P7.1 - Work Done

Question 1

What is energy?

Answer 1

Energy is a quantity in physics measured in Joules and provides information about what is possible to happen.

Question 2

How is energy stored?

Answer 2

• Energy stored is potential energy.
• Gravitational potential.
• Elastic potential.
• Potential energy could do work under certain conditions!

Question 3

What is the difference between fuel and energy store?

Answer 3

• Fuel is something we burn to produce motion or heat.

- Energy store describes an amount of Joules that we can calculate.

Question 4

How is energy transferred between stores?

Answer 4

• Mechanically using forces - Work done = Force x Distance
• Electrically using electric current - Energy Transferred = Electrical power x Time
• Heating by particles
• Heating by radiation

Question 5

What is the Law of Conservation of Energy?

Answer 5

Energy cannot be created or destroyed or destroyed. Energy can only be transferred between stores.

Question 6

What is a closed system in terms of energy?

Answer 6

A closed system does not exchange energy with the external environment. It has zero interaction with the external environment.

Question 7

Describe the difference between energy in a store and energy transfer?

Answer 7

Energy store is an amount of energy in Joules that we can calculate.

Energy transfer happens when energy changes from one store to another.

Question 8

In these three situations the energy in the store is decreasing.
Name the store in each situation:
1) A cup of tea cooling down
2) A woman on a bungee cord that is getting shorter
3) Wood burning on a fire

Answer 8

1) Thermal energy
2) Elastic potential energy
3) Chemical energy

Question 9

Suggest why the water is warmer at the bottom of a waterfall?

Answer 9

• The water at the top has gravitational potential energy.
• This gets transferred to kinetic energy.
• Due to the collision between the water and the rocks, heat is produced.
• This increases the temperature of the rock.
• Consequently the water gets warmed as well.

Question 10

How do you analyse a situation using energy?

Answer 10

• Choose two points in the process. (It is usually the initial and the final position.
• Identify which energy stores have more of less energy in them at those points.
• Work out which type of transfer has transferred the energy between the stores.

Question 11

Analyse the energy store in a car at the start and end of a race?

Answer 11

Point 1 - Start of the race
Stationary car, more fuel + oxygen
No energy in the kinetic store, more energy in the chemical store

TRANSFER
An engine exerts a constant force
Mechanically

Point 2 - End of the race
Moving car at the end of the track, less fuel + oxygen
Some energy in the kinetic store, less energy in the chemical store

Question 12

Analyse the energy change in stores for a braking car.

Answer 12

Point 1 - A moving car
A moving car and cool surroundings
More energy the the kinetic store than the thermal store

TRANSFER
The brakes exert a force over a distance
Mechanically
Heating by particles, heating by radiation

Point 2 - A stationary car
A stationary can with warmer surroundings
No energy in the kinetic store as it all gets transferred into the thermal store.

Question 13

Analyse the energy changes in stores for a gymnast in the air going on a springboard.

Answer 13

Point 1 - Gymnast in the air above springboard
Uncompressed spring, the gymnast running and jumping
No energy in the elastic store, some in the kinetic store, and some in the gravitational store

TRANSFER
Gymnast exerts a changing force over a distance
Mechanically

Point 2 - Gymnast momentarily stationary on compressed springboard
Compressed spring stationary gymnast at a lower height
Lots of energy in the elastic store, no energy in the kinetic store, some energy in the gravitational store

Question 14

Analyse the energy changes in stores for throwing a ball upwards.

Answer 14

Point 1 - As the ball leaves your hand
A moving tennis ball close to Earth
Some energy in its kinetic store and little energy in its gravitational store

TRANSFER
The Earth exerts a force over a distance
Mechanically

Point 2 - When the ball stops momentarily
A stationary tennis ball further from the Earth
No energy in its kinetic store and a low of energy in its gravitational store

Question 15

What do you also need to consider when analysing transfer between energy stores?

Answer 15

In reality the system is not closed, and some energy will end up in a thermal store because of air resistance.

Question 16

How can you increase energy in a thermal store?

Answer 16

You can increase the energy in a thermal store by heating such as by burning a fuel or using an electric current to transfer energy from a fuel.

Question 17

When will thermal energy transfer occu

Answer 17

Thermal energy transfer will always occur when there exists a temperature difference between two points/media.

Any temperature difference produces a transfer of energy, which continues until the objects are in equilibrium.

Question 18

Storage heaters in a house contain a large piece of concrete that heats up during the night. Analyse the energy transfers between stores for a storage heater

Answer 18

Point 1 - At the start of the day
A hot piece of concrete in a cold room
High energy in the thermal store of the concrete and low energy in the thermal store of the room

TRANSFER
The radiator gets hot, emits radiation, heats the air
Heating (Infrared, conduction, convection)

Point 2 - At the end of the day
A cool piece of concrete in a warm toom
The concrete has lower energy in its thermal store whereas the room now how high energy in its thermal store.

Question 19

What is dissipation (of energy)?

Answer 19

The transfer of energy to stores that are not useful (e.g. the thermal store of the surroundings), which cannot be used for working or heating.

Question 20

What is the effect of:

• Friction between parts of a car engine
• Conduction between water in a kettle and the plastic of the kettle
• Radiation from the front of a hot oven

Answer 20

1) The parts of the engine heat up, so energy ends up in the thermal store of the engine parts.
2) The plastic of the kettle gets hot, so energy ends up in the thermal store of the kettle.
3) The room that the over is in warms up, so energy ends up in the thermal store of the surroundings.

Question 21

How can you reduce the energy dissipated due to friction?

Answer 21

Lubrication - Using a lubricant such as oil to reduce energy dissipation by friction.

Oil - Placing a layer of fluid between two solid surfaces. The surfaces are no longer in direct contact, and friction is reduced.

Question 22

How can you reduce the energy dissipated due to heating?

Answer 22

Insulation - A material, such as foam, that reduces energy dissipation by heating

Insulation - Placing a material that does not conduct very well between a hot and cold object, so the rate of energy transfer is reduced.

Question 23

What are the three main ways heat energy is lost from a house and how could you minimise this heat loss?

Answer 23

Walls - Cavity wall insulation
Roof - Loft insulation
Windows - Double glazing

Question 24

How does fibreglass (glass wool) reduce heat loss?

Answer 24

Fibreglass traps air and stops it moving

Question 25

How does cavity wall insulation reduce heat loss?

Answer 25

Cavity wall insulation traps air and stops it moving

Question 26

How does a draught excluder reduce heat loss?

Answer 26

A draught excluder is put in places like underneath doors. It stops air from moving out.

Question 27

How does aluminium foil between a radiator panel and a wall reduce heat loss?

Answer 27

It reflects radiation back from the wall.

Question 28

How does double glazing a window reduce heat loss?

Answer 28

Double-glazed windows traps air and stops it moving.

Question 29

What does the rate at which heat energy is transferred through a wall depend on?

Answer 29

• The temperature difference between the two sides of the wall.
• The temperature gradient.
• The area of the walls.
• The thickness of the wall.
• The thermal conductivity of the material of the wall.

Question 30

How does temperature difference affect heat energy transfer through walls?

Answer 30

The greater the temperature difference between the two sides of the wall, the greater the rate of heat energy transfer.

Question 31

How does the area affect heat energy transfer through walls?

Answer 31

The greater the area of the walls, the greater the rate of heat energy transfer.

Question 32

How does thermal conductivity affect heat energy transfer through walls?

Answer 32

The lower the thermal conductivity of the wall (the better the insulator), the lower the rate of heat energy transfer.

Question 33

What is thermal conductivity?

Answer 33

• The heat energy transferred, per second, through a 1m thick wall, of area 1m2, when a temperature difference of 1 degrees Celsius exists across it.
• Units W/mK

Question 34

What does efficiency tell us and why are all appliances not ever 100% efficient?

Answer 34

Efficiency tells us how good the appliance is at doing its job, and is expressed as a ratio or percentage.

• No appliance is always 100% efficient because energy is always dissipated.
• More efficient devices are better at transferring energy between stores that do the job we want, rather than a job we do not want.

Question 35

What is the equation for the efficiency of an appliance?

Answer 35

Efficiency = Useful Output Energy Transfer / Input Energy Transfer

Question 36

What is a sankey diagram?

Answer 36

• A diagram to show the efficiency of an appliance.

- The width of the arrow shows the amount of energy transferred

Question 37

How do you increase efficiency?

Answer 37

If you want to increase efficiency you need to reduce the wasted energy.

More efficient devices also operate at a lower power, so they use up fuels more slowly.

You can:

1) Use insulation to reduce heating of the surroundings.
2) Make devices from materials that reduce unwanted energy transfer.
3) Use technology to produce devices that are better at their job like LEDs.

Question 38

What is conduction?

Answer 38

Conduction is the method of heat transfer in solids.

It is explained in terms of the particles, and their kinetic energy.

Question 39

Explain how conduction works.

Answer 39

Particles in a solid start to vibrate faster when heated.

These vibrating particles then collide with neighbouring particles transferring kinetic energy.

The neighbouring particles then vibrate faster and collide with further particles, transferring heat energy across the entire material.

Question 40

Why are metals the best conductors?

Answer 40

Metals are the best conductors because they contain delocalised electrons which are free to move throughout the structure.

The ions in a metal start to vibrate faster when heated.

Free electrons in a metal start to move faster when heated.

The vibrating ions then collide with neighbouring particles, transferring kinetic energy.

The moving electrons travel throughout the metal and collide with other electrons and ions, transferring kinetic energy faster than the ions vibrating alone.

Question 41

What is convection?

Answer 41

Convection is a method of heat energy transfer in fluids (liquid or gas).

Question 42

How does convection in gases work?

Answer 42

Air is heated up.

Air particles begin to move faster and further apart.

The hot air particles becomes less dense.

The hot air rises above the colder surrounding air.

The hot air cools as it rises and becomes more dense and falls again.

The cycle continues.

Question 43

How does convection in liquids work?

Answer 43

When water is heated it expands and becomes less dense and rises.

Cooler water moves into the bottom and is heated in turn and rises.

As the warm water rises it cools and becomes more dense and falls again.

The cycle continues.