Energy Transfer Pathways

Energy is involved whenever something changes: an object speeds up, a lamp switches on, water gets warmer, or sunlight reaches Earth. However, energy does not simply disappear from one place and appear in another. It is transferred between energy stores.

A useful model is:

ENERGY STORE → TRANSFER PATHWAY → ENERGY STORE

There are four main energy transfer pathways:

• Mechanical work
• Electrical work
• Heating
• Waves

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Learning Targets

• I can identify the four main energy transfer pathways: mechanical work, electrical work, heating, and waves.
• I can explain how energy is transferred through each pathway and describe real-world examples of each.
• I can distinguish between energy stores and energy transfer pathways in physical systems.
• I can analyze energy transfers in everyday situations and identify the stores and pathways involved.
• I can construct and interpret energy flow diagrams that show how energy is transferred between stores.

1. Energy Stores

An energy store describes where energy is held within a system.

The main energy stores are:

• Kinetic store – associated with moving objects
• Gravitational potential store – associated with an object's position in a gravitational field
• Elastic potential store – associated with stretched or compressed objects
• Chemical store – found in fuels, food and batteries
• Thermal store – associated with the internal energy and temperature of an object
• Magnetic store – associated with interacting magnetic fields
• Electrostatic store – associated with interacting electric charges
• Nuclear store – associated with atomic nuclei

For example, a moving bicycle has energy in its kinetic store, while a battery has energy in its chemical store.

Energy stores tell us where energy is.

Transfer pathways tell us how energy moves between stores.

2. Stores Are Not Pathways

This distinction is extremely important.

Suppose a battery powers a lamp.

The battery contains a chemical energy store.

Energy is transferred from the battery electrically.

The lamp becomes hot and produces light.

We could represent part of this process as:

Chemical store → electrical work → thermal store

Notice that electrical describes the transfer, not the store.

A useful rule is:

STORE = where energy is

PATHWAY = how energy gets somewhere else

3. The Four Transfer Pathways

These four pathways allow us to describe a huge range of physical systems.

4. Mechanical Work

Energy is transferred mechanically when a force acts and causes an object to move through a distance.

Examples include:

• Lifting a backpack
• Pushing a shopping cart
• Kicking a football
• Stretching a spring
• A falling object accelerating
• A crane lifting a load
• A cyclist pedalling uphill

Example: Lifting a Book

Imagine lifting a book from the floor onto a shelf.

Your muscles contain a chemical energy store.

Your muscles exert a force on the book and move it upwards.

Energy is transferred mechanically:

Chemical store in body → mechanical work → gravitational potential store of book

The book now has a larger gravitational potential energy store because it has been raised.

5. Mechanical Work and Moving Objects

Mechanical work does not always increase gravitational potential energy.

Suppose you push a stationary shopping cart.

Your muscles exert a force and the cart begins moving.

Chemical store → mechanical work → kinetic store

Some energy will also be transferred to thermal stores because of friction.

Therefore, a more realistic description is:

Chemical store → mechanical work → kinetic store + thermal stores

Energy has not been destroyed. It has simply been distributed among different stores.

6. Electrical Work

Energy is transferred electrically when electric charges move through a potential difference.

This commonly happens when an electric current flows through a circuit.

Examples include:

• A battery powering a lamp
• Charging a phone
• An electric fan operating
• A television operating
• An electric heater warming a room
• A motor turning

Example: Battery and Lamp

A battery begins with energy in its chemical store.

When the circuit is completed, energy is transferred electrically to the lamp.

The lamp becomes hot and produces light.

A simplified description is:

Chemical store → electrical work → thermal store of lamp

Energy then leaves the lamp through heating and electromagnetic waves.

7. Example: Electric Fan

An electric fan receives energy through an electrical pathway.

The motor causes the blades to rotate.

The blades therefore gain energy in their kinetic store.

We can represent this as:

Electrical work → motor → mechanical work → kinetic store of blades

Not all of the transferred energy becomes useful kinetic energy.

Some increases the thermal stores of:

• the motor
• the wires
• the surrounding air

Some energy is also carried away by sound waves.

That is why an electric fan produces both heat and sound as well as moving air.

8. Heating

Energy is transferred by heating when there is a temperature difference.

Energy moves from a region at a higher temperature toward a region at a lower temperature.

Imagine putting a cold metal spoon into hot soup.

The soup is hotter than the spoon.

Energy is transferred:

Thermal store of soup → heating → thermal store of spoon

As a result:

• The soup cools slightly.
• The spoon becomes warmer.
• Their temperatures move toward thermal equilibrium.

Heating between materials can occur through processes such as conduction and convection.

9. Examples of Heating

Heating occurs constantly in everyday life.

Examples include:

• A radiator warming a room
• Hot tea warming a cup
• Your hand warming an ice cube
• A frying pan heating food
• A hot engine warming the surrounding air
• Warm water melting an ice cube

In each case, energy is transferred because there is a temperature difference.

For example:

Thermal store of hot pan → heating → thermal store of food

Eventually, if no other processes interfere, the temperatures become closer together.

10. Transfer by Waves

Energy can also be transferred by waves.

Electromagnetic waves are particularly important because they can transfer energy through empty space.

Examples include:

• Visible light
• Infrared radiation
• Microwaves
• Radio waves
• Ultraviolet radiation
• X-rays

Mechanical waves such as sound waves can also carry energy through matter.

A major example is energy arriving at Earth from the Sun.

Space between the Sun and Earth is mostly empty, so conduction and convection cannot carry energy across it.

Instead, energy is transferred by electromagnetic waves.

11. Example: Sun to Earth

The Sun contains an enormous nuclear energy store.

Nuclear reactions in the Sun allow energy to be transferred outward.

Eventually, electromagnetic waves carry energy through space.

A simplified model is:

Nuclear store in Sun → electromagnetic waves → thermal and other stores on Earth

When sunlight is absorbed by the ground, oceans or buildings, their thermal stores can increase.

This is an important example because it demonstrates that energy can be transferred without matter travelling from the source to the receiver.

12. Example: Sound

Imagine a loudspeaker playing music.

Electrical energy is supplied to the speaker.

The speaker cone vibrates.

Those vibrations produce sound waves in the surrounding air.

We can describe the sequence as:

Electrical work → kinetic store of speaker cone → sound waves → surroundings

The sound waves carry energy away from the speaker.

13. Analyzing an Energy Transfer

When you encounter an energy-transfer problem, ask three questions:

Question 1: Where is the energy initially stored?

Identify the starting store.

Question 2: How is the energy transferred?

Identify the pathway.

Question 3: Where does the energy end up?

Identify the final store or stores.

This gives us:

Starting store → pathway → final store

This store–pathway–store approach is a useful way of analysing physical systems.

14. Worked Example: Falling Ball

Imagine holding a ball above the ground.

Initially, the ball has energy in its:

Gravitational potential store

Release the ball.

Gravity exerts a force on the ball while the ball moves downward.

Therefore, energy is transferred mechanically.

The ball speeds up, increasing its kinetic energy store.

Gravitational potential store → mechanical work → kinetic store

When the ball hits the ground, energy is transferred again.

Some increases thermal stores, while some is carried away by sound waves.

15. Worked Example: Phone Charging

Consider a phone connected to a charger.

Energy is transferred electrically through the charger and into the phone.

Much of this energy increases the battery's chemical energy store.

Electrical work → chemical store of battery

However, the phone may also become slightly warm.

Therefore:

Electrical transfer
       ↓
     Phone
    ↙     ↘
Chemical   Thermal
 store      store

This reminds us that energy can be transferred into more than one store.

16. Worked Example: Cyclist Going Uphill

A cyclist obtains energy from the chemical store associated with food.

The cyclist's muscles exert forces while pedalling.

Energy is transferred mechanically.

As the cyclist climbs the hill, the cyclist-bike system gains gravitational potential energy.

Chemical store → mechanical work → gravitational potential store

If the cyclist also speeds up:

Chemical store → mechanical work → kinetic store

Some energy is also transferred to thermal stores because of friction and air resistance.

17. Energy Flow Diagrams

An energy flow diagram represents how energy is transferred between stores.

A simple diagram might look like this:

Chemical store
      ↓
mechanical work
      ↓
Kinetic store

Or:

Chemical store in battery
          ↓
    electrical work
          ↓
   Thermal store
       of lamp

The boxes or labels represent stores.

The arrows represent energy transfers.

18. More Complex Energy Flow Diagrams

Real systems often have several outputs.

Consider an electric motor:

             Chemical store
               in battery
                    ↓
             electrical work
                    ↓
                  Motor
                ↙       ↘
       mechanical       heating
           work
           ↓               ↓
     Kinetic store     Thermal stores
       of shaft        of surroundings

The diagram shows that energy does not necessarily follow only one pathway.

19. Worked Example: Roller Coaster

At the top of a hill, a roller coaster has a large gravitational potential energy store.

As it travels downhill, gravity does mechanical work.

Gravitational potential store → mechanical work → kinetic store

The roller coaster speeds up.

Friction and air resistance also transfer energy into thermal stores.

Therefore:

Gravitational potential store
             ↓
       mechanical work
          ↙       ↘
   kinetic store   thermal stores

This explains why a roller coaster cannot return indefinitely to exactly the same height without an external energy input.

20. Worked Example: Hydroelectric Power

Hydroelectric power provides an excellent real-world example of several energy transfers.

Water stored behind a dam has a large gravitational potential energy store.

When released, the water falls and accelerates.

Its kinetic store increases.

The moving water turns a turbine, transferring energy mechanically.

The turbine drives a generator, which transfers energy electrically.

A simplified sequence is:

Gravitational potential store → kinetic store → mechanical work → electrical work

This demonstrates how several stores and pathways can be involved in a single system.

21. Other Real-World Examples

Wind Turbine

Moving air has a kinetic energy store.

The air exerts forces on the turbine blades.

Kinetic store of wind → mechanical work → kinetic store of turbine → electrical work

Electric Kettle

Energy is transferred electrically to the heating element.

The element becomes hot and transfers energy to the water.

Electrical work → thermal store of element → heating → thermal store of water

Car

Fuel has a chemical energy store.

Combustion allows energy ultimately to be transferred mechanically to the vehicle.

Chemical store → mechanical work → kinetic store

Some energy is also transferred to thermal stores and through sound waves.

22. Conservation of Energy

Energy can move between stores, but the total amount of energy is conserved.

Energy cannot be created or destroyed.

Therefore:

Total energy before = total energy after

Suppose a system initially has 500 J of energy.

After a transfer:

• 350 J enters a useful store.
• 100 J enters thermal stores.
• 50 J is transferred elsewhere.

Then:

All of the original energy can still be accounted for.

23. Useful and Dissipated Energy

Not every energy transfer produces the result we want.

Consider a motor.

We want energy transferred to the kinetic store of the moving parts.

But friction also increases the thermal stores of the motor and surroundings.

This energy becomes more spread out and less useful for doing further work. We say that energy has been dissipated.

It is important not to say that this energy has been "lost."

It still exists.

It has simply been transferred to stores where it is less useful.

24. Common Mistakes

Mistake 1: Treating a pathway as a store

Incorrect:

Electrical energy store

Better:

Energy is transferred electrically.

Electrical work describes a pathway.

Mistake 2: Saying energy is used up

Energy is not destroyed.

Instead say:

Energy is transferred to another store.

Mistake 3: Saying energy is lost

Energy may become dissipated into thermal stores in the surroundings, but it is not destroyed.

Mistake 4: Forgetting the pathway

Do not simply write:

Chemical → kinetic

A better description is:

Chemical store → mechanical work → kinetic store

Mistake 5: Confusing heating with a thermal store

Thermal describes an energy store.

Heating describes a transfer pathway.

25. Key Terms

Energy store – a way energy is held within a system.

Energy transfer pathway – a process through which energy moves between stores.

Mechanical work – energy transfer caused by a force acting through a distance.

Electrical work – energy transfer associated with moving electric charges.

Heating – energy transfer resulting from a temperature difference.

Waves – disturbances that can carry energy from one place to another.

Kinetic store – energy associated with movement.

Chemical store – energy associated with chemical substances such as fuels, foods and batteries.

Thermal store – internal energy associated with the particles in an object.

Gravitational potential store – energy associated with position in a gravitational field.

Dissipation – energy becoming spread into less useful stores, commonly thermal stores of the surroundings.

26. Check Your Understanding

1. Name the four energy transfer pathways.

2. Is a chemical energy store a store or a pathway?

3. What pathway transfers energy when you lift a backpack?

4. What pathway transfers energy from a battery through a circuit?

5. A hot cup of tea cools on a table. Which store decreases, and where does the energy go?

6. Explain why sunlight reaching Earth is an example of energy transfer by waves.

7. Describe the energy stores and transfer pathway when a ball falls.

8. Draw an energy flow diagram for a battery-powered toy car.

9. Draw an energy flow diagram for an electric kettle heating water.

10. A student says, "The energy disappeared because the motor got hot." Explain why this statement is incorrect.

Key Takeaways

• Energy is held in stores and transferred between stores.
• The four main transfer pathways are mechanical work, electrical work, heating, and waves.
• Mechanical transfer occurs when a force acts through a distance.
• Electrical transfer occurs through moving electric charges.
• Heating transfers energy because of a temperature difference.
• Waves can carry energy from one place to another.
• Stores and pathways are different concepts.
• Energy-flow diagrams help us track energy from its initial store through transfer pathways to final stores.
• Real systems usually involve several energy transfers at the same time.
• Energy may become dissipated, but it is never created or destroyed.

Did You Know?

Almost all of the energy used in ecosystems ultimately arrives from the Sun by electromagnetic waves. Plants transfer some of this energy into chemical stores through photosynthesis, and those chemical stores then support most food chains on Earth.