Energy transfer and conservation

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Potential and Kinetic Energy

Potential energy is the energy an object has because of its position or shape in a system.
Examples: a brick lifted above the ground, a stretched elastic band, water at the top of a dam.
Kinetic energy is the energy of movement.
Examples: a moving car, water flowing down a waterfall, a running refrigerator motor.
An object can have both potential and kinetic energy at the same time, e.g., a high jumper at the peak of the jump has potential energy and some kinetic energy.

A girl kicking a ball, shown pulling her leg back and following through

Law of Conservation of Energy

The Law of Conservation of Energy states that energy cannot be created or destroyed, only transferred or transformed.
Energy is transferred from one part of a system to another, or transformed from one form to another.
In the elastic band and matchbox example, the potential energy of the stretched band is transferred to the matchbox as kinetic energy.
The total energy in a closed system remains constant.

Energy Transfers in Systems

A system is made up of different parts that work together or affect each other.
Systems have an input energy and an output energy.
Energy can be transferred within mechanical, thermal, electrical, and biological systems.
In a mechanical system like a swing, potential energy at the top of the arc becomes kinetic energy as it swings down.
In a thermal system like boiling water, chemical potential energy in gas is transferred to kinetic energy of water particles.
In an electrical system like a fan, electrical energy is transferred to kinetic energy of the fan blades.
In a biological system, radiant energy from the Sun is stored as potential energy in plants, then transferred to animals that eat them.

A lady using an electric drill to make a hole in an aeroplane part

Useful and Wasted Energy

Systems often produce useful energy output (e.g., light from a bulb, movement from a fan) and wasted energy (e.g., heat, sound).
Wasted energy is transferred to the surroundings and is not useful for the intended purpose.
An efficient system has a useful output energy only slightly smaller than the input energy.
An inefficient system has a large amount of wasted energy compared to useful output.
For example, a filament light bulb wastes most energy as heat, while a fluorescent bulb wastes less and produces more light.

A car engine

Sankey Diagrams

A Sankey diagram shows the input energy and how it splits into useful and wasted energy outputs.
The width of the arrows represents the amount of energy.
The width of the useful and wasted arrows together equals the width of the input arrow, because energy is conserved.
Sankey diagrams are drawn to scale to visually compare efficiency.
Example: An electric torch with 100 J input, 10 J light (useful) and 90 J heat (wasted).

A Sankey diagram showing input energy split into useful output energy and wasted output energy

Energy Transformations

Kinetic energy can be transformed into potential energy and vice versa.
When you throw a ball upward, kinetic energy from your hand is transferred to the ball, then as it rises, kinetic energy becomes potential energy.
At the top of its path, the ball has maximum potential energy and minimum kinetic energy.
As it falls, potential energy is transformed back into kinetic energy.
Friction can transform kinetic energy into heat energy, e.g., rubbing hands together makes them warm.

A skateboarder speeding up and slowing down on the ramps of a skate park

Examples of Energy Transfers

High jumper: At the peak of the jump, she has maximum potential energy; just before landing, she has maximum kinetic energy.
Skateboarder: Going down a ramp, potential energy becomes kinetic energy, making them speed up; going up, kinetic energy becomes potential energy, slowing them down.
Water slide: At the top, the slider has potential energy; as they slide down, it changes to kinetic energy.
Hydropower plant: Water high in a dam has potential energy; as it flows down, it becomes kinetic energy, turning turbines to generate electricity.

A hydropower plant: dam, powerhouse, turbine, generator, power lines and city

Energy in Food Chains

Plants use radiant energy from the Sun to make food via photosynthesis, storing it as potential energy (starch).
When animals eat plants, they obtain that stored potential energy.
During respiration, animals release the energy to power movement and life processes, transforming potential energy to kinetic energy.
Energy is transferred from the Sun to plants to animals, and is conserved throughout the biological system.

An impala grazing; it stores energy from food and releases it as kinetic energy when it runs

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1.Which of the following is the correct statement of the Law of Conservation of Energy?
Easy
- AEnergy can be created but not destroyed.
- BEnergy can be destroyed but not created.
- CEnergy cannot be created or destroyed, only transferred or transformed.
- DEnergy can be created and destroyed in equal amounts.
2.Which of the following is an example of potential energy?
Easy
- AA moving car
- BA stretched rubber band
- CA spinning fan
- DA flying bird
3.Which of the following is an example of kinetic energy?
Easy
- AA book on a shelf
- BA charged battery
- CA ball rolling down a hill
- DA compressed spring
4.In a closed system, energy can be created or destroyed.
Easy
True or false?
5.Thermal energy in a substance is related to the total kinetic energy of its particles.
Easy
True or false?
6.A ball is thrown upwards. At the highest point of its motion, what type of energy does it have?
Easy
- AKinetic energy only
- BPotential energy only
- CBoth kinetic and potential energy
- DNo energy
7.In a pendulum, at which position does the bob have the most kinetic energy?
Easy
- AAt the highest point on the left
- BAt the highest point on the right
- CAt the lowest point of the swing
- DMidway between the highest and lowest points
8.When you rub your hands together, what energy transformation occurs?
Easy
- AKinetic energy to thermal energy
- BThermal energy to kinetic energy
- CPotential energy to kinetic energy
- DChemical energy to potential energy

Mehr Fragen

9.A plant gets its energy from the Sun. What type of energy does the plant store in its leaves and fruit?
Easy
- AKinetic energy
- BThermal energy
- CPotential energy
- DElectrical energy
10.Which of the following are examples of systems that can have potential energy? (Select all that apply)
Easy
- AA stretched elastic band
- BA book sitting on a high shelf
- CA moving car
- DWater stored behind a dam
- EA spinning top
11.Which of the following statements about energy are true? (Select all that apply)
Medium
- AEnergy can be transferred from one object to another.
- BEnergy can be transformed from one form to another.
- CEnergy is always conserved in a closed system.
- DEnergy can be created in nuclear reactions.
- EWasted energy is destroyed.
12.In an energy transfer, the total amount of energy before and after the transfer is always the same.
Medium
True or false?
13.A system that wastes a lot of energy is considered efficient.
Medium
True or false?
14.Match each energy form with an example.
Easy
- Kinetic energy
- Potential energy
- Thermal energy
- A moving car
- A stretched spring
- Hot water
15.Arrange the following events in the correct order for a roller coaster car starting at the top of a hill:
Medium
- Car moves down the hill, speeding up
- Car is at the top of the hill, at rest
- Car reaches the bottom, moving fastest
- Car climbs the next hill, slowing down
16.What type of energy does the water in the dam have?
Medium
- AKinetic energy
- BPotential energy
- CThermal energy
- DChemical energy
17.As the water flows down the outlet, what energy transfer occurs?
Medium
- APotential to kinetic
- BKinetic to potential
- CThermal to kinetic
- DChemical to potential
18.The flowing water turns a turbine. What type of energy does the turbine have?
Medium
- APotential energy
- BKinetic energy
- CThermal energy
- DElectrical energy
19.Match each type of system with a correct example.
Easy
- Mechanical system
- Thermal system
- Electrical system
- Biological system
- Pulley and rope
- Boiling water
- Electric fan
- Food chain
20.Arrange the energy transfers in a hydropower plant in the correct order.
Medium
- Potential energy of water in dam
- Kinetic energy of flowing water
- Kinetic energy of turbine
- Electrical energy from generator
21.Which of the following are examples of energy transfers in a mechanical system? (Select all that apply)
Medium
- AA stretched elastic band snaps and pushes a matchbox.
- BA ball is thrown upward and slows down.
- CWater is heated in a kettle.
- DA pulley lifts a heavy object.
- EA plant makes food using sunlight.
22.A girl pulls her leg back to kick a ball. When her leg is at its highest point, what type of energy does it have?
Medium
- AKinetic energy
- BPotential energy
- CBoth kinetic and potential energy
- DNo energy
23.In a thermal system where water is heated by a Bunsen burner, what is the main energy transfer that causes the water temperature to rise?
Medium
- AKinetic energy to potential energy
- BChemical potential energy to thermal energy
- CThermal energy to kinetic energy
- DElectrical energy to thermal energy
24.In the investigation of heating water, what is the independent variable?
Medium
- ATemperature of water
- BTime
- CVolume of water
- DType of thermometer
25.A filament light bulb uses 10% of input energy for light and wastes 90% as heat. Which Sankey diagram correctly represents this?
Medium
- AA thick arrow for light, thin arrow for heat
- BA thin arrow for light, thick arrow for heat
- CArrows of equal width
- DOnly one arrow for light
26.The temperature of a substance depends on the kinetic energy of its particles.
Easy
True or false?
27.In the investigation of heating water, the thermometer should not touch the sides of the beaker. Why is this important?
Medium
- ATo prevent the thermometer from breaking
- BTo ensure the thermometer measures the temperature of the water, not the glass
- CTo allow the thermometer to float freely
- DTo avoid the thermometer being heated by the Bunsen burner directly
28.In the heating water investigation, after the water starts to boil, what happens to the temperature?
Medium
- AIt continues to increase rapidly
- BIt stays constant
- CIt decreases slowly
- DIt fluctuates randomly
29.Arrange the following steps in the correct order for the heating water investigation.
Hard
- Pour 200 ml of water into a beaker
- Place the beaker on a tripod and gauze
- Light the Bunsen burner
- Measure the temperature every 30 seconds
- Plot a graph of time vs. temperature
30.Match each system type with the correct description of its energy transfer.
Medium
- Mechanical system
- Thermal system
- Electrical system
- Energy transferred through movement and forces
- Energy transferred as heat due to temperature difference
- Energy transferred through electric currents
31.Complete the sentence:
Easy
The Law of Conservation of Energy states that energy cannot be ____ or destroyed, it can only be transferred or transformed.
32.What is the name for the energy stored in an object due to its position or shape?
Easy
33.What is the name for the energy of moving objects?
Easy
34.What is the name for the energy that is not usefully transferred and is lost to the surroundings?
Easy
35.What type of diagram is used to show the input, useful output, and wasted energy in a system?
Easy
36.An electric torch converts 100 J of electrical energy into 10 J of light energy and the rest as thermal energy. How much thermal energy is produced?
Medium
37.A television has an energy input of 500 J. It produces 400 J of light and 50 J of sound. How much energy is wasted as thermal energy?
Medium
38.Complete the sentences about energy in a plant.
Easy
A plant receives energy from ____. It uses this energy to make ____, which stores ____ energy.
39.Complete the energy transfer description for a car engine.
Medium
The input energy is ____ energy from petrol. The useful output is ____ energy. Wasted energy is mostly ____ energy and ____ energy.
40.In the heating water investigation, the temperature is recorded every 30 seconds. If the water starts at 25°C and after 3 minutes it reaches 85°C, what is the temperature increase in degrees Celsius?
Medium
41.Explain why a high jumper has the most potential energy at the highest point of her jump.
Medium
42.Describe the energy transfer that occurs when a ball is thrown upwards and then falls back down.
Medium
43.In the heating water investigation, describe the energy transfer that occurs as the water is heated over the flame. Include the terms 'potential energy', 'kinetic energy', and 'conservation of energy' in your answer.
Hard
44.An electric torch converts 100 J of electrical energy into 10 J of light energy and the rest as thermal energy. How much thermal energy is produced?
Medium
- A90 J
- B108 J
- C99 J
- D81 J
45.A television has an energy input of 500 J. It produces 400 J of light and 50 J of sound. How much energy is wasted as thermal energy?
Medium
- A50 J
- B55 J
- C60 J
- D45 J
46.In the heating water investigation, the temperature is recorded every 30 seconds. If the water starts at 25°C and after 3 minutes it reaches 85°C, what is the temperature increase in degrees Celsius?
Medium
- A72 °C
- B66 °C
- C54 °C
- D60 °C
47.An athlete kicks a soccer ball. In position X, the ball is at rest on the ground just before being kicked. In position Y, the ball is in the air just after being kicked. Which statement best describes the energy of the ball?
Easy
- AThe ball gains kinetic energy from the kick.
- BThe ball loses gravitational potential energy from the kick.
- CThe ball has more kinetic energy in position X than in position Y.
- DThe ball has more gravitational potential energy in position X than in position Y.
48.A crane uses an electromagnet to lift a car straight up off the ground. A team of students claims that the crane did work on the car because the car moved from the ground to a higher position. Which statement about the energy of the Earth-car system best supports this claim?
Medium
- AThe work done by the crane decreased the energy of the Earth-car system.
- BThe work done by the crane did not transfer energy to the Earth-car system.
- CThe work done by the crane increased the potential energy of the Earth-car system.
- DThe work done by the crane changed the thermal energy of the Earth-car system.

Lernnotizen

## Energy Transfer and Conservation ### Key Concepts - **Energy** is the ability to do work or cause change. It can be transferred from one object to another or transformed from one form to another. - **Law of Conservation of Energy**: In an isolated system, the total energy remains constant. Energy cannot be created or destroyed; it can only change form or be transferred. - When an object's kinetic energy changes, energy is transferred to or from that object. ### Forms of Energy - **Kinetic Energy (KE)**: Energy of motion. KE = ½ mv² (m = mass, v = speed). - **Potential Energy (PE)**: Stored energy due to position or condition. Examples: gravitational PE (PE = mgh), elastic PE, chemical PE. - **Thermal Energy**: Energy due to temperature (related to particle motion). - **Sound Energy**: Energy carried by sound waves. - **Light Energy**: Energy carried by electromagnetic waves. ### Energy Transfers and Transformations - Energy can be transferred from one object to another (e.g., a moving ball hits a stationary ball, transferring kinetic energy). - Energy can be transformed from one type to another (e.g., chemical energy in a battery becomes electrical energy, then light and heat in a bulb). - In any process, the total energy before equals the total energy after (conservation). ### Examples - **Pendulum**: At the highest point, PE is maximum; at the lowest point, KE is maximum. Energy converts between PE and KE, but total energy (ignoring friction) stays constant. - **Explosion**: Chemical energy converts to KE, sound, and thermal energy. The sum of all energies after equals the chemical energy before. - **Roller Coaster**: As the car descends, PE converts to KE; as it ascends, KE converts back to PE. Friction converts some mechanical energy to thermal energy. ### Important Notes - In real-world systems, friction and air resistance cause some mechanical energy to become thermal energy (heat). The total energy still remains constant. - A **perpetual motion machine** (a machine that runs forever without energy input) is impossible because energy cannot be created from nothing. - Mass and energy are related by Einstein's equation E = mc², but at middle school level, we focus on energy conservation without mass conversion. ### Key Equations - Kinetic Energy: KE = ½ mv² - Gravitational Potential Energy: PE = mgh (m = mass, g = gravity ≈ 9.8 m/s², h = height) - Conservation of Energy: Total energy before = Total energy after (in a closed system)

Energy: The ability to do work or cause change; measured in joules (J).
Kinetic Energy: Energy of motion; KE = ½ mv².
Potential Energy: Stored energy due to position or condition; e.g., gravitational PE = mgh.
Law of Conservation of Energy: In an isolated system, total energy remains constant; energy cannot be created or destroyed, only transferred or transformed.
Energy Transfer: Movement of energy from one object or place to another.
Energy Transformation: Change of energy from one form to another (e.g., chemical to kinetic).
Thermal Energy: Energy associated with the random motion of particles; often called heat energy.
Perpetual Motion Machine: A hypothetical machine that can run indefinitely without an external energy source; impossible due to conservation of energy.

Karteikarten

What does the law of conservation of energy state?

The total energy of an isolated system remains constant; energy can neither be created nor destroyed, only transformed or transferred.

What is an isolated system?

A system that does not exchange energy or matter with its surroundings.

What happens to energy when an object's kinetic energy changes?

Energy is transferred to or from the object, and the total energy in the system remains constant.

Give an example of energy transformation from chemical to kinetic energy.

When a stick of dynamite explodes, chemical energy is converted to kinetic energy of the pieces, as well as heat and sound.

What is the equation for mass-energy equivalence?

E = mc², where E is energy, m is mass, and c is the speed of light.

What does mass-energy equivalence imply?

Mass can be converted to energy and vice versa; mass-energy as a whole is conserved.

What is a perpetual motion machine of the first kind?

A system that delivers unlimited energy to its surroundings without an external energy supply, which is impossible due to conservation of energy.

What is kinetic energy?

Energy associated with the motion of an object.

What is potential energy?

Stored energy that depends on the position or configuration of an object.

How does the height of a moving body relate to energy conservation?

On a frictionless surface, the height a moving body rises equals the height from which it falls, converting kinetic energy to potential energy and back.

What is an elastic collision?

A collision where kinetic energy is conserved; total kinetic energy before and after collision is the same.

What is an inelastic collision?

A collision where kinetic energy is not conserved; some energy is transformed into heat, sound, or deformation.

What did Huygens discover about collision?

He found that the sum of linear momenta and the sum of kinetic energies are conserved in collisions (though later distinguished elastic vs inelastic).

What is Torricelli's Principle?

The center of gravity of a heavy object or collection cannot lift itself; used to analyze pendulum motion.

What is Noether's theorem?

It states that conservation laws arise from symmetries; conservation of energy comes from time translation symmetry.

Can energy be created or destroyed?

No, energy can only be transformed or transferred from one form to another.

What is an energy transfer?

Energy moving from one object or system to another without changing form (e.g., heat transfer).

What is an energy transformation?

Energy changing from one form to another (e.g., chemical to kinetic).

Give an example of energy transfer in a closed system.

A hot cup of coffee transferring heat to the surrounding air; total energy remains constant.

What is the difference between a closed system and an isolated system?

A closed system can exchange energy but not matter with surroundings; an isolated system exchanges neither.

What did Galileo observe about a pendulum?

The height a moving body rises equals the height from which it falls, showing conversion between potential and kinetic energy.

What is mass defect?

The difference between the mass of a nucleus and the sum of its individual nucleons, converted to binding energy (E=mc²).

Why can't a perpetual motion machine exist?

Because energy cannot be created from nothing; any machine would eventually run down due to energy transfers and transformations.

What is the role of friction in energy conservation?

Friction converts kinetic energy into thermal energy, but total energy remains conserved.

How does the law of conservation of energy apply to a roller coaster?

At the top, potential energy is high; as it descends, potential energy converts to kinetic energy; total mechanical energy is conserved if friction is ignored.

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