Grade 7 Physics
Chapter 28: Energy Stores and Energy
Transfer
Part 1
Chapter Introduction
Everything we do requires energy. We need energy to walk, run, play sports, cook food,
charge our phones and even think. Plants also need energy to grow, while machines need
energy to operate. Scientists study how energy is stored and transferred because
understanding energy helps us design better homes, safer vehicles, cleaner power stations
and more efficient technology. In this chapter you will learn that energy is never created or
destroyed; instead, it moves from one energy store to another.
Learning Objectives
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Know that energy is measured in joules (J) and kilojoules (kJ).
Describe the meaning of energy and explain why it is important.
Identify thermal, chemical and kinetic energy stores.
Recognise everyday examples of different energy stores.
Use scientific vocabulary correctly when describing energy.
Key Vocabulary
Word
Energy
Joule (J)
Kilojoule (kJ)
Energy Store
Thermal Energy
Chemical Energy
Kinetic Energy
Meaning
The ability to cause change or do work.
The SI unit used to measure energy.
1000 joules.
A way in which energy is held.
Energy associated with the movement of
particles.
Energy stored in chemicals such as food
and fuels.
Energy possessed by moving objects.
Lesson 1: What is Energy?
Energy is the ability to make things happen. It allows objects to move, change speed, change
temperature, produce light, create sound and perform work. Although we cannot see energy
directly, we can observe its effects everywhere around us.
Imagine kicking a football. Your muscles use energy from food. The ball starts moving
because energy has been transferred from your muscles to the ball. Similarly, a torch shines
because energy stored in its battery is transferred to the lamp, producing light.
Scientists describe energy as something that can be stored and transferred. Energy is not
used up completely; it simply changes from one form or store to another.
Science in Everyday Life
• A mobile phone uses energy stored in its battery.
• A cyclist uses energy from food to pedal a bicycle.
• A toaster transfers electrical energy into thermal energy.
• A plant uses energy from sunlight to make food by photosynthesis.
Lesson 2: Measuring Energy
The standard international (SI) unit of energy is the joule (J). Small amounts of energy are
often measured in joules, while larger amounts are measured in kilojoules (kJ).
1 kilojoule (1 kJ) = 1000 joules (1000 J).
Worked Example:
A chocolate bar contains 850 kJ of energy.
To convert to joules:
850 × 1000 = 850,000 J.
Worked Example:
A battery stores 12,000 J of energy.
To convert to kilojoules:
12,000 ÷ 1000 = 12 kJ.
Lesson 3: Energy Stores
Scientists describe energy as being held in different energy stores. In this part of the chapter
we will study three important energy stores.
1. Thermal Energy Store
All objects contain thermal energy because the particles inside them are always moving.
The hotter an object becomes, the faster its particles move and the greater its thermal
energy store.
Examples include a cup of hot tea, boiling water, a hot oven and warm sand on a sunny
beach.
2. Chemical Energy Store
Chemical energy is stored in substances such as food, fuels and batteries. This energy is
released during chemical reactions. Humans release chemical energy from food through
respiration, while engines release chemical energy by burning fuel.
Examples include bread, rice, petrol, diesel, natural gas, coal, firewood and rechargeable
batteries.
3. Kinetic Energy Store
Any moving object has kinetic energy. Faster objects have more kinetic energy than slower
objects of the same mass.
Examples include a running athlete, a moving bicycle, flowing river water, a flying aeroplane
and a rolling football.
Comparing the Three Energy Stores
Energy Store
Thermal
Chemical
Kinetic
Where it is found
Hot objects
Food, fuels, batteries
Moving objects
Example
Boiling water
Petrol
Cyclist riding a bike
Did You Know?
Even ice contains thermal energy because the particles inside it are still moving. They
simply move much more slowly than the particles in hot water.
Chapter Summary (Part 1)
• Energy is the ability to cause change or do work.
• Energy is measured in joules (J) and kilojoules (kJ).
• 1 kJ = 1000 J.
• Energy is stored in different ways called energy stores.
• Thermal energy is associated with hot objects.
• Chemical energy is stored in food, fuels and batteries.
• Kinetic energy belongs to moving objects.