GCSE · Physics · Edexcel · Spec 1PH0
Closed system - no net change
Move money between pockets in a sealed account and the total never changes. Energy can do the same, if you draw the boundary right.
Physics · Energy
Is the system closed?
Pick a system, then pick a box. Does any energy enter or leave across its boundary?
Still to sort
Closed — no energy enters or leaves (0)
Energy can move between stores inside the boundary, but nothing crosses it.
Where the line is: One transfer across the boundary, in or out, and it is no longer closed.
Not closed — energy is transferred to or from the surroundings (0)
At least one transfer crosses the boundary, in or out.
Where the line is: Energy moving around inside the boundary does not count. Only a crossing does.
Every card tells you where the boundary is. Ask one question: does any energy cross it?
Balance the books
Problem
A toy car rolls down a ramp inside a sealed, insulated box. The system is the car, the ramp, the box and the air inside it. Illustrative numbers: the car's gravitational potential store decreases by 12 J as it rolls to the bottom. At that moment its kinetic store has increased by 8 J. Show that the total energy of the system has not changed.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- A system is an object, or a group of objects, that you choose to study.
- Energy can be transferred between the energy stores of the objects inside the system.
- A closed system has no energy entering it and none leaving it. There is no transfer with the surroundings.
Have a goYour friend says a radiator is a closed system because 'it just sits there on the wall'. What should you ask about the energy before agreeing?
Does any energy enter or leave? A hot radiator transfers energy to the room, so it is not closed.
Sitting still is not the test. Closed means no energy transfer between the system and its surroundings.
- You pick the system, so the same situation can be closed or not depending on where you draw the boundary.
- Energy cannot be created or destroyed. It can only be transferred from one store to another.
- In a closed system there is no net change in total energy. A decrease in one store equals the total increase in the others.
Have a goA closed system's gravitational store falls by 30 J. The kinetic store gains 21 J, and the only other store that changes is thermal. How much does thermal gain?
9 J
The gains must add up to the 30 J decrease, so 21 J + thermal = 30 J. The leftover energy has not vanished, it is in the thermal store.
- Energy moved to a less useful store, such as heating parts of the system, is still in the total.
- So the useful energy may decrease while the total energy stays exactly the same.
Have a goIn a closed system the useful energy drops from 50 J to 20 J. Does the total drop by 30 J too? Say where the 30 J is.
No. The total is unchanged. The 30 J has moved to less useful stores inside the system, like heating.
Less useful energy is still inside the system, so it still counts. The useful part falls but the total does not.
The big picture
A system is whatever you choose to study. If no energy enters or leaves it, it is closed. Then energy can only move between stores inside it, so the total does not change, even when some of the energy ends up in a less useful store like heating.
Key points
Worked example
Problem
A kettle and the water in it are chosen as the system. The kettle is plugged in and switched on. Is this a closed system, and can we assume its total energy stays the same?
⚠ Watch out
Thinking energy that heats parts of the system is lost, so the total falls. The useful energy falls, but the heated store is still inside the system and still counts, so the total is unchanged.
Memory hook
Sealed account: shuffle money between pockets and the total never changes. Even the money in the 'junk drawer' pocket is still yours, just less useful.
Check yourself
A closed system's store loses 100 J and another gains 70 J. Your friend says 30 J has vanished. Are they right? Explain how you know.
Flashcards
(8)What is a system in physics?
What can happen to energy inside a system?
What is a closed system?
Why can the same situation be closed or not closed?
Conservation of energy: what is the rule?
What happens to the total energy of a closed system?
Energy is transferred to heating parts of a closed system. Is it still in the total?
A closed system has one store that decreases by 20 J. What is the total increase in the other stores?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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