Energy in collisions & systems (Sankey/energy transfers)
Practice Flashcards
Flip to reveal answersState the principle of conservation of energy.
Track your progress — Sign up free to save your progress and get smart review reminders based on spaced repetition.
All 12 Flashcards — Energy in collisions & systems (Sankey/energy transfers)
Sign up free to track progress and get spaced-repetition review schedules.
Question
State the principle of conservation of energy.
Answer
Energy cannot be created or destroyed — it is only **transferred** from one store to another. The total amount stays the same.
Question
What does it mean that energy is 'degraded' or 'wasted'?
Answer
It has been transferred to a **less useful** store — almost always **thermal energy (heat)** — that spreads out and can't easily be reused. It is NOT destroyed.
Question
Define efficiency.
Answer
The **useful fraction** of the energy (or power) supplied: η = useful output ÷ total input. It has no unit and is often given as a %.
Question
Formula for efficiency?
Answer
$\eta = \dfrac{\text{useful out}}{\text{total in}}$ — useful energy (or power) out ÷ total energy (or power) in.
Question
What is a Sankey diagram?
Answer
An arrow diagram showing how the input energy splits into useful and wasted branches; the **width** of each arrow shows the amount of energy.
Question
On a Sankey diagram, how do the branch widths relate to the input?
Answer
The useful and wasted branches **add up to the input arrow** — energy is conserved, so nothing is missing.
Question
What form does wasted energy usually take?
Answer
**Thermal energy (heat)** — and sometimes **sound** in moving parts. It spreads into the surroundings.
Question
Can efficiency ever be more than 100%? Why or why not?
Answer
No — the useful output can never be larger than the total input, so efficiency is always between **0 and 1** (0–100%).
Question
How do you find the wasted energy of a machine?
Answer
wasted = total energy in − useful energy out.
Question
A motor takes in 500 J and gives 350 J of useful kinetic energy. Find its efficiency.
Answer
η = useful ÷ total = 350 ÷ 500 = 0.70 = 70%.
Question
A lamp uses 60 J and emits 9 J of light. How much is wasted, and as what?
Answer
Wasted = 60 − 9 = 51 J, transferred as thermal energy (heat).
Question
Why is it wrong to say energy is 'lost' in a machine?
Answer
Because energy is **conserved** — it isn't lost, only **transferred** to a less useful store (heat). The total is unchanged.
Read the notes
Full study notes for Energy in collisions & systems (Sankey/energy transfers)
Topic 1.3 hub
Work, energy and power
More from Topic 1.3
All flashcards in this topic
Physics exam skills
Paper structures & tips
Track your progress with spaced repetition
Sign up free — Aimnova tells you exactly which cards to review and when, so you remember everything before your IB exam.
Start Free