Conservation and dissipation of energy

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Lektionsnotizen

Conservation of Energy

Energy can be transferred usefully, stored, or dissipated, but it can never be created or destroyed.
In a closed system, there is no net change to the total energy — energy transfers just move energy between stores.
Energy is transferred by heating and by radiation, and these transfers tend to spread energy out to the surroundings.
Never say energy is 'lost' — it implies energy is not conserved; instead say it is dissipated to the surroundings.
Energy can be transferred mechanically, electrically, by heating, or by radiation between stores.

Conservation of energy: a bat hitting a ball

Wasted Energy and Dissipation

Useful energy is an energy transfer that serves an intended purpose.
Wasted energy is an energy transfer that is not useful for the intended purpose and is dissipated to the surroundings.
Dissipation means energy spreads out to the surroundings, usually by heating or radiation.
Dissipated energy is very difficult to gather and use again, so it becomes less useful.
Mechanical processes become wasteful when they cause a rise in temperature, often through friction.
When friction acts, energy is transferred from the kinetic store by heating to the objects and the surroundings.
Whenever a process produces unwanted heating, light, or sound, energy is dissipated and essentially wasted.

Useful and Wasted Transfers: Examples

Bat hitting a ball: energy transfers usefully from the kinetic store of the bat to the kinetic store of the ball; some is dissipated by heating to the thermal store of the bat, ball, and surroundings.
Electric motor lifting a load: energy transfers usefully from the kinetic store of the motor to the gravitational potential store of the load.
In the motor, friction between components transfers energy from the kinetic store of the motor to its thermal store, dissipated to the surroundings.
Television: useful transfers are electrical energy dissipated as visible light and sound; wasted transfer is electrical energy to the thermal store, dissipated by heating.
Electric heater: useful transfer is electrical energy to the thermal store of the heating element, dissipated by heating; wasted transfer is dissipated by radiation as visible light.
The same energy transfer can be useful in one device and wasted in another — it depends on the intended purpose.

Conduction of Heat

Thermal conduction is the process where energy is transferred by vibrating particles in a substance.
Vibrating particles transfer energy from their kinetic store to the kinetic store of neighbouring particles.
Energy is always transferred by conduction from hot to cold.
The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it.
Conduction is the main method of energy transfer by heating in solids; metals are extremely good thermal conductors.
Non-metals are poor thermal conductors, and liquids and gases are extremely poor; poor conductors are called insulators.
Objects continue to lose heat until they reach thermal equilibrium (equal temperature) with their surroundings.

A household radiator: hot water inside heats the metal by conduction, and the metal then radiates thermal energy into the room.

Factors Affecting the Rate of Energy Transfer

The rate of energy transfer by conduction depends on the thickness of the material.
It also depends on the thermal conductivity of the material.
It depends on the temperature difference between two areas of the material, such as the internal and external surfaces of a wall.
The rate of energy transfer is reduced by increasing the thickness of the material.
It is reduced by decreasing the thermal conductivity of the material.
It is reduced by decreasing the temperature difference across the material.

Reducing Unwanted Energy Transfers

Lubrication reduces friction between parts that rub together, cutting wasted heating.
Insulation reduces energy transfers by conduction.
A good insulator has low thermal conductivity, low density, and greater thickness.
In a denser material, particles are closer together, so they transfer energy to one another more easily and conduction is greater.
Fibreglass loft insulation works well because air trapped between the fibres makes it a good insulator.
Cavity wall insulation fills the gap between external walls with foam, lowering conduction of heat from inside to outside.
Insulating a loft lowers the rate of cooling of a house, so less energy is transferred to the surroundings.

Efficiency

Efficiency is a measure of the amount of wasted energy in an energy transfer.
It is the ratio of the useful energy output from a system to its total energy output.
Equation: efficiency = useful energy transferred by the device ÷ total energy supplied to the device.
Efficiency can also be calculated using power: efficiency = useful power output ÷ total power input.
Efficiency can be given as a decimal (between 0 and 1) or as a percentage (between 0 and 100%).
Efficiency has no units (only % when expressed as a percentage).
High efficiency means most energy transferred is useful; low efficiency means most is wasted.
By conservation of energy: total energy transferred = useful energy transferred + wasted energy.

Improving Efficiency (Higher Tier)

Efficiency is improved by increasing useful energy transfers and reducing wasted energy transfers.
Machines waste energy due to friction between moving parts, air resistance, electrical resistance, and noise.
Friction can be reduced by adding bearings so components do not directly rub together, and by lubricating parts.
Electrical resistance causes heating in wires and components; it can be reduced by using lower-resistance components and reducing the current.
Air resistance opposes motion and causes heating; it can be reduced by streamlining shapes, such as a racing cyclist's posture, clothing, and bike design.
Noise is reduced by tightening loose parts to reduce vibration and by lubricating parts.
A more efficient device still wastes some energy — it does not waste zero energy.

Energy Resources

Energy resources are large stores of energy used to generate electricity and heat homes and businesses.
A renewable resource is replenished at a faster rate than it is used, so it will not run out.
Renewable resources include solar, wind, bio-fuel, hydroelectricity, geothermal, and tidal.
Non-renewable resources include fossil fuels (coal, oil, natural gas) and nuclear fuel.
A reliable resource can produce energy at any time; non-reliable resources only produce energy some of the time (e.g. wind).
The three main uses of energy resources are transport, electricity generation, and heating.

Energy from the Sun is the ultimate source of most of Earth's energy resources

Generating Electricity from Energy Resources

Electricity is generated in similar ways regardless of the resource: a turbine is turned, which turns a generator.
Fossil fuels: chemical store of fuel → thermal store of water → kinetic store of turbine → kinetic store of generator → transferred electrically to the National Grid.
Nuclear fuel: nuclear store of fuel → thermal store of water → kinetic store of turbine → kinetic store of generator.
Hydroelectric dam: gravitational potential store of water → kinetic store of water → kinetic store of turbine → kinetic store of generator.
Wind: kinetic store of moving wind → kinetic store of turbine → kinetic store of generator → transferred electrically to the National Grid.
Geothermal: cold water is pumped down to hot rocks and returns as steam to turn turbines.
The only thing that differs between resources is how the turbine is made to turn.

Wind turbines use the kinetic energy of moving air to generate electricity

Comparing and Using Energy Resources

Each energy resource has advantages and disadvantages for large-scale electricity production.
Most vehicles run on petroleum products (petrol, diesel, kerosene) from crude oil, a fossil fuel.
Electric vehicles produce zero carbon emissions while driven, but charging connects to the National Grid, which uses a mix of renewable and non-renewable sources.
Biofuel is renewable, but the claim that it is carbon-neutral is largely controversial.
The majority (84%) of the world's energy is still produced by non-renewable, carbon-emitting sources, harming the environment.
Most homes in cold countries use natural gas central heating, but gas is non-renewable; geologically active countries such as Iceland can use geothermal heating instead.
Generating energy reliably requires a range of different energy resources.

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Übungsfragen

Gratis-Vorschau — 8 von 63 Fragen. Registriere dich, um alle zu sehen.

1.Which statement best describes what happens to energy in a closed system?
Easy
- AEnergy can be transferred usefully, stored or dissipated, but the total energy never changes
- BEnergy is gradually destroyed as it is dissipated to the surroundings
- CEnergy can be created by friction between moving parts
- DEnergy is only conserved if no energy is wasted
2.Which row correctly defines useful energy and wasted energy?
Easy
- AUseful energy serves an intended purpose; wasted energy is not useful for the intended purpose and is dissipated to the surroundings
- BUseful energy is energy that is stored; wasted energy is energy that is destroyed
- CUseful energy is always thermal energy; wasted energy is always kinetic energy
- DUseful energy is energy transferred by heating; wasted energy is energy transferred electrically
3.Energy that is dissipated to the surroundings is destroyed.
Easy
True or false?
4.A device with an efficiency of 100% would waste no energy at all.
Easy
True or false?
5.A student calculates the efficiency of a motor as 1.5. Why must this be incorrect?
Medium
- AEfficiency cannot be greater than 1 (or 100%) because the useful output can never exceed the total input
- BEfficiency must always be given in joules
- CThe motor must have been switched off
- DEfficiency is always a whole number
6.Which of the following are ways of reducing unwanted energy transfers in a machine? (select all that apply)
Medium
- ALubricating the moving parts
- BAdding bearings so components do not rub directly together
- CStreamlining the shape of a moving object
- DIncreasing the current in the circuit
- ETightening loose parts to reduce vibration
7.Which of the following materials would transfer energy by conduction at the highest rate?
Medium
- AA metal with high thermal conductivity
- BA metal with low thermal conductivity
- CA gas with low thermal conductivity
- DA non-metal with low thermal conductivity
8.The thicker a material is, the better it insulates and the lower the rate of energy transfer through it.
Medium
True or false?

Mehr Fragen

9.The more dense an insulating material is, the better it insulates.
Medium
True or false?
10.Why does lubricating the moving parts of a machine reduce wasted energy?
Medium
- AIt reduces friction, so less energy is transferred by heating to the machinery and the surroundings
- BIt increases the thermal conductivity of the parts
- CIt makes the machine heavier so it moves more slowly
- DIt converts wasted thermal energy back into useful kinetic energy
11.A lamp is supplied with 60 J of energy. 16 J is transferred to the thermal store of the surroundings. What is the efficiency of the lamp?
Medium
- A0.73
- B1.27
- C0.27
- D73 J
12.A wind turbine receives 6.2 kJ of energy from the wind each second and delivers 2.2 kJ each second to the National Grid. What is the efficiency of the turbine?
Medium
- A0.35
- B0.65
- C2.8
- D35 J
13.Match each key term to its correct meaning.
Medium
- Useful energy
- Wasted energy
- Efficiency
- Thermal conductivity
- An energy transfer that serves an intended purpose
- An energy transfer that is not useful for the intended purpose and is dissipated to the surroundings
- The ratio of the useful energy output from a system to its total energy input
- A property of a material that determines the rate of energy transfer by conduction across it
14.Place the energy transfers in the correct order for generating electricity from fossil fuels.
Medium
- Chemical store of the fuel
- Thermal store of the water
- Kinetic store of the turbine
- Kinetic store of the generator
- Transferred electrically to the National Grid
15.Which of the following are renewable energy resources? (select all that apply)
Medium
- ASolar energy
- BWind
- CCoal
- DBio-fuel
- ENuclear fuel
16.Match each energy resource to its correct category.
Medium
- Solar
- Coal
- Wind
- Nuclear fuel
- Bio-fuel
- Renewable
- Non-renewable
17.Place the energy transfers in the correct order for a hydroelectric dam generating electricity.
Medium
- Gravitational potential store of water
- Kinetic store of water
- Kinetic store of turbine
- Kinetic store of generator
- Electrical transfer to National Grid
18.Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.
Easy
True or false?
19.Which of the following factors affect the rate of energy transfer by conduction through a material? (select all that apply)
Medium
- AThe thickness of the material
- BThe thermal conductivity of the material
- CThe temperature difference across the material
- DThe colour of the material
- EThe mass of the material
20.Match each energy transfer to whether it is useful or wasted in an electric heater.
Medium
- Energy transferred to the thermal store of the heating element
- Energy dissipated as visible light
- Energy dissipated by heating to the surroundings
- Useful
- Wasted
21.Place the steps in the correct order for calculating the efficiency of a device when the total energy input and wasted energy are known.
Medium
- Calculate useful energy output = total input - wasted energy
- Identify total energy input
- Use efficiency = useful output / total input
- Convert to percentage if needed
22.A machine is supplied with 500 J of energy and wastes 100 J. What is its efficiency as a decimal?
Medium
- A0.2
- B0.8
- C1.25
- D0.5
23.Which of the following are ways to increase the efficiency of a car? (select all that apply)
Hard
- AStreamlining the car's shape
- BIncreasing the car's mass
- CLubricating the engine parts
- DUsing tyres with higher friction
- EReducing air resistance
24.Match each method of reducing unwanted energy transfer to the type of energy transfer it reduces.
Medium
- Lubrication
- Streamlining
- Insulation
- Tightening loose parts
- Friction
- Air resistance
- Conduction
- Noise
25.Place the energy transfers in the correct order for generating electricity from nuclear fuel.
Medium
- Nuclear store of fuel
- Thermal store of water
- Kinetic store of turbine
- Kinetic store of generator
- Electrical transfer to National Grid
26.The higher the thermal conductivity of a material, the lower the rate of energy transfer by conduction across it.
Easy
True or false?
27.Which of the following is the correct equation for efficiency?
Medium
- Aefficiency = total energy supplied / useful energy transferred
- Befficiency = useful energy transferred / total energy supplied
- Cefficiency = useful energy transferred × total energy supplied
- Defficiency = total energy supplied - useful energy transferred
28.Which of the following statements about dissipated energy are correct? (select all that apply)
Medium
- ADissipated energy is spread out to the surroundings.
- BDissipated energy is destroyed.
- CDissipated energy becomes less useful.
- DDissipated energy can easily be gathered and reused.
- EDissipated energy is often wasted energy.
29.Match each energy resource to its description.
Medium
- Wind
- Coal
- Hydroelectricity
- Nuclear fuel
- Renewable but not reliable
- Non-renewable fossil fuel
- Renewable using flowing water
- Non-renewable but no carbon emissions
30.Place the steps in the correct order to calculate the percentage efficiency of a fan motor when 300 J is supplied and 85 J is wasted.
Hard
- Calculate useful energy = 300 - 85 = 215 J
- Identify total energy input = 300 J
- Calculate efficiency = 215/300 = 0.717
- Convert to percentage: 0.717 × 100 = 71.7%
31.A student investigates how the rate of cooling of a hot drink changes when the drink is placed in a sealed vacuum flask. The student records the temperature every minute. Which statement best explains why the temperature of the drink falls very slowly in the flask?
Medium
- AThe vacuum between the walls reduces energy transfer by conduction and convection.
- BThe vacuum between the walls absorbs the thermal energy from the drink.
- CThe vacuum between the walls increases the thermal conductivity of the walls.
- DThe vacuum between the walls reflects thermal energy back into the drink.
32.A table shows the energy transferred each second by four different lamps. Which lamp is the most efficient? Lamp A: 100 J input, 10 J useful output. Lamp B: 100 J input, 25 J useful output. Lamp C: 100 J input, 40 J useful output. Lamp D: 100 J input, 60 J useful output.
Medium
- ALamp A
- BLamp B
- CLamp C
- DLamp D
33.A student is investigating the effectiveness of different materials as thermal insulators. They wrap identical beakers of hot water in different materials and record the temperature every minute. Which of the following would improve the validity of the investigation? (select all that apply)
Hard
- AUsing the same starting temperature of water in each beaker.
- BUsing a different volume of water in each beaker.
- CKeeping the room temperature constant throughout the investigation.
- DUsing the same thickness of insulating material around each beaker.
- EUsing a different type of thermometer for each beaker.
34.A material with a higher thermal conductivity will transfer energy by conduction at a higher rate than a material with a lower thermal conductivity, provided all other factors are the same.
Easy
True or false?
35.Match each energy transfer in a television to whether it is useful or wasted.
Medium
- Energy transferred to the thermal store of the surroundings by heating
- Energy transferred to the surroundings by radiation as visible light
- Energy transferred to the surroundings by heating as sound waves
- Wasted energy
- Useful energy
36.Place the steps in the correct order to calculate the percentage efficiency of a device when the total energy input and the wasted energy are known.
Medium
- State the equation: efficiency = (useful energy output / total energy input) × 100%
- Write down the total energy input and the wasted energy.
- Calculate the useful energy output by subtracting the wasted energy from the total energy input.
- Substitute the values into the equation and calculate the efficiency as a percentage.
37.A group of students investigate the rate of cooling of a hot drink in four identical cups. Cup A has no lid. Cup B has a lid. Cup C has a lid and is wrapped in a thick layer of insulation. Cup D has a lid, is wrapped in insulation, and is placed on a wooden mat. The starting temperature of the drink is the same in each cup. Which cup would be expected to cool down the slowest?
Hard
- ACup A
- BCup B
- CCup C
- DCup D
38.A hydroelectric dam transfers energy from the gravitational potential store of water to its kinetic store to turn a turbine. A student claims that the total energy of the water in the dam system is conserved. Which statement best supports this claim?
Hard
- AThe gravitational potential energy of the water is converted entirely into kinetic energy of the turbine.
- BEnergy is transferred from the gravitational potential store of the water to the kinetic store of the water, and some is dissipated to the surroundings, so the total energy remains constant.
- CThe water loses gravitational potential energy, which is destroyed as it falls.
- DThe turbine creates new energy as it turns.
39.A car engine wastes energy through several mechanisms. Which of the following are ways to increase the efficiency of a car? (select all that apply)
Hard
- ALubricating the moving parts of the engine.
- BStreamlining the shape of the car to reduce air resistance.
- CIncreasing the mass of the car.
- DUsing tyres with higher rolling resistance.
- EReducing the electrical resistance in the car's circuits.
40.When energy is dissipated to the surroundings, it becomes less useful because it is spread out and difficult to gather and use again.
Medium
True or false?
41.A steam engine is supplied with 500 J of energy each second. It transfers 375 J usefully. Calculate the efficiency of the steam engine as a decimal.
Medium
42.A fan motor is supplied with 300 J of energy in one second. 85 J is wasted due to friction and sound. Calculate the percentage efficiency of the motor.
Medium
43.Complete the definition of a renewable energy resource.
Easy
A renewable energy resource is replenished at a faster ____ than the rate at which it is being ____.
44.Give the term for energy that is spread out to the surroundings and becomes less useful.
Easy
45.Name the process that transfers energy by vibrating particles in a substance, from hot to cold.
Medium
46.A motor is supplied with 250 J of energy and wastes 50 J. Calculate its efficiency as a percentage.
Medium
47.A light bulb is supplied with 100 J of energy and transfers 20 J usefully as light. Calculate the energy wasted as heat in joules.
Medium
48.Complete the sentence about conduction.
Easy
Conduction is the main method of energy transfer by heating in ____.
49.An electric heater takes 50 s to heat the water in a tank. The power supplied by the electric heater is 2.0 kW. Calculate the energy transferred to the water in 50 s. Use the equation P = E/t. Give your answer in kilojoules (kJ).
Medium
50.A model train has a mass of 8.0 kg and travels at a speed of 1.5 m/s. Calculate the kinetic energy of the model train. Use the equation: kinetic energy = 1/2 × mass × (speed)². Give your answer in joules (J).
Medium
51.Complete the sentence about the conservation of energy in a closed system.
Medium
In a closed system, the total energy before an energy transfer is ____ to the total energy after the transfer.
52.Name the process by which energy is transferred through a solid material by vibrating particles, from a hotter region to a cooler region.
Medium
53.A student measures the energy transferred to a motor and the energy output by the motor. The student says the motor has an efficiency of 1.5. Explain how another student knows this answer is incorrect.
Hard
54.A car uses energy from the chemical store in fossil fuels to move. Some energy is dissipated in the wheels of the car. Suggest a method to increase the efficiency of these moving parts.
Hard
55.A television is switched on. State the useful energy transfer and the main wasted energy transfer occurring in the television.
Hard
56.Explain how lubricating the moving parts of a machine reduces wasted energy.
Hard
57.Explain why the efficiency of a device can never be greater than 1 (or 100%).
Hard
58.A steam engine is supplied with 500 J of energy each second. It transfers 375 J usefully. Calculate the efficiency of the steam engine as a decimal.
Medium
- A-0.25
- B1.75
- C2.75
- D0.75
59.A fan motor is supplied with 300 J of energy in one second. 85 J is wasted due to friction and sound. Calculate the percentage efficiency of the motor.
Medium
- A65 %
- B86 %
- C72 %
- D79 %
60.A motor is supplied with 250 J of energy and wastes 50 J. Calculate its efficiency as a percentage.
Medium
- A96 %
- B88 %
- C80 %
- D72 %
61.A light bulb is supplied with 100 J of energy and transfers 20 J usefully as light. Calculate the energy wasted as heat in joules.
Medium
- A88 J
- B80 J
- C96 J
- D72 J
62.An electric heater takes 50 s to heat the water in a tank. The power supplied by the electric heater is 2.0 kW. Calculate the energy transferred to the water in 50 s. Use the equation P = E/t. Give your answer in kilojoules (kJ).
Medium
- A120 kJ
- B100 kJ
- C110 kJ
- D90 kJ
63.A model train has a mass of 8.0 kg and travels at a speed of 1.5 m/s. Calculate the kinetic energy of the model train. Use the equation: kinetic energy = 1/2 × mass × (speed)². Give your answer in joules (J).
Medium
- A11 J
- B10 J
- C9 J
- D8 J

Karteikarten

State the law of conservation of energy.

Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed.

What is meant by a closed system in terms of energy?

A closed system is one where no energy can enter or leave, so there is no net change to the total energy inside it.

What is dissipation of energy?

Dissipation is the spreading out of energy to the surroundings, often by heating or radiation, so that it becomes less useful.

Why is dissipated energy described as 'wasted'?

It is not useful for the intended purpose and is spread out to the surroundings, making it difficult to gather and use again.

Give an example of a useful energy transfer and a wasted energy transfer when a bat hits a ball.

Useful: energy is transferred from the kinetic store of the bat to the kinetic store of the ball. Wasted: energy is dissipated by heating to the thermal store of the bat, ball and surroundings.

In an electric motor lifting a load, what is the useful energy transfer?

Energy is transferred from the kinetic store of the motor to the gravitational potential store of the load.

In an electric motor lifting a load, what is the main wasted energy transfer?

Friction between components causes heating, so energy is transferred from the kinetic store of the motor to the thermal store of the motor and dissipated to the surroundings.

State the equation for efficiency in terms of energy.

efficiency = useful energy transferred by the device ÷ total energy supplied to the device

State the equation for efficiency in terms of power.

efficiency = useful power output ÷ total power input

A device transfers 300 J of energy and wastes 85 J. Calculate its efficiency as a percentage.

Useful energy = 300 − 85 = 215 J. Efficiency = (215 ÷ 300) × 100% = 72%.

Why can efficiency never be greater than 1 (or 100%)?

Because energy cannot be created, so the useful energy output can never be greater than the total energy input.

What does a high efficiency tell you about a device?

Most of the energy transferred is useful, so only a small proportion is wasted.

How can friction between moving parts be reduced?

By lubricating the parts that rub together and by adding bearings to prevent components from directly rubbing.

How can unwanted energy transfer by air resistance be reduced?

By streamlining the shapes of moving objects, for example a racing cyclist adopting a more streamlined posture.

How can unwanted energy transfer due to electrical resistance be reduced?

By using components with lower resistance and by reducing the current.

How can unwanted energy transfer by noise be reduced?

By tightening loose parts to reduce vibration and by lubricating parts.

What is thermal conduction?

Thermal conduction is the transfer of energy by vibrating particles in a substance, from hot to cold.

How does the thermal conductivity of a material affect the rate of energy transfer by conduction?

The higher the thermal conductivity, the higher the rate of energy transfer by conduction across the material.

What three factors affect the rate of energy transfer through a material?

The thickness of the material, its thermal conductivity, and the temperature difference between the two sides.

How does the thickness of a material affect the rate of energy transfer by conduction?

Increasing the thickness reduces the rate of energy transfer.

Why does a denser insulator allow more conduction?

In a denser material, particles are closer together, so they can transfer energy to one another more easily.

Define a renewable energy resource.

An energy source that is replenished at a faster rate than the rate at which it is being used, so it will not run out.

Name three renewable energy resources.

Any three from: solar, wind, biofuel, hydroelectricity, geothermal, tidal.

Name two non-renewable energy resources.

Any two from: coal, oil, natural gas (fossil fuels) and nuclear fuel.

What are the three main uses of energy resources?

Transport, electricity generation, and heating.

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