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ScienceGrade 7 5 min read

⚡ Energy Transfer & Transformation: Following the Flow of Energy

Discover how energy moves between objects and changes from one form to another, from a cup of hot cocoa cooling down to a bouncing ball — and learn why energy is never created or destroyed.

Learning Goals

By the end of this lesson, you will be able to:

✓
Identify the main forms of energy (kinetic, thermal, light, sound, chemical, gravitational potential).
✓
Describe how energy is transferred between objects and transformed from one form to another.
✓
Apply the Law of Conservation of Energy to everyday situations.
✓
Trace energy flow using simple energy chains and basic calculations.

Before You Begin

Before this lesson, you should know:

→
Knowing that energy is the ability to do work or cause change.
→
Recognising common forms of energy such as heat, light, and movement.
→
Basic arithmetic including addition, subtraction, and simple percentages.

Make sure you are comfortable with these ideas first.

Notation & Symbols

Key notation and symbols used in this topic:

E

Energy — usually measured in joules (J).

J

Joule — the standard unit of energy.

E_{k}

Kinetic energy — the energy of a moving object.

E_{p}

Gravitational potential energy — energy stored due to height.

\to

Transforms into — used in energy chains to show one form changing to another.

\sum E

The total energy — the sum of all energy forms in a system.

These symbols will appear throughout the lesson.

How Energy Moves and Changes

Energy transfer is the movement of energy from one object or place to another, while energy transformation is the change of energy from one form into another. In both cases, the total amount of energy stays the same.

Forms of Energy: Energy exists in several forms, including kinetic ( $E_{k}$ , movement), thermal (heat), light, sound, chemical (stored in food and fuel), and gravitational potential ( $E_{p}$ , stored due to height).
Energy Transfer: Energy can move between objects without changing form — for example, heat flowing from a hot mug into your cold hands by conduction.
Energy Transformation: Energy can change form — a torch transforms chemical energy in the battery into light and thermal energy.
Conservation of Energy: Energy cannot be created or destroyed, only transferred or transformed. The total stays constant: $\sum E_{before} = \sum E_{after}$ .
Dissipation: In real processes, some energy is always spread out as unwanted heat or sound, so not all energy ends up in the useful form we want.

Example

When you eat a banana and then run, chemical energy stored in the food is transformed into kinetic energy of your moving body, plus thermal energy that warms you up. If the banana supplied

400 J

, then

400 J

is shared between movement and heat — none is lost, just spread out:

E_{chemical} \to E_{k} + E_{thermal}

100%

A torch transforms chemical energy in the battery into electrical energy, then into light and heat, with total energy conserved.

Worked Example

Basic

A cup of hot cocoa transfers

2400 J

of thermal energy to the surrounding air. How much energy does the air gain, assuming no energy is lost elsewhere?

Worked Example

Intermediate

A student eats a snack storing

500 J

of chemical energy. During exercise,

300 J

is transformed into useful movement (kinetic energy) and the rest becomes thermal energy. How much thermal energy is produced, and what is the efficiency of the energy transformation?

Worked Example

Advanced

A falling apple of mass

0.2 kg

drops from a branch

3 m

high. Using

g = 10 m/s^{2}

, find its gravitational potential energy at the top, then state its kinetic energy just before hitting the ground if

15%

of the energy is lost to air resistance (sound and heat).

Why It Works

Understanding the reasoning behind the rule:

The principle of conservation of energy explains why we can track energy as it changes form. Let us see why total energy stays constant during a transformation.

Your Turn

Try this one yourself — use the hint if you get stuck:

A torch (flashlight) battery supplies

800 J

of chemical energy. The bulb transforms

120 J

into light energy and the rest into heat. How much energy becomes heat, and what is the torch's efficiency as a light source?

Watch Out For...

✗

Thinking energy is 'used up' or destroyed during a transfer, for example believing a battery's energy simply vanishes.

Apply the law of conservation of energy:

E_{in} = E_{u se f u l} + E_{w a s t e d}

. The energy is transformed (often into heat and sound), not destroyed.

✗

Confusing energy transfer (moving energy between objects) with energy transformation (changing the form of energy).

Transfer = energy moving place to place (e.g., hot pan to hand). Transformation = form changing (e.g., chemical energy in food

\to

kinetic energy in muscles).

✗

Assuming a device can be 100% efficient.

Real devices always waste some energy, usually as heat. Efficiency

= \frac{E _{u se f u l}}{E _{in}} \times 100%

is always less than 100%.

In the Real World

🌱

Plants perform photosynthesis, transforming light energy from the Sun into chemical energy stored in sugars — the start of nearly every food chain in your local environment.

🏃

When you exercise, chemical energy from food is transformed into kinetic energy of movement and thermal energy, which is why you feel hot and sweat after running.

💡

A light bulb transforms electrical energy into light energy, but an older filament bulb wastes much of it as heat — modern LED bulbs are far more efficient, so

\frac{E _{u se f u l}}{E _{in}}

is higher.

Extended Practice

Test yourself with a mix of multiple choice and short-answer questions:

Q1.

A toaster takes in 1000 J of electrical energy and produces 850 J of useful thermal energy. What is its efficiency?

MCQ

Q2.

Which process transforms light energy into chemical energy?

MCQ

Q3.

A ball is held at the top of a hill. As it rolls down, describe the main energy transformation taking place.

Gravitational potential energy is transformed into kinetic energy as the ball speeds up going downhill. Some energy is also transformed into thermal and sound energy due to friction.

Q4.

A kettle uses 2000 J of electrical energy and transfers 1600 J usefully to heat the water. How much energy is wasted, and in what form?

Wasted energy

= 2000 - 1600 = 400

J, mostly as thermal energy lost to the surroundings and some as sound.

Q5.

Which statement correctly describes the law of conservation of energy?

MCQ

Extension Challenge

Advanced

A harder challenge that connects to the next skill:

A solar-powered garden light charges during the day and glows at night. Trace the full chain of energy transformations from the Sun to the glowing light, and explain where energy is wasted along the way.

Key Takeaways

1.Energy cannot be created or destroyed, only transferred or transformed: $E_{in} = E_{u se f u l} + E_{w a s t e d}$ .
2.Transfer means energy moving between objects; transformation means energy changing form.
3.Real devices are never 100% efficient — efficiency $= \frac{E _{u se f u l}}{E _{in}} \times 100%$ , and wasted energy is usually heat.
4.Everyday examples — plants, exercise, kettles — all follow these same energy rules.

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Ready to Practice?

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