speed (m/s))^2

KE = 1/2mv^2

Question 3

What does a diagram showing energy transfers look like?

Answer 3

(tinyurl.com/energydiagram)

A box with the initial energy store described within it with arrows going towards the destinations of the energy transfers described within other boxes

Question 4

What is meant by conservation of energy?

Answer 4

Energy can be stored, transferred between stores, and dissipated - but it can never be created or destroyed. The total energy of a closed system has no net change.

Question 5

Describe the energy transfer that takes place when an object is projected upwards or up a slope

Answer 5

The object does work against the gravitational force, so energy is transferred mechanically from the kinetic energy store of the object to its gravitational potential energy store

Question 6

Describe the energy transfers that take place when a moving object hits an obstacle

Answer 6

Energy is transferred mechanically from the kinetic energy store of the object to the kinetic energy store of the obstacle, and some (also mechanically) to the thermal energy stores of the obstacle and the object

Question 7

Describe the energy transfer that takes place when an object is being accelerated by a constant force (i.e. gravity)

Answer 7

As the force (gravity) does work on the object, energy is transferred from the object’s gravitational potential energy store to the object’s kinetic energy store

Question 8

Describe the energy transfer that takes place when a vehicle is slowing down

Answer 8

Energy is transferred mechanically, and then by heating, from the kinetic energy store of the car to the thermal energy stores of the car and the road

Question 9

Describe the energy transfers that take place when a kettle brings water to a boil

Answer 9

Energy is transferred electrically from the mains to the heating element of the kettle, and then by heating to the thermal energy store of the water

Question 10

In what energy store is the energy within food held?

Answer 10

Chemical

Question 11

What type of energy attracts or repels charges?

Answer 11

Electrostatic

Question 12

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

Answer 12

1) Mechanically - a force acting on an object
2) Electrically - a charge doing work against resistance
3) By hearing - energy transferred from a hotter object to a colder one
4) By radiation - energy transferred by waves

Question 13

When there are energy transfers within a closed system, does the total energy in that system net increase or net decrease?

Answer 13

There is never a net change

Question 14

When is energy useful?

Answer 14

When it is transferred from one store to a useful store

Question 15

When do mechanical processes become wasteful?

Answer 15

When they cause a rise in temperature, dissipating energy in heating the surroundings

Question 16

Why in all system changes is energy dissipated so that it is stored in less useful ways?

Answer 16

Whenever work is done mechanically, frictional forces have to be overcome, including things like moving parts rubbing together, and air resistance. The energy needed to overcome these frictional forces is transferred to the thermal energy stores of whatever’s doing the work and the surroundings.

Question 17

How can unwanted energy transfer by friction be reduced?

Answer 17

Lubricants (usually liquids so they can flow easily between objects and coat them) can be used to reduce the friction between objects’ surfaces when they move

Question 18

How can unwanted energy transfer by heating be reduced?

Answer 18

Thermal insulation - in a building this means lowering the thermal conductivity of the walls. One method is using cavity walls which are made up of an inner and an outer wall with an air gap between them because air has a very low thermal conductivity. Thicker walls help too, lowering the rate of energy transfer and cooling.

Question 19

efficiency = ?

Answer 19

efficiency = (useful energy transferred by the device (J)) / (total energy supplied to the device (J))

Question 20

How can efficiency be increased?

Answer 20

By reducing unwanted energy transfer with methods such as insulation and lubrication

Question 21

What are non-renewable energy resources?

Answer 21

They are resources that cannot be renewed and will run out i.e. fossil fuels and nuclear fuel (uranium and plutonium) - they currently provide most of the world’s energy

Question 22

What are fossil fuels?

Answer 22

Natural resources that form underground over millions of years that are typically burnt to provide energy. The three main fossil fuels are coal, oil and natural gas.

Question 23

What are the pros of fossil fuels?

Answer 23

• Reliable - currently plenty of fuel to meet demand, always have fuel in stock, can respond quickly to changes in energy demand
• Cost to extract is low
• Fossil fuel plants are relatively cheap to build and run

Question 24

What are the cons of fossil fuels?

Answer 24

• Slowly running out
• Release carbon dioxide when burned which adds to greenhouse effect, and contributes to global warming
• Burning coal and oil can release sulphur dioxide which causes acid rain
• Oil spillages cause serious environmental problems, affecting animals that live in and around the sea

Question 25

What are the pros of nuclear fuel?

Answer 25

• Reliable - currently plenty of fuel to meet demand, always have fuel in stock, can respond quickly to changes in energy demand
• Nuclear power is clean and environmentally friendly

Question 26

What are the cons of nuclear fuel?

Answer 26

• Nuclear power plants are pretty costly to build and to safely decommission
• Nuclear waste is very dangerous and difficult to dispose of
• Risk of major catastrophe like the Fukushima disaster in Japan

Question 27

What are renewable energy resources? Give 5 examples

Answer 27

They are resources that will never run out and can be renewed as they are used. There is some concern as to how reliable some types of resources are.

Examples:

1) Bio-fuels
2) Wind
3) The Sun (solar)
4) Hydro-electricity
5) Tides

Question 28

What are biofuels?

Answer 28

Renewable energy resources created from either plant products or animal dung. They can be solid, liquid, or gas and can be burnt to produce electricity or run cars in the same way as fossil fuels.

Question 29

Are biofuels carbon neutral?

Answer 29

Supposedly, although there is some debate about this as it’s only really true if you keep growing plants (that absorb carbon dioxide as they grow) at the same rate that you’re burning things (which releases carbon dioxide)

Question 30

What are some issues with biofuels?

Answer 30

• The cost to refine them is very high
• Some worry that growing crops specifically for biofuels will mean there isn’t enough space or water to meet the demand for food crops
• In some regions, large areas of forest have been cleared to make room to grow biofuels, resulting in lots of species losing their natural habitats
• The decay or burning of this cleared vegetation also increases methane and carbon dioxide emissions

Question 31

How is wind power generated? How much pollution does it create?

Answer 31

Using wind turbines which each have a generator inside them. Wind rotates the blades which turn the generators and produce electricity - so there’s no pollution (although there can be pollution created when the turbines are made).

Question 32

Is generating wind power expensive?

Answer 32

Initial costs are high but running costs are minimal

Question 33

What are some issues with wind turbines?

Answer 33

• Lots of them are needed to produce as much power as, for example, a coal power plant - so they can spoil the view
• They only work when it’s windy so aren’t reliable - can’t always supply electricity, can’t respond to high demand

Question 34

How is solar power generated?

Answer 34

Using solar cells which are made from materials that use energy transferred by light to create an electric current

Question 35

Where is solar power often used?

Answer 35

In remote places where there’s not much choice (e.g. the Australian outback) and to power electric road signs and satellites

Question 36

Is generating solar power expensive?

Answer 36

Initial costs are high but there are basically no running costs

Question 37

In what countries and on what scale are solar cells most suitable for generating energy?

Answer 37

They’re mainly used to generate electricity on a relatively small scale (e.g. in homes) and solar power is most suitable for sunny countries but it can be used in cloudy countries like Britain although it is less reliable

Question 38

Why isn’t solar power reliable?

Answer 38

Its availability is dependent on natural sunlight which is out of human control - it cannot be produced at night or when it’s cloudy/dark, and production cannot be increased when there’s extra demand

Question 39

Along with not adding to the greenhouse effect, what’s another environmental benefit that can be gained from using solar power?

Answer 39

It causes basically no pollution (although solar cells do use quite a lot of energy to make)

Question 40

How is hydro-electricity produced?

Answer 40

Production usually involves a high reservoir (often created through building a large dam). Rainwater is caught and allowed out through turbines.