MYP Physics

E.1 Energy Forms and Transfers

Investigating different forms of energy, such as kinetic, potential, and chemical energy, and how they transform.

Questions to explore

? Where does energy go when it seems to disappear?
? How efficient are the machines we depend on every day?

💡 Key ideas, explained simply

🔋 Energy is never lost — only moved

The law of conservation of energy says energy cannot be created or destroyed, only transferred between stores. When you drop a ball, gravitational store → kinetic store. When it hits the ground, kinetic → heat and sound.

Adding up all the energy before and after always gives the same total.

⚙️ Useful vs wasted energy

No machine is perfect. Some input energy always ends up in a store you did not want — usually heat from friction. Efficiency measures the fraction that becomes useful:

$\text{efficiency} = \dfrac{\text{useful energy out}}{\text{total energy in}} \times 100\%$

A Sankey diagram for a filament lamp: the arrow width shows how much energy goes each way. Only 10% becomes useful light; 90% is wasted as heat.

📖 Key terms

Energy store: Where energy is held: kinetic, gravitational, elastic, chemical, thermal, nuclear.
Kinetic energy: Energy of a moving object, $E_k = \frac{1}{2}mv^2$ .
Gravitational PE: Energy from height, $E_p = mgh$ .
Conservation of energy: Total energy stays constant; it is only transferred, never lost.
Efficiency: Useful energy out ÷ total energy in, as a percentage.
Dissipation: Energy spreading out uselessly, usually as heat.

1 Key Formulas

Kinetic Energy

E_k = \frac{1}{2}mv^2

Gravitational Potential Energy

E_p = mgh

✏️ Worked example

Kinetic energy

A $0.5\text{ kg}$ ball moves at $4\text{ m/s}$ . Calculate its kinetic energy.

1

Write the formula

$E_k = \tfrac{1}{2}mv^2$
2

Substitute

$E_k = \tfrac{1}{2} \times 0.5 \times 4^2 = 0.25 \times 16$

✓

Eₖ = 4 J. Note the v² term: doubling the speed would give four times the energy.

🔬

Interactive Lab

Heat Engine Lab

Watch energy transfer from a hot store into useful work — and see where it is wasted.

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💡 Physics around you

An old filament bulb is only about 10% efficient — most of the electricity becomes heat, not light. An LED is over 80% efficient, which is why swapping bulbs saves so much energy on your electricity bill.

🎯 Nail it in the exam

Energy Transfers and Sankey Diagrams

Energy transfers are the backbone of MYP energy questions. You must be able to identify useful and wasted (dissipated) energy for any device.

Sankey diagrams: The width of the arrows is proportional to the amount of energy. Useful energy goes right, wasted energy goes down.

Efficiency ( $\eta$ ): $\eta = \frac{\text{Useful output energy}}{\text{Total input energy}} \times 100\%$ , or $\eta = \frac{\text{Useful power output}}{\text{Total power input}} \times 100\%$ .

Common exam command: *Calculate the efficiency* → show working and express as a percentage.

Pro Exam Strategy

No real device is 100% efficient; always some energy is dissipated as thermal energy.
When analysing a roller coaster or pendulum, ignore friction unless stated – then use conservation of energy to find speeds or heights.
In ‘describe’ questions, always name the energy type at each stage (e.g., ‘chemical energy in the battery → electrical energy in the circuit → light + thermal energy in the bulb’).

Work and Power (Often Combined with Energy)

Work done = force × distance moved in the direction of the force ( $W = Fd$ ). 1 joule = 1 newton·metre.

Power = work done / time taken ( $P = W/t$ ) or energy transferred / time.

Many past papers combine work done against gravity ( $mgh$ ) with power calculations. Always check the time given and the mass.

Pro Exam Strategy

If a force is perpendicular to the movement (e.g., holding a book still), no work is done.
Use g = 10 m/s² unless 9.8 is specified – MYP often allows 10 for simplicity.
Write the formula, rearrange if needed, and always include units in the final answer.

🧠 Check your understanding

Tap an answer to see if you're right — and why.

Q1. Which of the following is a form of stored (potential) energy?

Light energy
Sound energy
Chemical energy
Kinetic energy

Q2. An object of mass 2 kg is raised to a height of 5 meters. What is its gravitational potential energy? (Take g = 9.8 m/s²)

$10\, J$
$49\, J$
$98\, J$
$196\, J$

📝 Exam-style questions

Try each one, then reveal the model answer.