Study Companion
Topic 07 | Physical sciences

Energy forms & transformations

Year 8 (Levels 7-8 band): the main forms of energy, how energy transfers (conduction, convection, radiation) and transforms, and how to calculate efficiency of a simple system.

45-60 min Printable practice Answer key Challenge included
VC2S8U15
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What you will learn

Worked example 0 Real-world example: a phone charging

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

  1. Electrical energy flows from the power point into the charger.
  2. The charger transforms electrical energy into chemical energy stored in the phone’s battery.
  3. Some energy is unavoidably lost as heat (you can feel the charger is warm).
  4. 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.

FormDescriptionExample
KineticEnergy of motionA moving car, a thrown ball
Gravitational potentialEnergy stored by height above groundA book on a shelf
Elastic potentialEnergy stored by stretching or compressingA drawn bowstring, a spring
Thermal (heat)Random motion of particlesA hot cup of tea
ChemicalEnergy stored in chemical bondsFood, petrol, a battery
ElectricalEnergy of moving chargesCurrent in a wire
Light (radiant)Energy carried by electromagnetic wavesSunlight, torch beam
SoundEnergy of vibrating particles in a mediumA speaker, a guitar
NuclearEnergy stored in atomic nucleiReactors, 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.

  1. (a) Conduction — heat passes directly from hot hotplate to cold pan by contact.
  2. (b) Convection — warm air is less dense, rises, and circulates.
  3. (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:

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.

  1. Starting motor: electrical →\to kinetic to lift the car up.
  2. At the top: the car has maximum gravitational potential energy.
  3. Going down: gravitational PE →\to kinetic energy (car speeds up).
  4. Rising loops: kinetic →\to gravitational PE (car slows).
  5. 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
η=EusefulEtotal input×100%.\eta = \dfrac{E_{\text{useful}}}{E_{\text{total input}}} \times 100\%.
Interpreting the number

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

Worked example 3 Lightbulb efficiency

An old incandescent bulb draws 6060 J of electrical energy per second. Only 33 J per second actually becomes visible light. Find its efficiency.

η=360×100%=5%.\eta = \dfrac{3}{60} \times 100\% = 5\%.

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

Worked example 4 Efficient motor

A motor uses 500500 J of electrical energy and lifts a 22 kg mass to a height where it gains 300300 J of gravitational PE. Find the efficiency and the wasted energy.

  1. Efficiency: η=300500×100%=60%\eta = \dfrac{300}{500} \times 100\% = 60\%.
  2. Wasted energy: 500−300=200500 - 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 200200 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

    1. 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.
    2. Classify each as kinetic or potential: (a) a flying arrow, (b) a drawn bow, (c) water behind a dam, (d) wind.
    3. Name four common forms of energy used in the home.
    4. What form of energy does the Sun mostly deliver to Earth?
    5. Give one example each of sound, light, and chemical energy.
Fluency

Heat transfer

    1. 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.
    2. Which mode of heat transfer can occur in a vacuum?
    3. Why are metal handles on cookware often covered in plastic?
    4. Explain how a convection current forms in a pot of water on a stove.
    5. Which colour absorbs more radiant energy: black or white?
Fluency

Transformations

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

Efficiency

    1. An LED bulb takes 1010 J of electrical energy and produces 44 J of light. Find the efficiency.
    2. A heater uses 10001000 J and delivers 950950 J as heat to the room. Find the efficiency.
    3. A car engine is 25%25\% efficient. If 20002000 J of chemical energy are burned, how much becomes useful kinetic energy?
    4. A motor is only 50%50\% efficient. Where does the other 50%50\% of the energy go?
Problem solving

Applied contexts

    1. A skateboarder at the top of a ramp has 600600 J of gravitational PE. At the bottom, they have 540540 J of kinetic energy. Find the energy “lost” and explain where it went.
    2. A pole-vaulter runs fast and uses a flexible pole to reach a height of 55 m. Trace the energy transformations from the run to the jump.
    3. A solar panel receives 20002000 J of sunlight per minute and outputs 400400 J of electrical energy per minute. Find its efficiency.
    4. Explain why double-glazed windows improve the energy efficiency of a house.

Challenge

Reasoning

Harder reasoning

    1. 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.
    2. A car is travelling at 6060 km/h, then brakes sharply to a stop. The kinetic energy was 200 000200\,000 J. Where did this energy go?
    3. A well-insulated hot-water tank holds 300300 kJ of thermal energy initially and drops to 280280 kJ after 1010 hours of standing. Estimate the efficiency of the insulation over this period and explain your reasoning.
    4. A coal-fired power station has an overall efficiency of about 35%35\%. For every 10001000 MJ of coal energy burned, how much becomes electrical energy, and how is the rest dissipated?
Answers

Answer key

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Year 8 answers

Fluency

Forms of energy

    1. (a) Chemical, (b) elastic potential, (c) kinetic, (d) gravitational potential, (e) nuclear.
    2. (a) Kinetic, (b) potential (elastic), (c) potential (gravitational), (d) kinetic.
    3. Any four of: electrical, chemical, thermal, light, sound, kinetic.
    4. Radiant (light / electromagnetic) energy.
    5. Sound: a speaker. Light: a lamp. Chemical: food or a battery.
Fluency

Heat transfer

    1. (a) Conduction, (b) convection, (c) radiation.
    2. Radiation.
    3. Plastic is a poor conductor (insulator), so the handle stays cool enough to hold while the metal stays hot in the food.
    4. The water at the bottom heats first, expands and becomes less dense, so it rises. Cooler water sinks to the bottom to take its place. This cycle repeats, creating a convection current that spreads heat through the pot.
    5. Black — it absorbs more radiant energy (while white reflects more).
Fluency

Transformations

    1. Chemical (battery) →\to electrical →\to light + heat.
    2. Chemical (petrol) →\to heat (combustion) →\to kinetic (moving pistons/wheels) + sound + heat.
    3. Torch: waste heat (bulb warms). Car: waste heat in engine and exhaust, plus sound.
    4. Energy cannot be created or destroyed, only transferred or transformed from one form to another.
    5. Friction (at the pivot) and air resistance continuously transform kinetic energy into heat. The swing’s motion decreases until it stops.
Reasoning

Efficiency

    1. η=4/10×100%=40%\eta = 4/10 \times 100\% = 40\%.
    2. η=950/1000×100%=95%\eta = 950/1000 \times 100\% = 95\%.
    3. Useful energy =25%×2000=500= 25\% \times 2000 = 500 J.
    4. Into waste heat (mostly), sound, and vibration.
Problem solving

Applied contexts

    1. Lost energy =600−540=60= 600 - 540 = 60 J. It became heat (friction between wheels and ramp, and air resistance) and a small amount of sound.
    2. Chemical (in muscles from food) →\to kinetic (running) →\to elastic potential (bending pole) →\to gravitational potential (height) →\to kinetic (falling back down).
    3. η=400/2000×100%=20%\eta = 400/2000 \times 100\% = 20\%.
    4. Double glazing traps a layer of air between two panes of glass. Air is a poor conductor, so less heat escapes in winter (or enters in summer). Lower heat loss means heaters and coolers run less, saving energy.
Reasoning

Challenge

    1. Real pendulums constantly transfer kinetic energy into heat (friction at the pivot, air resistance) and eventually stop. To run forever they would need a new energy source — which violates conservation of energy because no net energy can appear from nothing.
    2. The 200 000200\,000 J becomes heat in the brake discs and tyres (they get very hot) and a small amount of sound (screeching). Energy has not disappeared — it has spread out as thermal energy.
    3. Useful energy retained =280= 280 kJ out of 300300 kJ, so the insulation kept ∼93%\sim 93\% of the thermal energy over 10 hours. The remaining ∼7%\sim 7\% (20 kJ) leaked out via conduction through the walls and radiation from the surface.
    4. Useful electrical =35%×1000=350= 35\% \times 1000 = 350 MJ. The remaining 650650 MJ is lost mostly as waste heat at the boiler and turbine (typically carried away by cooling water or cooling towers), with smaller losses in the generator and transmission.

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