D. Energy and Society · Work, Energy and Power

Forms of Energy and Energy Transformations

D3.1 D2.1 D3.3About 65 minutes

Learning goals

I can name and describe the main forms of energy, including kinetic, gravitational potential, thermal, chemical, electrical, radiant, sound and nuclear energy.
I can tell the difference between an energy transfer and an energy transformation.
I can describe the energy transformations in everyday devices and natural events with an energy flow chain.
I can state the law of conservation of energy and use it to find a missing amount of energy.

Words to know

energy
The ability to do work, that is, to cause a change such as moving, heating or lighting something. Energy is a scalar measured in joules (J).

joule (J)
The SI unit of energy and of work. 1 J is the work done by a 1 N force pushing something 1 m along the line of the force (1 J = 1 N·m).

kinetic energy (Ek)
The energy an object has because it is moving.

gravitational potential energy (Eg)
The energy an object has because of its height above a reference level.

mechanical energy
The sum of an object's kinetic energy and potential energy.

thermal energy
The total kinetic and potential energy of the particles in a substance. Hotter objects of the same material and mass have more thermal energy.

energy transfer
Energy moving from one object to another, without changing form (for example, a moving puck passing kinetic energy to another puck).

energy transformation
Energy changing from one form to another (for example, chemical energy in gasoline becoming kinetic energy of a car).

law of conservation of energy
Energy cannot be created or destroyed. It can only be transferred or transformed, so the total energy of an isolated system stays the same.

Where did the energy go?

On a cold January morning a family heats the house with natural gas, makes toast, drives to the rink and watches a slapshot rattle off the crossbar. Every one of those events is energy changing form. The gas becomes warm air. The toaster turns electrical energy into thermal energy. The car turns chemical energy into motion. The slapshot turns the energy in a player's muscles into a puck that can fly faster than highway traffic.

This unit is about how physicists keep track of energy: how it is transferred by doing work, how fast it moves (power), how much of it ends up useful (efficiency), and the rule that never breaks: energy is conserved. We start by naming the forms of energy and following energy through a chain of changes.

Remember from Grades 8 and 9

In Grade 8 (Systems in Action) you calculated work = force × distance and the efficiency of machines. In Grade 9 (Electricity) you followed energy transformations in electrical devices, calculated P = E ÷ t and kilowatt-hours, drew Sankey diagrams and compared the ways Ontario produces electrical energy. In SPH3U you will put all of this on a firm footing with equations for kinetic energy, gravitational potential energy, work, power and efficiency, and use the law of conservation of energy to solve problems.

What is energy?

Energy is the ability to do work, which means the ability to cause a change: to make something move, speed up, heat up, light up or make a sound. Energy is a scalar: it has a size but no direction. Its SI unit is the joule (J). One joule is small. Lifting a 100 g apple up 1 m takes about 1 J. So we often use kilojoules (1 kJ = 10³ J) and megajoules (1 MJ = 10⁶ J).

Energy comes in two big families:

Energy of motion. Kinetic energy of an object, thermal energy (the random motion of particles), sound (vibrating particles) and electrical energy (moving charges).
Stored (potential) energy. Gravitational potential energy (height), elastic potential energy (a stretched or compressed spring, a bent hockey stick), chemical potential energy (stored in the bonds of food, fuel and batteries) and nuclear energy (stored in the nucleus of an atom).

Radiant energy (light, infrared, microwaves) is energy carried by electromagnetic waves. It can travel through empty space, which is how the Sun's energy reaches Earth.

Form	What it depends on	Canadian example
Kinetic (Ek)	mass and speed	a snowmobile crossing a frozen lake
Gravitational potential (Eg)	mass and height above a reference level	water behind a dam on the Niagara River
Elastic potential	how far an elastic object is stretched or bent	a hockey stick flexing during a slapshot
Chemical potential	the bonds in a substance	maple syrup, gasoline, a phone battery
Thermal	temperature, mass and material	a mug of hot chocolate
Electrical	moving charges	current in a toaster element
Radiant	electromagnetic waves	sunlight on a solar panel
Sound	vibrations of particles	a goal horn at the arena
Nuclear	the forces holding a nucleus together	uranium fuel in an Ontario nuclear station

Forms of energy. Kinetic plus potential energy is called mechanical energy.

Transfer or transformation?

These two words sound alike but mean different things.

An energy transfer moves energy from one object to another without changing its form. When a curling stone hits another stone, kinetic energy is transferred to the second stone. When a hot pan warms your hand, thermal energy is transferred to you. (A transfer of thermal energy because of a temperature difference is called heat.)
An energy transformation changes energy from one form to another. A toaster transforms electrical energy into thermal energy. Your muscles transform chemical energy into kinetic energy.

Most real events include both. We show them with an energy flow chain: boxes for each form, arrows for each change, and side arrows for energy that becomes a form we did not want (usually thermal energy and sound).

An energy flow chain for a hydroelectric station. Each arrow is a transformation or transfer. The dashed arrows show energy that leaves as thermal energy and sound. None of it is destroyed.

The law of conservation of energy

Scientists have measured energy changes in countless experiments, from steam engines to particle accelerators, and have never found a case where energy appears from nothing or vanishes. This is the law of conservation of energy:

Energy cannot be created or destroyed. It can only be transferred from one object to another or transformed from one form to another.

In a closed (isolated) system, where no energy enters or leaves, the total energy stays constant. For any device or event:

total energy in = useful energy out + wasted energy out

When people say energy was "lost", they mean it became a form that is hard to use, usually thermal energy spread out into the surroundings. It still exists. In later lessons of this unit you will give numbers to every box in a chain like the one above.

A ball falls from 10 m with air resistance ignored. Its gravitational potential energy (Eg) becomes kinetic energy (Ek), but the total stays 100 J at every height.

Worked example

Example 1: The energy chain of a slapshot

A hockey player takes a slapshot. Describe the energy transformations and transfers from the player's breakfast to the puck flying toward the net.

Start with the stored form: the food the player ate holds chemical potential energy.
Muscles transform chemical energy into kinetic energy of the arms and stick (and a lot of thermal energy, which is why the player warms up).
The stick hits the ice behind the puck and bends. Some kinetic energy is transformed into elastic potential energy of the flexed stick.
The stick springs back. Elastic potential energy is transformed back into kinetic energy, which is transferred to the puck.
Along the way some energy becomes sound (the crack of the shot) and thermal energy (stick, puck and ice warm very slightly).

Chemical → kinetic → elastic potential → kinetic (stick) → kinetic (puck), with some energy becoming sound and thermal energy at each step.

Worked example

Example 2: Using conservation to find a missing amount

In 10 s a toaster takes in 12 000 J of electrical energy. It gives out 11 200 J of thermal energy to the bread and the air inside, and 650 J of radiant energy (the orange glow). The rest leaves as thermal energy through the case. How much energy leaves through the case?

Energy is conserved, so total in = total out.
12 000 J = 11 200 J + 650 J + E(case)
E(case) = 12 000 J − 11 200 J − 650 J = 150 J

150 J of thermal energy leaves through the case. The total out (11 200 + 650 + 150) is exactly the 12 000 J that went in.

Just for fun

A physics student was told to clean their room. "I can't," they said. "The mess can't be destroyed. It can only be transformed from one form to another." (Nice try. Moving the mess to the closet is just an energy transfer.)

Hands-on activity

Energy detective at home

You need: a notebook, a pencil and a small rubber ball.

Walk through your home and find eight things that transform energy: for example the stove, a phone charger, a fridge, a lamp, a radio, a furnace, a bicycle and yourself.
For each one, draw a short energy flow chain: the form going in, the useful form coming out, and the wasted forms (side arrows).
Hold the ball at shoulder height and drop it onto a hard floor. Watch how high it bounces, then how high the next bounce is.
Write two sentences: where did the ball's energy go on each bounce, and was any energy destroyed?

What to look for: a lamp might be electrical → radiant (useful) + thermal (wasted); a furnace chemical → thermal; a phone charger electrical → chemical (in the battery) + thermal. The ball bounces lower each time because, on every bounce, some of its energy is transformed into thermal energy (the ball and floor warm a tiny bit) and sound. None is destroyed; it becomes forms that do not lift the ball.

Your turn

Name the form of energy stored in: (a) a stretched elastic band, (b) a granola bar, (c) a snowball sitting on a roof.

Is each one a transfer or a transformation? (a) A moving curling stone hits a stationary stone and stops, and the second stone moves off. (b) A solar panel charges a battery.

Draw (in words) the energy flow chain for a gas barbecue.

A kettle takes in 180 000 J of electrical energy. 162 000 J heats the water. How much energy goes elsewhere, and what form is it mostly?

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

What is mechanical energy?

Why do people say a car "loses" energy when it brakes, and what really happens to it?

More practice: warm-up

What is the SI unit of energy?

How many joules are in 4.5 MJ?

What form of energy does a moving snowmobile have because it is moving?

Name the useful energy output of a ceiling fan.

Is energy a vector or a scalar?

More practice: on your own

Write an energy flow chain for a person riding a bicycle up a hill.

A phone battery stores 40 000 J. A call uses all of it: 2000 J becomes sound and radiant energy from the screen, and the rest becomes thermal energy. How much thermal energy is produced? Explain.

A friend says a battery that is "dead" has had its energy destroyed. Correct the statement.

Explain why a bouncing basketball eventually stops bouncing.

Sort these into energy of motion or stored energy: thermal, chemical, sound, nuclear, gravitational, electrical.

Challenge

In a hydroelectric station, 1.00 × 10⁸ J of gravitational potential energy of water becomes 9.0 × 10⁷ J of electrical energy. The rest becomes thermal energy and sound. (a) How much energy is not electrical? (b) What fraction of the input becomes electrical energy?

A student claims to have invented a machine that, once started, runs forever and also powers a light. Use the law of conservation of energy to explain why this cannot work.

A car travels at a steady speed on a flat highway. Its engine is burning fuel the whole time, yet its kinetic energy is not increasing. Where is the fuel's chemical energy going?

Check your understanding

Quiz

Which statement is the law of conservation of energy?

A curling stone slides into another stone and stops; the second stone slides away. This is mainly an example of:

Which pair adds to give mechanical energy?

What is the main useful energy transformation in a solar panel?

A motor takes in 500 J and gives out 380 J of kinetic energy. The rest is thermal energy and sound. How much is that?

When a ball bounces lower each time, the missing mechanical energy has:

For parents and teachers

Big idea: This first lesson sets up the vocabulary of the unit (D2.1) and the law of conservation of energy, which every later calculation depends on (D3.1).

How to help: Point to everyday devices and ask "What goes in? What useful form comes out? What is wasted?" The furnace, the car, the kettle and the TV all make good examples. Ask your teen to say transfer or transformation each time.

Common difficulties:

Saying energy is "used up" or "lost". Push for the scientific version: it became thermal energy (or sound) in the surroundings.
Mixing up transfer (same form, different object) and transformation (different form).
Forgetting that heat is a transfer of thermal energy caused by a temperature difference, not a form of energy stored in an object. The Thermal and Nuclear Energy unit develops this fully.

Safety: the bouncing-ball activity is safe; keep it away from breakable things.