Energy forms & transformations

What you will learn

the main forms of energy (kinetic, potential, thermal, chemical, etc.),
the three modes of heat transfer: conduction, convection, radiation,
what the law of conservation of energy says,
how to calculate the efficiency of a device,
how to track energy through an everyday chain of transformations.

Worked example 0 Real-world example: a phone charging

Trace the energy transformations when you plug a phone into a wall charger.

Electrical energy flows from the power point into the charger.
The charger transforms electrical energy into chemical energy stored in the phone’s battery.
Some energy is unavoidably lost as heat (you can feel the charger is warm).
Later, when you use the phone, chemical energy becomes electrical, then light (screen), sound (speaker), and more heat.

Key idea: every transformation “leaks” a little energy as heat. Energy is never destroyed — but once it spreads out as heat, it is hard to recover.

1. Forms of energy

Energy comes in many forms. All of them can be converted into one another.

Form	Description	Example
Kinetic	Energy of motion	A moving car, a thrown ball
Gravitational potential	Energy stored by height above ground	A book on a shelf
Elastic potential	Energy stored by stretching or compressing	A drawn bowstring, a spring
Thermal (heat)	Random motion of particles	A hot cup of tea
Chemical	Energy stored in chemical bonds	Food, petrol, a battery
Electrical	Energy of moving charges	Current in a wire
Light (radiant)	Energy carried by electromagnetic waves	Sunlight, torch beam
Sound	Energy of vibrating particles in a medium	A speaker, a guitar
Nuclear	Energy stored in atomic nuclei	Reactors, the Sun

Energies can also be sorted into kinetic (movement now) or potential (stored, ready to move).

2. Energy transfers: conduction, convection, radiation

Conduction — heat moves by direct contact between particles. Best in solids, especially metals. Example: a metal spoon getting hot in soup.

Convection — heat moves with a flowing fluid (liquid or gas). Warm fluid is less dense and rises; cool fluid sinks, creating a current. Example: hot air rising above a radiator.

Radiation — heat transfers as electromagnetic waves (infrared light). It needs no medium and can travel through vacuum. Example: the Sun warming Earth.

Worked example 1 Which kind of heat transfer?

Identify the main mode of heat transfer in each: (a) a saucepan heating on a stove; (b) warm air filling a room; (c) heat from a campfire on your face across the fire.

(a) Conduction — heat passes directly from hot hotplate to cold pan by contact.
(b) Convection — warm air is less dense, rises, and circulates.
(c) Radiation — infrared waves carry energy from the flames to your skin without moving air.

Key idea: solids = conduction; fluids = convection; across empty space = radiation.

3. Conservation of energy and transformations

The Law of Conservation of Energy says energy cannot be created or destroyed, only transformed. The total energy in a closed system is constant.

When tracing a chain of transformations, write them in a flow diagram:

Torch: chemical (battery) $\to$ electrical $\to$ light + heat.
Car engine: chemical (fuel) $\to$ heat $\to$ kinetic (moving car) + sound + heat.
Hair dryer: electrical $\to$ kinetic (fan) + heat + sound.
Plant: light (sun) $\to$ chemical (glucose).

Worked example 2 Tracking energy in a roller coaster

A roller coaster car is lifted to the top of the first hill, released, and then coasts through loops. Describe the energy transformations.

Starting motor: electrical $\to$ kinetic to lift the car up.
At the top: the car has maximum gravitational potential energy.
Going down: gravitational PE $\to$ kinetic energy (car speeds up).
Rising loops: kinetic $\to$ gravitational PE (car slows).
Throughout: a small amount of energy becomes heat (from friction) and sound (rattling).

Key idea: the coaster never reaches the height it started at because friction constantly drains kinetic energy into heat.

4. Energy efficiency

Most devices transform energy with some loss — usually as waste heat. Efficiency is the fraction of input energy that becomes useful output.

Efficiency

Efficiency as a percentage

\eta = \dfrac{E_{\text{useful}}}{E_{\text{total input}}} \times 100\%.

Interpreting the number

An efficiency of $80\%$ means $80\%$ of the input becomes useful energy and $20\%$ is wasted (usually as heat).

Worked example 3 Lightbulb efficiency

An old incandescent bulb draws $60$ J of electrical energy per second. Only $3$ J per second actually becomes visible light. Find its efficiency.

\eta = \dfrac{3}{60} \times 100\% = 5\%.

$95\%$ of the input is wasted as heat — that is why old bulbs got hot. Modern LEDs deliver the same light output from only $\sim 8$ W of electrical power (about $40\%$ efficient).

Worked example 4 Efficient motor

A motor uses $500$ J of electrical energy and lifts a $2$ kg mass to a height where it gains $300$ J of gravitational PE. Find the efficiency and the wasted energy.

Efficiency: $\eta = \dfrac{300}{500} \times 100\% = 60\%$ .
Wasted energy: $500 - 300 = 200$ J (mostly heat and sound in the motor).

5. Where does “lost” energy go?

Energy is never really lost. In a motor, that $200$ J goes into heating the bearings, vibrating the frame (sound), and warming the air. In a bulb, “lost” energy heats the room. This is why engineers work so hard on reducing friction, insulation losses and waste heat — efficient devices cost less to run and waste fewer resources.

Practice: Year 8

Fluency

Forms of energy

Name the energy form stored in (a) a battery, (b) a stretched spring, (c) a moving car, (d) a book on a shelf, (e) a piece of uranium.
Classify each as kinetic or potential: (a) a flying arrow, (b) a drawn bow, (c) water behind a dam, (d) wind.
Name four common forms of energy used in the home.
What form of energy does the Sun mostly deliver to Earth?
Give one example each of sound, light, and chemical energy.

Fluency

Heat transfer

Match the example to conduction, convection or radiation: (a) a metal bar heated at one end; (b) a hot-air balloon rising; (c) the warmth of a fireplace on your face.
Which mode of heat transfer can occur in a vacuum?
Why are metal handles on cookware often covered in plastic?
Explain how a convection current forms in a pot of water on a stove.
Which colour absorbs more radiant energy: black or white?

Fluency

Transformations

Trace the energy transformations in a torch from battery to light.
Trace the transformations in a petrol car from fuel to motion.
State one unavoidable “waste” energy in each of the examples above.
State the law of conservation of energy in one sentence.
Explain why a swing eventually stops without a push.

Reasoning

Efficiency

An LED bulb takes $10$ J of electrical energy and produces $4$ J of light. Find the efficiency.
A heater uses $1000$ J and delivers $950$ J as heat to the room. Find the efficiency.
A car engine is $25\%$ efficient. If $2000$ J of chemical energy are burned, how much becomes useful kinetic energy?
A motor is only $50\%$ efficient. Where does the other $50\%$ of the energy go?

Problem solving

Applied contexts

A skateboarder at the top of a ramp has $600$ J of gravitational PE. At the bottom, they have $540$ J of kinetic energy. Find the energy “lost” and explain where it went.
A pole-vaulter runs fast and uses a flexible pole to reach a height of $5$ m. Trace the energy transformations from the run to the jump.
A solar panel receives $2000$ J of sunlight per minute and outputs $400$ J of electrical energy per minute. Find its efficiency.
Explain why double-glazed windows improve the energy efficiency of a house.

Challenge

Reasoning

Harder reasoning

A student claims a pendulum is a “perpetual motion machine” because it keeps swinging. Explain, using conservation of energy, why no such machine can exist.
A car is travelling at $60$ km/h, then brakes sharply to a stop. The kinetic energy was $200\,000$ J. Where did this energy go?
A well-insulated hot-water tank holds $300$ kJ of thermal energy initially and drops to $280$ kJ after $10$ hours of standing. Estimate the efficiency of the insulation over this period and explain your reasoning.
A coal-fired power station has an overall efficiency of about $35\%$ . For every $1000$ MJ of coal energy burned, how much becomes electrical energy, and how is the rest dissipated?