E. Earth and Space Systems · Conservation of Energy and Resources

Energy Changes Form: The Law of Conservation of Energy

E2.2 E2.3 A1.3 A1.5About 45 minutes

Learning goals

I can describe energy transformations in everyday devices.
I can draw an energy chain with arrows.
I can explain the law of conservation of energy: energy cannot be created or destroyed, only changed from one form to another.
I can use the engineering design process to build and improve a marble run.

Words to know

energy transformation
When energy changes from one form to another, like electrical energy changing into light.

energy chain
A list of energy forms joined by arrows that shows how energy changes, step by step.

law of conservation of energy
Energy cannot be created or destroyed. It can only be changed from one form to another (or moved from place to place).

turbine
A machine with blades that spin when wind, water or steam pushes them. Turbines are used to make electricity.

engineering design process
Steps for solving a problem by designing: plan, build, test, and improve.

The amazing changing energy

Flip on a flashlight. Light shines out. But a flashlight has no light inside it when it is off! Where does the light come from?

The energy was there all along, as chemical energy in the batteries. When you switch on the flashlight, that energy changes form. Energy changing from one form to another is called an energy transformation.

A switched-on metal flashlight shining a beam across a wooden table, with two batteries beside it. — Inside a flashlight, chemical energy in the batteries changes into electrical energy, then into light and a little heat. AI-generated image

Energy chains

Scientists show energy transformations with arrows. Each arrow means "changes into".

Flashlight: chemical (battery) → electrical → light (and a little heat)

Some more examples:

Solar panel: light → electrical
Toaster: electrical → thermal (heat) and a little light (the wires glow)
Wind turbine: kinetic (moving air) → kinetic (spinning blades) → electrical
Doorbell: electrical → sound
You, riding a bike: chemical (food) → kinetic (motion) and thermal (you get warm)
Hydroelectric dam: gravitational potential (high water) → kinetic (falling water and spinning turbine) → electrical

Energy chain for a flashlight: chemical energy in the battery changes into electrical energy in the wires, which changes into light (the useful part) and a little heat.

The law of conservation of energy

Here is one of the most important rules in all of science:

Energy cannot be created or destroyed. It can only be transformed (changed) from one form to another.

This is called the law of conservation of energy. Conservation here means "kept": the total amount of energy is always kept the same.

So when your tablet battery runs "out", the energy did not disappear. It was changed into light from the screen, sound from the speaker, and heat (the tablet felt warm). That energy spread out into the room.

When people say they "used up" energy, they really mean they changed it into forms that are hard to use again.

Counting energy with pretend units

Scientists measure energy in units called joules. To keep the math simple, this unit uses pretend energy units. They are made-up numbers that help you see the rule: the energy that goes in always equals the total energy that comes out.

Example: a toy car has 100 units of stored energy. It ends up with 70 units of motion, 20 units of heat and 10 units of sound. 70 + 20 + 10 = 100. Nothing was created or destroyed!

Worked example

Let's trace the energy in a wind turbine

A wind turbine near Lake Huron turns in the breeze and sends electricity to homes. Make an energy chain, then use pretend units to check the law of conservation of energy. Pretend the wind gives the turbine 500 units of energy. 300 units become electrical energy and 150 units become heat in the moving parts. How many units become sound?

Start: wind is moving air, so it has kinetic energy.
The wind pushes the blades, so the blades spin: kinetic energy.
The spinning blades turn a generator, which makes electrical energy.
Energy chain: kinetic (wind) → kinetic (blades) → electrical (plus some heat and sound).
Energy can't be destroyed, so the outputs must add up to 500. 300 + 150 = 450.
500 − 450 = 50 units must be sound (the whooshing of the blades).

Kinetic → kinetic → electrical, and 50 units of sound. 300 + 150 + 50 = 500 units, the same as what went in.

Both bars are the same length. The energy changed form, but none was created or destroyed: 300 + 150 + 50 = 500 units.

Just for fun

You are an energy-changing machine! Right now your body is turning the chemical energy from your breakfast into motion, heat, and maybe even sound energy... like the giggle you're about to make.

Hands-on activity

Design challenge: a paper-tube marble run

The challenge: design a marble run where a marble takes as long as possible to get from the top to the bottom, and goes around at least one turn.

You need: paper towel or toilet paper tubes, cardboard, scissors, masking tape, a wall or the side of a cardboard box, a marble, and a timer.

Use the engineering design process (A1.3):

Plan: sketch your run. Where will the marble start? Where will it turn?
Build: cut tubes in half lengthwise to make open chutes. Tape them to the wall or box, sloping down.
Test: time the marble 3 times.
Improve: change one thing (a gentler slope, an extra turn) and test again. Did the time get longer?
Explain (A1.5): write the energy chain for your marble run. Where does the marble have the most stored energy? The most motion energy? Did you hear any sound? (That's energy changing form too!)

What to look for: an energy chain like gravitational potential (top) → kinetic (rolling) → a little sound and heat. A gentler slope usually makes the marble roll more slowly, so the run takes longer.

Safety: ask a grown-up to help with cutting the tubes. Use masking tape (painter's tape) on walls so the paint doesn't peel. Marbles are a choking hazard: keep them away from babies and young children, and pick them all up when you are done.

Your turn

What is an energy transformation?

State the law of conservation of energy in your own words.

Write the energy chain for a flashlight.

What energy transformation happens in a solar panel?

What energy transformation happens in a toaster?

Write the energy chain when you ride a bike.

More practice: warm-up

Name the transformation: a wind turbine.

Name the transformation: a burning candle.

Name the transformation: a doorbell.

Name the transformation: an electric fan.

True or false: energy can be destroyed.

More practice: on your own

A wind-up toy starts with 50 pretend units of stored energy. 30 units become motion and 15 units become heat. The rest becomes sound. How many units of sound?

Leo says, "My tablet battery is dead, so its energy was destroyed." What would you tell him?

Write the energy chain for a hydroelectric dam.

A hair dryer blows warm air. Name two forms of energy it changes electrical energy into.

A company says its new machine puts out more energy than it takes in. Should you believe it? Why?

Challenge

Trace the energy all the way from the Sun to you riding your bike.

An inventor says his wheel will keep spinning forever without any energy being added. Use what you know to explain why a scientist would doubt this.

A kettle uses 200 pretend units of electrical energy. The water ends up with 150 units of thermal energy. The rest did not disappear. Where could it have gone?

Check your understanding

Quiz

The law of conservation of energy says energy...

What is the energy chain for a solar panel?

In a flashlight, the energy starts as...

A toy has 80 pretend units of stored energy. It ends with 50 units of motion and 20 units of heat. How many units of sound?

An electric fan changes electrical energy mostly into...

When your tablet battery runs out, its energy has...

For parents and teachers

Big idea (E2.2, E2.3): energy is transformed from one form to another, and the law of conservation of energy says it cannot be created or destroyed. The "pretend energy units" are a teaching model, not real measurements; real energy is measured in joules. The marble run uses the engineering design process (A1.3) and asks children to communicate an energy chain (A1.5).

How to help: at home, point to a device and ask, "What goes in? What comes out?" Have your child draw the chain with arrows. Use the pretend-unit questions as quick mental math: the outputs must add up to the input.

Common difficulties:

Saying energy is "used up" or "gone". Gently correct: it changed form and spread out.
Leaving heat and sound out of chains. Almost every transformation also makes a little heat.
Mixing up the start of the chain. Ask "Where was the energy stored before the device was turned on?"

Safety: help with cutting cardboard tubes. Marbles are a choking hazard for young children; count them out and back in.