Energy changes in a system, and the ways energy is stored before and after such changes

邊玩邊學

回答這些題目賺取能量，接著就能釣魚、探索。不需要帳號。

給教育者：為 Energy changes in a system, and the ways energy is stored before and after such changes（Science、Physics）準備好可直接使用的課程投影片, 複習筆記——用於你的課程，或把這個主題當成互動班級活動，讓學習者以即時遊戲的方式進行。

課程筆記

Energy Stores

Energy is stored in objects in different energy stores.
Kinetic store: moving objects have energy in their kinetic store.
Gravitational potential store: objects gain energy here when lifted through a gravitational field.
Elastic potential store: stretched, squashed or bent objects have energy here.
Thermal store: all objects have energy here; the hotter the object, the more energy it has.
Other stores include chemical, nuclear, magnetic and electrostatic.

Eight energy stores, each shown with one everyday example.

Energy Transfer Pathways

Energy is transferred between stores by different energy transfer pathways.
The four pathways are mechanical working, electrical working, heating and radiation.
Mechanical working: a force acts on an object (e.g. pulling, pushing, stretching, squashing).
Electrical working: a charge moves through a potential difference (e.g. current).
Heating: energy is transferred from a hotter object to a colder one (e.g. conduction).
Radiation: energy is transferred by electromagnetic waves (e.g. visible light).

Energy-transfer pathways and pendulum motion

Systems and Conservation of Energy

A system is an object or a group of objects.
When there is a change in a system, energy is transferred.
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another.
In a closed system, the total amount of energy remains constant.
Total energy in = useful energy out + wasted energy.
Energy is never 'lost'; it can be dissipated (spread out) to the surroundings, often by heating and radiation.

Conservation of energy: a bat hitting a ball

Energy Transfer Diagrams

Energy flow diagrams show the stores and the transfer pathways taking place within a system.
Sankey diagrams represent energy transfers using arrows whose widths are proportional to the amount of energy transferred.
The flat left-hand end of the arrow represents the energy transferred into the system.
The straight arrow pointing right represents the useful energy output.
Arrows that bend away represent the wasted energy.
A more efficient device has less wasted energy, shown by a smaller arrow bending away.

Changes in Energy

Energy can be transferred by heating, by mechanical work done by forces, or by electrical work done when a current flows.
Heating increases the energy in the kinetic store of the particles, which increases the thermal store of the object; this can raise temperature or cause a change of state.
Mechanical work is done when a force acts over a distance, e.g. pushing a box across the floor.
When a current flows, energy is transferred electrically from the power supply to the components in the circuit.
Changes in speed are related to kinetic energy; changes in height are related to gravitational potential energy; changes in shape are related to elastic potential energy.

Energy is transferred to an object's gravitational store as it is lifted through a height: ΔEp = mgΔh.

Work Done and Energy

Work is done when an object is moved over a distance by a force applied in the direction of its displacement.
If a force is applied but there is no movement, no work is done.
Work done = energy transferred.
The equation is E = F × d, where E is work done or energy transferred in joules (J), F is force in newtons (N), and d is distance in metres (m).
Example: a car's brakes apply a force of 500 N to stop the car over 23 m; work done = 500 × 23 = 11 500 J.

Work is done when a force moves an object over a distance: W = Fd

Gravitational Potential Energy and Kinetic Energy

Gravitational potential energy is the energy an object has due to its height in a gravitational field.
The change in gravitational potential energy is calculated using ΔGPE = m × g × Δh, where m is mass in kg, g is gravitational field strength in N/kg, and Δh is change in vertical height in m.
Kinetic energy is the energy an object has as a result of its mass and speed.
Kinetic energy is calculated using KE = ½ × m × v², where m is mass in kg and v is speed in m/s.
In a perfect energy transfer (no wasted energy), ΔGPE = KE; this is useful for finding speed or height when resistance is ignored.

A boy holding a brick out at arm's length has given it potential energy; the brick on the ground has none.

Dissipation of Energy

Wasted energy transfers are inevitable; there is no such thing as a perfect energy transfer.
Most wasted energy transfers result in heating of the objects and the surroundings.
Energy that is spread out to the thermal store of the surroundings is said to be dissipated.
Work done against air resistance, friction, and resistance in wires all result in heating.
Once energy is in the thermal store of the surroundings, it cannot be gathered for any specific use, so it is called wasted energy.
Friction is a major cause of wasted energy in machines; it can be reduced by lubrication.

Reducing Unwanted Energy Transfers

Unwanted energy transfers include keeping a house warm, keeping a hot drink hot or cold, and friction of mechanical parts.
Insulation reduces energy transfers by conduction.
The effectiveness of an insulator depends on its thermal conductivity (lower is better), its density (lower is better), and its thickness (thicker is better).
In a denser material, particles are closer together so they transfer energy more easily.
Loft insulation is often made from fibreglass; the 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 through the walls.

Power and Efficiency

Power is the rate of energy transfer, or the rate of work done.
Power is calculated using P = E / t, where P is power in watts (W), E is energy transferred or work done in joules (J), and t is time in seconds (s).
1 watt = 1 joule per second (1 W = 1 J/s); 1 kilowatt (kW) = 1000 W.
Efficiency is the ratio of useful energy output to total energy output.
Efficiency = useful energy transferred by the device / total energy supplied to the device, or useful power output / total power input.
Efficiency can be a decimal (0 to 1) or a percentage (0 to 100%); it has no units.
A system with high efficiency transfers most of its energy usefully; a system with low efficiency wastes most of it.

投影片

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

免費註冊登入

練習題

免費預覽——64 題中的 8 題。註冊即可查看全部。

1.Which energy store is associated with the position of an object in a gravitational field?
Easy
- AGravitational potential store
- BKinetic store
- CThermal store
- DElastic potential store
2.Energy can be created and destroyed.
Easy
True or false?
3.A ball is thrown upwards. Which energy transfer takes place as the ball rises?
Medium
- AKinetic store → gravitational potential store
- BGravitational potential store → kinetic store
- CKinetic store → thermal store
- DGravitational potential store → elastic potential store
4.A car of mass 1200 kg is travelling at 15 m/s. Calculate its kinetic energy.
Medium
- A135 000 J
- B270 000 J
- C18 000 J
- D9 000 J
5.Which of the following are energy transfer pathways? (Select all that apply)
Medium
- AMechanical working
- BElectrical working
- CHeating by radiation
- DKinetic store
- EGravitational potential store
6.Put the energy transfers in order for a ball being dropped from a height and bouncing back up (ignoring air resistance).
Medium
- Gravitational potential store at the top
- Kinetic store as it falls
- Elastic potential store as it deforms on impact
- Kinetic store as it bounces back up
- Gravitational potential store at the top of the bounce
7.Match each energy store with its correct description.
Medium
- Kinetic store
- Gravitational potential store
- Elastic potential store
- Thermal store
- Energy of a moving object
- Energy due to height in a gravitational field
- Energy of a stretched or squashed object
- Energy related to the temperature of an object
8.Which of the following statements about energy stores are correct? (Select all that apply)
Medium
- AA moving object has energy in its kinetic store
- BA stretched spring has energy in its elastic potential store
- CA hot object has more energy in its thermal store than a cold object
- DEnergy can be created when a force acts
- EMagnetic materials interacting have energy in a magnetic store

字卡

What is a system in physics?

A system is an object or a group of objects that you choose to focus on. Defining the system sets the starting point and stopping point for the energy transfers you consider.

State the principle of conservation of energy.

Energy cannot be created or destroyed, it can only be transferred from one store to another. In a closed system the total amount of energy remains constant.

Name the eight main energy stores.

Kinetic, gravitational potential, elastic potential, magnetic, electrostatic, chemical, nuclear and thermal.

What is meant by the kinetic energy store of an object?

It is the energy an object has because of its mass and its speed. Any moving object has energy in its kinetic store.

When does an object gain energy in its gravitational potential store?

When it is lifted through a gravitational field, i.e. raised to a greater height. Energy is transferred away from this store when the object falls.

What does the elastic potential store describe?

The energy stored in an object that has been stretched, squashed or bent. It depends on the spring constant and the extension.

Name the four energy transfer pathways.

Mechanical working, electrical working, heating (by particles) and heating by radiation.

Describe energy transfer by mechanical working.

A force acts on an object and moves it over a distance, for example pulling, pushing, stretching or squashing. The force does work on the object.

Describe energy transfer by electrical working.

A charge moves through a potential difference, for example a current flowing from a power supply through the components of a circuit.

Describe energy transfer by heating.

Energy is transferred from a hotter object to a colder one, for example by conduction. This raises the energy in the kinetic store of the particles, increasing the object's thermal store.

How is energy transferred by radiation?

Energy is carried by electromagnetic waves, such as visible light or infrared. No particles are needed for this transfer.

What is the equation for work done by a force?

E = F × d, where E is work done or energy transferred in joules (J), F is force in newtons (N) and d is distance in metres (m).

What does 'work done' mean?

Work is done when a force moves an object over a distance in the direction of the force. Work done equals the energy transferred.

What happens to the work done if a force is applied but the object does not move?

No work is done, because there is no movement in the direction of the force. No energy is transferred by that force.

Give the equation for gravitational potential energy.

ΔGPE = m × g × Δh, where m is mass in kg, g is gravitational field strength in N/kg and Δh is the change in vertical height in metres.

Give the equation for kinetic energy.

KE = ½ × m × v², where m is mass in kg and v is speed in m/s. Remember to square the speed and include the factor of one half.

Give the equation for elastic potential energy.

Ee = ½ × k × e², where k is the spring constant in N/m and e is the extension in metres. This assumes the limit of proportionality has not been exceeded.

What is the equation linking power, energy transferred and time?

P = E ÷ t, where P is power in watts (W), E is energy transferred or work done in joules (J) and t is time in seconds (s).

Define power.

Power is the rate at which energy is transferred, or the rate at which work is done. One joule transferred per second equals one watt.

What is the unit of power and what does it mean?

The watt (W). 1 W = 1 J/s, so a device with a power of 1 W transfers 1 joule of energy every second.

Two motors lift the same weight through the same height, but one does it faster. Which has more power?

The motor that lifts the weight faster has more power. Both transfer the same amount of energy, but the faster motor transfers it in less time.

What is efficiency?

Efficiency is the ratio of the useful energy output from a system to the total energy supplied to it. It can be written as a decimal between 0 and 1, or as a percentage.

Give the two equations for efficiency.

efficiency = useful energy transferred ÷ total energy supplied, and efficiency = useful power output ÷ total power input. Efficiency has no units.

What does it mean if a system has low efficiency?

Most of the energy transferred is wasted rather than useful. A high-efficiency system transfers most of its energy usefully.

What is meant by dissipated energy?

Energy that is spread out to the thermal store of the surroundings, often by heating or radiation. It cannot easily be gathered for a useful purpose, so it is described as wasted energy.

Why is energy never described as 'lost'?

Energy is conserved, so it is never destroyed. It is transferred to the surroundings, often by heating, where it is dissipated and no longer useful.

What causes wasted energy transfers in machines?

Friction. It transfers energy from the kinetic store of the moving parts to the thermal store of the objects and surroundings by heating, so more work is needed to keep the machine moving.

How can friction in machine parts be reduced?

By lubricating the parts that rub together. This reduces the wasted energy transferred to the thermal store of the surroundings.

What is a Sankey diagram used for?

To represent energy transfers, with the width of each arrow proportional to the amount of energy transferred. The straight arrow shows useful output and the bent arrows show wasted energy.

State the energy balance shown by a Sankey diagram.

Total energy in = useful energy out + wasted energy. This follows from the conservation of energy.

Describe the energy transfers when a bat hits a ball.

Energy is transferred from the kinetic store of the bat to the kinetic store of the ball. Some is transferred to the thermal store of the bat, ball and surroundings by heating, and is dissipated.

Describe the energy transfers when an electric kettle boils water.

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

Describe the energy transfers when a moving object hits an obstacle.

Energy is transferred from the kinetic store of the moving object to the kinetic store of the obstacle, and some to the thermal store of both objects and the surroundings by heating and sound.

Describe the energy changes for an object projected upwards.

Energy is transferred mechanically from the kinetic store of the object to its gravitational potential store as it rises. On the way down, energy is transferred back from the gravitational potential store to the kinetic store.

Describe the energy transfers when a vehicle slows down.

Energy is transferred from the kinetic store of the vehicle to the thermal store of the brakes and surroundings by heating due to friction. The brakes do work against the motion.

What is the equation for the change in thermal energy of a substance?

ΔE = m × c × Δθ, where m is mass in kg, c is specific heat capacity in J/kg °C and Δθ is the temperature change in °C.

Define specific heat capacity.

The amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius. Its unit is J/kg °C.

What does the symbol Δ mean in equations such as ΔGPE = m × g × Δh?

Δ is the Greek letter delta and stands for 'change in'. So Δh is the change in vertical height.

When can energy values be equated in a calculation?

When the energy transfer is assumed to be perfect, for example when a question says to ignore air resistance or friction. Then the energy lost from one store equals the energy gained by another.

How do you find the speed of an object from its kinetic energy?

Rearrange KE = ½ × m × v² to give v = √(2 × KE ÷ m). Substitute the kinetic energy in joules and the mass in kilograms.

Why does a ball bounce back to a lower height than it was dropped from?

Some energy is transferred to the thermal store of the ball, floor and surroundings by heating and sound on impact. This energy is dissipated, so less is available to transfer back to the gravitational potential store.

What is the difference between energy and power?

Energy is the total amount transferred, measured in joules. Power is the rate at which energy is transferred, measured in watts, so power = energy ÷ time.

How does the gravitational potential energy of an object change with height?

It increases linearly with height, because ΔGPE = m × g × Δh. A graph of GPE against height is a straight line through the origin.

Unlock all 64 questions, flashcards & more

建立免費帳號，即可查看這個主題的所有題目、投影片、字卡與複習筆記。

免費註冊登入

歷屆試題

這個主題的歷屆試題練習即將推出。

即將推出