Energy stores and transfers

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Big idea: energy is moved between stores

Big idea (key concept): Systems. A system is the group of objects we choose to look at. Energy moves around inside a system and out to its surroundings, and we can track every step.
Related concept: Energy. Energy is not a substance you can see. It is a quantity we use to describe what can happen: things can speed up, rise, heat up or light up when energy is transferred to them.
Global context: Globalization and sustainability. Every country relies on energy resources, and wasting less energy helps people everywhere.
Energy is measured in joules (J). A bigger unit is the kilojoule (kJ): 1 kJ = 1000 J.
Scientists think of energy as being held in stores, and moved between them by transfers. Nothing is 'used up': the energy ends up somewhere else.

Energy stores

Kinetic store: energy of anything that is moving.
Gravitational potential store: energy of an object because of its height above the ground.
Elastic potential store: energy in something stretched or squashed, such as a spring or a rubber band.
Chemical store: energy in food, fuels and batteries, released in chemical reactions.
Thermal store: energy of the moving particles inside an object. A hotter object, or one with more mass, has a bigger thermal store.
Nuclear store: energy inside the centre (nucleus) of atoms. The Sun and nuclear power stations release it.
Magnetic and electrostatic stores: energy held between magnets, or between charged objects, that attract or repel.

Eight energy stores with an everyday example of each

How big is a store?

A kinetic store gets bigger when the object has more mass or moves faster. Speed matters most: doubling the speed makes the store 4 times bigger.
A gravitational potential store gets bigger when the object has more mass or is lifted higher.
To calculate the change in a gravitational store: change in energy (J) = mass (kg) x gravitational field strength (N/kg) x change in height (m).
On Earth we use g = 10 N/kg (it is really about 9.8 N/kg, but 10 is close enough here).
Worked example: a 2 kg bag is lifted 1.5 m. Energy = 2 kg x 10 N/kg x 1.5 m = 30 J.
An elastic store gets bigger the more you stretch or squash a spring. A thermal store gets bigger when you heat something up.

Lifting a mass through a height raises its gravitational store

Energy transfers

A transfer moves energy from one store to another. There are four pathways.
Mechanically: a force moves something, like a person lifting a box or a bat hitting a ball.
Electrically: an electric current carries energy, for example from a battery to a motor.
By heating: thermal energy moves from a hotter object to a cooler one, like a hot cup warming your hands.
By radiation: waves such as light carry energy across a gap, even empty space. Sound waves also carry energy by vibrations in the air.
To describe a transfer, name the store energy leaves, the store it arrives in and the pathway. Example: a bat hits a ball. Energy goes from the kinetic store of the bat to the kinetic store of the ball (mechanically), and some also goes to thermal stores.

Useful and wasted transfers when a bat hits a ball

Conservation, wasted energy and efficiency

Conservation of energy: energy is never created or destroyed. The total amount before a change equals the total after it.
In every real change some energy goes into stores we do not want, usually the thermal store of the surroundings. This is wasted energy. It spreads out and is hard to get back, so we say it is dissipated.
Reducing friction, for example by oiling a bicycle chain, wastes less energy.
Efficiency compares useful energy with the total: efficiency = useful output energy / total input energy x 100%.
Worked example: a lamp takes in 20 J and 16 J becomes light, so efficiency = 16 / 20 x 100% = 80%. The other 4 J is wasted.
Efficiency can never be more than 100%. A Sankey diagram shows the input energy splitting into useful and wasted output.
Energy resources: fossil fuels (coal, oil, gas) are chemical stores that are non-renewable. Wind, solar and hydroelectric power are renewable, and burning less fuel also helps the environment.

A Sankey diagram: input split into useful and wasted energy

Think like a scientist: how much energy does a bouncing ball keep?

A student drops a ball from different heights and measures the bounce height. This tests whether more energy at the start means a higher bounce.
The independent variable (what you change) is the drop height. The dependent variable (what you measure) is the bounce height.
Control variables (keep the same): the same ball, the same floor, the same way of reading the height.
Repeat each drop at least 3 times and calculate the mean. A single reading can be a mistake.
Inquiry task: use your results to calculate bounce height / drop height x 100% for each drop. Does the ball keep the same fraction each time? Where does the rest of the energy go, and how could you improve the method?

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Soal latihan

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1.Which of these is an energy store?
Easy
- AA force
- BSpeed
- CKinetic
- DGravity
2.In which unit is energy measured?
Easy
- AJoule (J)
- BNewton (N)
- CKilogram (kg)
- DDegree Celsius (°C)
3.A stretched rubber band is holding energy in which store?
Easy
- AKinetic
- BChemical
- CNuclear
- DElastic potential
4.A new battery has not been connected to anything yet. In which store is its energy held?
Easy
- AThermal
- BChemical
- CElastic potential
- DKinetic
5.A cyclist is riding along a flat road. In which store is the energy of her movement?
Easy
- AGravitational potential
- BKinetic
- CChemical
- DNuclear
6.A book is lifted from the floor onto a high shelf. Which store of the book has increased?
Easy
- AKinetic
- BElastic potential
- CThermal
- DGravitational potential
7.Which pathway carries energy from the Sun to the Earth across empty space?
Easy
- ABy radiation (light)
- BBy conduction
- CMechanically
- DElectrically
8.Which of these energy resources is renewable?
Easy
- ACoal
- BOil
- CWind
- DNatural gas

Soal lainnya

9.A diver steps off a high board and falls. Which is the main change between stores as she falls?
Easy
- AKinetic to gravitational potential
- BGravitational potential to kinetic
- CChemical to elastic potential
- DThermal to nuclear
10.A phone charger feels warm while it works. What does the warmth show?
Easy
- ASome energy is being wasted to the thermal store of the surroundings
- BThe charger is creating extra energy
- CEnergy is being destroyed inside the charger
- DThe charger is storing all the energy as heat
11.Which statement is the law of conservation of energy?
Easy
- AEnergy is used up when a machine stops
- BEnergy is destroyed by friction
- CEnergy is never created or destroyed, only transferred between stores
- DEnergy is made whenever a force acts
12.A ball is dropped from 1.00 m and bounces back up to only 0.80 m. Which statement best explains the missing height?
Medium
- ASome energy was transferred to the thermal store of the ball, floor and air, and to sound
- BSome energy was destroyed when the ball hit the floor
- CGravity took the energy away from the ball
- DThe ball stored the missing energy as extra height
13.
Four machines each transfer energy as shown.
Machine Energy in (J) Useful energy out (J)
A 100 40
B 50 30
C 200 100
D 20 14
Which machine is the most efficient?
Medium
- AMachine A
- BMachine B
- CMachine C
- DMachine D
14.In science, what do we mean by a 'system'?
Medium
- AAny machine with a motor
- BThe objects we choose to include when we track energy
- CA set of rules for measuring energy
- DEverything in the universe at once
15.Why are fossil fuels described as non-renewable?
Easy
- AThey can only be used once in each country
- BThey take millions of years to form, so we use them far faster than they are replaced
- CThey are made only in power stations
- DThey are always more expensive than wind power
16.A cyclist doubles her speed but her mass stays the same. By what factor does her kinetic store change?
Hard
- AIt becomes 2 times larger
- BIt becomes 8 times larger
- CIt becomes 4 times larger
- DIt stays the same
17.
Use g = 10 N/kg. Four objects are lifted as follows.
Object Mass (kg) Height lifted (m)
P 3 1
Q 2 2
R 1 5
S 4 1
Which object gains the most gravitational potential energy?
Hard
- AObject P
- BObject Q
- CObject S
- DObject R
18.A skateboarder rolls down a ramp, speeds up, then slows to a stop on flat ground. A student says: 'The skateboarder has used up all the energy.' Which reply is most accurate?
Hard
- AThe student is right, because energy is destroyed by friction
- BThe kinetic store went down, but the energy was transferred to the thermal stores of the wheels, ground and air
- CThe energy went back into the skateboarder's gravitational store
- DThe energy was created again as sound
19.A student drops the same ball from heights of 0.5 m, 1.0 m, 1.5 m and 2.0 m onto a hard floor and measures how high it bounces. Which is the independent variable?
Medium
- AThe height the ball bounces to
- BThe mass of the ball
- CThe height the ball is dropped from
- DThe type of floor
20.In the same ball-drop investigation, which choice keeps the test fair?
Medium
- AUse the same ball and the same floor for every drop
- BUse a different ball for each drop height
- CUse a softer floor for the higher drops
- DDrop from whichever height is easiest each time
21.
In one experiment a ball was dropped once from each height.
Drop height (m) Bounce height (m)
0.5 0.38
1.0 0.74
1.5 1.10
2.0 1.46
Which conclusion is best supported by these results?
Hard
- AThe ball always bounces back to exactly half of its drop height
- BThe bounce height does not depend on the drop height
- CA higher drop gives a higher bounce, and the ball bounces back to roughly three-quarters of its drop height
- DNo energy is transferred away from the ball at each bounce
22.Which change would make the ball-drop results more reliable?
Medium
- ARepeat each drop three times and calculate the mean bounce height
- BDrop the ball only from the highest position
- CMeasure the bounce by eye and not write it down
- DUse a different ball for each repeat
23.Which of these are energy stores? (select all that apply)
Medium
- AKinetic
- BThermal
- CChemical
- DForce
- ESpeed
24.Which of these are ways of transferring energy? (select all that apply)
Medium
- AMechanically
- BElectrically
- CBy heating
- DBy radiation
- EBy insulating
25.A roller-coaster car rolls down from the top of a hill. Which stores increase as it goes down? (select all that apply)
Medium
- AGravitational potential store of the car
- BKinetic store of the car
- CThermal store of the track and air
- DNuclear store of the track
26.Which of these would reduce the wasted energy of a bicycle? (select all that apply)
Medium
- AOiling the chain
- BKeeping the tyres pumped up
- CLetting the chain get dry and rusty
- DPressing the brakes while pedalling
27.A machine can create new energy.
Easy
True or false?
28.A moving object has energy in its kinetic store.
Easy
True or false?
29.When a rolling ball stops, its energy has disappeared.
Medium
True or false?
30.A more efficient device wastes a smaller fraction of its input energy.
Easy
True or false?
31.An everyday machine can give out more useful energy than it takes in.
Medium
True or false?
32.If an object is lifted twice as high, the energy transferred to its gravitational store is doubled (same mass).
Hard
True or false?
33.Match each energy store to an example.
Easy
- Kinetic
- Gravitational potential
- Elastic potential
- Chemical
- A stretched spring
- A piece of fruit
- A rolling football
- A book on a high shelf
34.Match each pathway to an example.
Easy
- Mechanically
- Electrically
- By heating
- By radiation
- A hot cup warming your hands
- Light from a lamp reaching a desk
- A person lifting a box
- A battery lighting a bulb
35.Match each situation to the store that increases.
Medium
- Brakes pressed on a moving bike
- A compressed spring is released
- Climbing a flight of stairs
- Plants growing in sunlight
- Gravitational potential store of the climber
- Chemical store of the plant
- Thermal store of the brakes
- Kinetic store of the spring's load
36.Match each term to its meaning.
Hard
- Efficiency
- Wasted energy
- Conservation of energy
- Joule
- The total energy stays the same in any change
- The unit used to measure energy
- The fraction of input energy that is transferred usefully
- Energy transferred to stores we do not want
37.Put this energy chain for a wound-up toy car in order.
Easy
- Thermal store of the surroundings
- Elastic potential store of the spring
- Kinetic store of the car
38.Put the stages of a hydroelectric power station in order, starting with the store at the top.
Medium
- Electrical energy sent to the grid
- Kinetic store of the falling water
- Gravitational potential store of water behind the dam
- Kinetic store of the turbine and generator
39.
Put these devices in order from least efficient to most efficient.
Device Energy in (J) Useful energy out (J)
P 200 50
Q 80 64
R 500 200
S 250 150
Hard
- Device S
- Device Q
- Device P
- Device R
40.Complete the sentence.
Easy
Energy cannot be created or destroyed, only ____ from one store to another.
41.Complete the equation in words.
Medium
efficiency = useful output energy / ____ input energy x 100%
42.Complete the sentence.
Medium
Energy that is not transferred usefully is called ____ energy.
43.A ball is dropped and bounces three times, each bounce lower than the last. Describe the energy transfers that happen, naming the stores.
Medium
44.A swinging pendulum slowly comes to a stop. Explain why this does not break the law of conservation of energy.
Hard
45.Plan an investigation into whether the mass of a trolley changes how far it rolls after leaving the bottom of a ramp. State the independent variable, the dependent variable and two variables you would control.
Hard
46.A box of mass 3 kg is lifted 2 m onto a shelf. Use g = 10 N/kg and the equation change in gravitational potential energy = mass x g x height. How much energy is transferred to the box's gravitational store?
Medium
47.A snack label says it contains 250 kJ of energy. How many joules is this?
Medium
48.A machine takes in 200 J of energy and transfers 80 J usefully. What is its efficiency?
Medium
49.A device takes in 500 J of energy and 120 J of this is useful output. How much energy is wasted?
Medium
50.A climber of mass 60 kg climbs a 5 m wall. Use g = 10 N/kg. How much energy is transferred to the climber's gravitational store? Give your answer in kilojoules.
Hard
51.A lamp takes in 20 J of electrical energy. It wastes 4 J as thermal energy. What is its efficiency?
Hard

Catatan ulasan

## The big idea - A **system** is the objects we track. Energy moves between **stores** by **transfers** (**Systems**, **Energy**). ## Stores - **Kinetic**, **gravitational potential**, **elastic potential**, **chemical**, **thermal**, **nuclear**, plus magnetic and electrostatic. - Kinetic depends on mass and speed (double the speed, 4 times the store). Gravitational depends on mass and height. ## Calculations - change in gravitational energy (J) = mass (kg) x g (N/kg) x height change (m), with g = 10 N/kg. - efficiency = useful output / total input x 100%. - 1 kJ = 1000 J. ## Transfers - Four pathways: **mechanically**, **electrically**, **by heating**, **by radiation**. - Energy is **conserved**. Wasted energy ends up in the thermal store of the surroundings. - Fossil fuels are non-renewable; wind and solar are renewable. ## Think like a scientist - Change one variable, measure one variable, control the rest, repeat and take the mean.

Energy: A quantity that tells us what can happen; it is measured in joules.
Joule (J): The unit of energy. 1 kJ = 1000 J.
Energy store: A way of holding energy, such as kinetic, gravitational, elastic, chemical, thermal or nuclear.
Kinetic store: The energy of a moving object; it depends on mass and speed.
Gravitational potential store: The energy an object has because of its height; it depends on mass and height.
Elastic potential store: The energy in a stretched or squashed object that can spring back.
Chemical store: The energy in food, fuels and batteries.
Thermal store: The energy of the moving particles inside an object.
Transfer pathway: The way energy moves: mechanically, electrically, by heating or by radiation.
System: The group of objects chosen when describing energy changes.
Conservation of energy: Energy cannot be created or destroyed, only transferred between stores.
Wasted energy: Energy transferred to stores that are not useful, usually the thermal store of the surroundings.
Efficiency: Useful output energy divided by total input energy, as a percentage.
Renewable resource: An energy resource that will not run out, such as wind or solar power.

Kartu flash

What is energy measured in?

Joules (J). 1 kJ = 1000 J.

Name six energy stores.

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

What does a kinetic store depend on?

The mass of the object and its speed.

What happens to the kinetic store if speed doubles?

It becomes 4 times larger.

What does a gravitational store depend on?

The mass of the object and its height.

How do you calculate a change in gravitational energy?

Mass (kg) x g (N/kg) x change in height (m). Use g = 10 N/kg.

Name the four transfer pathways.

Mechanically, electrically, by heating and by radiation.

How do you describe a transfer?

Say which store energy leaves, which store it arrives in, and the pathway.

What is the law of conservation of energy?

Energy is never created or destroyed, only transferred between stores.

What is wasted energy?

Energy transferred to stores that are not useful, usually the thermal store of the surroundings.

Where does the energy of a stopped rolling ball go?

To the thermal stores of the ball, the ground and the air, and to sound.

How do you calculate efficiency?

Useful output energy / total input energy x 100%.

Can efficiency be more than 100%?

No. A machine cannot give out more energy than it takes in.

Give two renewable energy resources.

Wind and solar power (also hydroelectric).

Why are fossil fuels non-renewable?

They take millions of years to form, so they are used far faster than they are replaced.

What are the independent and dependent variables?

The independent variable is what you change; the dependent variable is what you measure.

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Segera hadir

Object	Mass (kg)	Height lifted (m)
P	3	1
Q	2	2
R	1	5
S	4	1

Drop height (m)	Bounce height (m)
0.5	0.38
1.0	0.74
1.5	1.10
2.0	1.46

Device	Energy in (J)	Useful energy out (J)
P	200	50
Q	80	64
R	500	200
S	250	150