Energy and Its Forms
Energy is never made and never destroyed. It only moves and changes form, which means every question about energy is really a question about where it went.
What energy is
Energy is what makes things happen, and it is measured in joules.
That definition is deliberately vague, and it is worth being honest with a child about why. Energy is not a substance you can hold up and point at. It is a quantity that can be counted, and the reason it is so useful is that the total never changes: whatever you start with, you still have at the end, just arranged differently.
The one rule that matters
Energy cannot be created and it cannot be destroyed. It can only be transferred from one place to another, or changed from one form into another.
So "where did the energy go?" always has an answer.
The forms it takes
These are the ones worth knowing by name at this stage.
- Chemical, stored in food, fuel and batteries.
- Kinetic, the energy of anything moving.
- Gravitational potential, stored in anything lifted up.
- Elastic potential, stored in anything stretched or squashed.
- Thermal, or heat.
- Light and sound.
- Electrical, carried by a current.
Notice how many of them are stored rather than doing anything. A stretched elastic band, a book on a high shelf and a full battery are all holding energy that has not been used yet, and that is what "potential" means.
Transfer chains
A transfer chain is a way of writing down what became what. It is the most commonly asked thing in this topic and it is easy marks once you know the shape of the answer.
Write the energy chain for a ball thrown straight up, at the moment it is highest.
- The energy came from your arm, which is chemical energy from food.
- Throwing gives the ball movement, so chemical becomes kinetic.
- As it rises it slows, so the kinetic is turning into gravitational potential.
- At the top it has stopped for an instant, so almost all of it is stored as height.
chemical to kinetic to gravitational potential
Where the rest goes
No transfer is perfect. A torch bulb is meant to give light and it also gets warm, and that heat was not the point of the torch. Energy that ends up somewhere useless is called wasted energy, though it is not gone, only spread out.
Almost all wasted energy ends up as heat, and friction is usually how. That is the link back to the friction page: every time something rubs, movement is being converted into warmth, and the warmth spreads into the surroundings where nobody can get it back.
Why an old bulb was worse
An old filament bulb turned about 5% of its electrical energy into light and the rest into heat. An LED turns most of it into light. Neither one destroys any energy; the difference is entirely about how much ends up where you wanted it.
Common mistakes
1. Saying energy is used up
Everyday language is against us here: we talk about using up a battery and running out of energy. Nothing was destroyed. The chemical energy in the battery became electrical, then light and heat, and it is now spread thinly through the room.
2. Leaving heat out of every chain
Writing "electrical to light" for a bulb and stopping. Nearly every real transfer produces some heat as well, and mark schemes usually want it. If you cannot think where the wasted energy went, heat is the answer more often than not.
3. Calling electricity a form of energy source
Electricity carries energy, it does not make it. Trace any plug back far enough and you reach burning gas, falling water, wind or sunlight. Answering "electricity" to where energy comes from stops one step too early.
Practice questions
Seven questions. One attempt each, and the explanation appears once the answer is in.
Check yourself
No sign up. Nothing is sent anywhere, the score stays on this device.
What to learn next
A circuit is the neatest example of a transfer chain there is, and it is the one you can build on a kitchen table.
For parents
Pick anything in the room that is switched on and ask for the chain, ending with "and where does the wasted bit go?" A phone charger, a kettle, a lamp. The answer is heat almost every time, and noticing that is worth more than memorising the list of forms.
Still stuck on this?
Send us the question and a teacher will answer it. No charge, and you do not have to be a student here.
What to learn next
Everything in the Learning Hub
Every topic is free and every one ends with practice questions. Start anywhere.
Fractions
Math · 9 topics
What Is a Fraction?Equivalent FractionsSimplifying FractionsComparing FractionsAdding and Subtracting Fractions With the Same DenominatorAdding Fractions With Different DenominatorsFinding a Fraction of a NumberMixed Numbers and Improper FractionsFractions to DecimalsDecimals and percentages
Math · 7 topics
What Are Decimals?Comparing and Ordering DecimalsAdding and Subtracting DecimalsRounding DecimalsWhat Are Percentages?Finding a Percentage of a NumberFractions, Decimals and PercentagesScience: matter and the water cycle
Science · 5 topics
What Is Matter?Solids, Liquids and GasesChanging StateThe Water CycleMixtures and SolutionsWhole numbers
Math · 10 topics
Place Value to 1,000,000Rounding Whole NumbersAdding and Subtracting With RegroupingTimes Tables That Actually StickMultiplying by 10, 100 and 1000Long Multiplication, Step by StepShort DivisionLong Division, Step by StepFactors, Multiples and PrimesOrder of OperationsForces and energy
Science · 6 topics
Forces and MotionFrictionGravity, Mass and WeightLevers and Simple MachinesEnergy and Its FormsSimple CircuitsStuck on this topic? Get one class, free.
A tutor works through it live with your child, one on one, and sends you a short written report afterwards. No card needed.