GCSE · Physics · Edexcel · Spec 1PH0

Closed system - no net change to total energy

A ball falls and speeds up. Its gravitational potential store is emptying, so where is all that energy going?

A falling object: where does the energy go?

Stage 1 of 5: Released. Total energy in the system: 100 J. paused

Released · 1/5Drag along the fall and check each stage: what was lost equals what was gained.

Gravitational potential 100 J. Kinetic 0 J. Thermal 0 J. Total 100 J.

Scrub down the fall. Illustrative numbers for a system holding 100 J. They follow the balance rule exactly, but they are not measured data.

Physics · Energy

Closed, or not closed?

Sort each situation. Pick it, then pick a box, and read why.

Still to sort

Closed: total is fixed (0)

No energy crosses the boundary, so energy only moves between stores inside.

Where the line is: If nothing crosses the edge, the system is closed. Stores can change as much as they like inside it.

Not closed: total can change (0)

Energy crosses the boundary, for example through heating from outside or work done by an external force.

Where the line is: If anything can carry energy across the edge, the total can go up or down.

6 of 6 still to sort.

The answer depends on where the edge of the system is drawn. Ask one question each time: is any energy crossing that edge?

Physics · Energy

Where does dissipated energy go?

A ball falls inside a closed system. As its gravitational potential store decreases, a bit of the energy is dissipated, warming parts of the system.

Which is closest to what you think happens to that dissipated energy?

It's used up. Once energy is dissipated it's gone, so the total is a bit smaller.It leaves the system for good, so the system's total falls.It's still around, but it doesn't count. Only the useful energy counts towards the total.It's in a thermal store inside the system. Less useful, but still counted, so the total is unchanged.

How sure are you?

I'm sureI think soI'm guessing

It's used up. Once energy is dissipated it's gone, so the total is a bit smaller.

This is a common misconception

This is the big one, and it's easy to see why it feels right: dissipated energy is no use to you any more. But 'no use' is not the same as 'gone'. Energy is never made or destroyed, only moved from one store to another, and dissipating is just one more move. It's now warming parts of the system.

It leaves the system for good, so the system's total falls.

This is a common misconception

Mixing up 'dissipated' with 'crossed the boundary'. In a closed system, no energy crosses the boundary at all. The dissipated energy warms parts of the system, so it is still inside it and still in the total.

It's still around, but it doesn't count. Only the useful energy counts towards the total.

This is a common misconception

Half right: it is still around. But the total counts every store in the system, useful or not. Dissipated energy is less useful, but it still counts towards the total.

It's in a thermal store inside the system. Less useful, but still counted, so the total is unchanged.

This matches the evidence

That's the idea. The decrease in the gravitational potential store equals the increase in the kinetic store plus the increase in the thermal stores. Nothing is missing, so the total is unchanged.

Dissipated energy is less useful, not lost. It still counts towards the total.

Physics · Energy

Find the slip

A ball falls in a closed system. Its gravitational potential store decreases by 40 J. Its kinetic store increases by 32 J and its thermal stores increase by 8 J. A student writes down their reasoning. One line goes wrong. Which?

A student's answer — which line goes wrong?

The gravitational potential store decreases by 40 J.
The kinetic store increases by 32 J and the thermal stores increase by 8 J.
Two stores have gone up, so the system must now hold more energy than it started with.
So 40 J of energy has been created.

Physics · Energy

Now you explain it

A pendulum swings from its highest point down to its lowest point. It is a closed system. Explain the energy changes. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The gravitational potential store decreases. (1 mark)
The kinetic store increases. (1 mark)
Some energy is dissipated, warming parts of the system, so a thermal store increases and the energy is still inside the system. (1 mark)
The decreases and the increases balance, so the total energy stays the same. (1 mark)

Model answer

As the pendulum swings down, its gravitational potential store decreases and its kinetic store increases. Some energy is dissipated, warming parts of the system, so a thermal store also increases. The energy dissipated is still in the system. The decrease in the gravitational potential store is balanced by the increases in the kinetic and thermal stores, so the total energy is unchanged.

A full explanation names what goes down, names what goes up, and says the changes balance.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Keep your eye on the total. It's the one thing that never moves.

What you need to know

A closed system is an object, or objects considered together, that exchanges no energy with anything outside it.
So any energy transfers happen only between stores inside the system.
The total energy stays the same: whatever one store loses, the other stores gain by exactly the same amount.
That's the principle of conservation of energy: energy is never made or destroyed, only moved from one store to another.
Have a goSam is sure that when one store in a closed system loses 30 J, the 30 J has simply vanished. How much do the other stores gain between them, and is Sam right?
30 J between them. Sam is wrong.
In a closed system, whatever one store loses the other stores gain by exactly the same amount, so nothing has vanished.
Some energy may be dissipated, for example by warming parts of the system.
Dissipated energy is less useful, but it still counts towards the total, so the overall total is unchanged.
Have a goIn a closed system one store loses 50 J. One store gains 45 J, and 5 J is dissipated, warming part of the system. How many joules do the stores inside the system gain altogether?
50 J (45 J + 5 J).
The dissipated 5 J is less useful but still inside the system, so it counts in the total and the changes balance.
To explain a change, name the stores that decrease, name the stores that increase, and say the decreases and increases balance.
Falling object: the gravitational potential store decreases, the kinetic store increases, and any dissipated energy sits in thermal stores inside the system.
A system that is not closed can gain or lose energy across its boundary, so its total can go up or down.
Heating from outside and work done by an external force are two ways energy can cross a boundary.
Have a goWhich of these is NOT energy crossing a boundary: heating from outside, work done by an external force, or one store inside the system gaining what another store inside it lost?
The third one: one store gaining what another store inside lost.
That is energy moving between stores inside the system, so it doesn't cross the boundary and doesn't change the total.

The big picture

In a closed system, energy only moves between stores, so the total never changes. Whatever one store loses, the other stores gain by exactly the same amount, and that includes energy that gets dissipated. A system that isn't closed can gain or lose energy across its boundary, so its total can change.

Key points

1Closed system: no energy crosses its boundary, so transfers happen only between stores inside it.

2In a closed system the total energy stays the same, because decreases and increases balance exactly.

3Dissipated energy is less useful but still counts towards the total.

4A system that isn't closed can gain or lose energy across its boundary, so its total can change.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

A trolley rolls down a ramp. Take the trolley, ramp and Earth together as a closed system. The trolley's kinetic store increases by 36 J and its thermal stores increase by 4 J. By how much does the gravitational potential store decrease?

⚠ Watch out

Adding up only the 'useful' increases and then announcing that some energy has gone missing. Count every store that increases, thermal ones included, and the sums balance.

🧠

Memory hook

A closed system is a locked cash box. Money moves between the tins inside, nothing goes in or out, so the total on the lid never changes. Dissipated energy is the tin that's hard to spend, but it's still in the box.

✓

Check yourself

Cover the page. One store loses 80 J and two others gain, one by 55 J. How much does the other gain? (25 J, and the total is unchanged.)

Flashcards

(8)

What is a closed system?

An object, or a set of objects considered together, that exchanges no energy with anything outside it.

How does the energy lost by one store compare with what the other stores gain?

It is exactly the same amount, so the total energy stays the same.

State the principle of conservation of energy.

Energy is never made or destroyed. It is only moved from one store to another (including being dissipated).

What happens to dissipated energy inside a closed system?

It warms parts of the system. It is less useful, but it still counts towards the total, so the total is unchanged.

An object falls in a closed system. Which stores decrease and increase?

The gravitational potential store decreases and the kinetic store increases, with any dissipated energy in thermal stores still inside the system.

What three things go in an explanation of energy changes in a closed system?

The stores that decrease, the stores that increase, and a statement that the decreases and increases balance.

What can happen to the total energy of a system that is not closed?

It can go up or down, because energy can cross the boundary.

Give two ways energy can cross a system's boundary.

Heating from outside, and work done by an external force.

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.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

A falling object: where does the energy go?

Closed, or not closed?

Where does dissipated energy go?

Find the slip

Now you explain it

More Edexcel GCSE Physics topics