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All 12 Flashcards — First law of thermodynamics
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Question
Define internal energy U of a gas.
Answer
The **total energy of all the particles**: their random **kinetic energy** + the **potential energy** of the forces between them.
Question
What does the internal energy of an **ideal gas** depend on?
Answer
**Temperature only** — an ideal gas has no inter-particle PE, so U is fixed by the random KE of the particles.
Question
State the first law of thermodynamics.
Answer
$Q = \Delta U + W$ — the heat **added** equals the rise in **internal energy** plus the **work done by** the gas.
Question
Rearrange the first law for ΔU.
Answer
$\Delta U = Q - W$ (heat in **minus** work done by the gas).
Question
In Q = ΔU + W, what is the sign of Q when heat is **removed**?
Answer
**Negative** — Q is the heat **added** to the gas, so heat leaving makes Q < 0.
Question
In Q = ΔU + W, what is the sign of W when the gas is **compressed**?
Answer
**Negative** — W is the work done **by** the gas; on compression the surroundings do work on it, so W < 0.
Question
Work done by a gas at constant pressure?
Answer
$W = P\,\Delta V$ — pressure times the change in volume.
Question
Units for W = PΔV?
Answer
P in **pascals (Pa)**, ΔV in **cubic metres (m³)**, giving W in **joules (J)**.
Question
Internal energy vs heat — what's the difference?
Answer
**Internal energy** is energy a gas **already has** inside; **heat** is energy **flowing** in or out due to a temperature difference.
Question
For an ideal gas at **constant temperature**, what is ΔU?
Answer
**ΔU = 0** — U depends on temperature alone, so no temperature change means no change in internal energy.
Question
500 J heat added, gas does 200 J work — find ΔU.
Answer
$\Delta U = Q - W = 500 - 200 = 300$ J (the gas warms).
Question
Quick way to handle the signs in the first law?
Answer
Write each sign in **words** first ('heat removed → Q negative', 'gas compressed → W negative'), then plug into $\Delta U = Q - W$.
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Full study notes for First law of thermodynamics
Topic 2.4 hub
Thermodynamics (HL)
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