9702 · 16.2
The first law of thermodynamics flashcards
Revision flashcards for Cambridge 9702 The first law of thermodynamics (syllabus 16.2). Flip, recall, then mark a real past-paper question.
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What is internal energy (U)?
The total sum of the random kinetic and potential energies of all the particles within a body or system.
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How is the average kinetic energy of particles related to temperature?
It is directly proportional to the system's absolute temperature (in Kelvin).
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State the First Law of Thermodynamics.
It states that the change in a system's internal energy (\(\Delta U\)) equals the heat transferred to it (\(Q\)) plus the work done on it (\(W\)).
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Write the formula for the First Law of Thermodynamics.
\(\Delta U = Q + W\)
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What does a positive \(Q\) mean in the First Law equation?
Energy is transferred *into* the system via heating.
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If a gas expands and does work, what is the sign of \(W\) in the equation \(\Delta U = Q + W\)?
\(W\) is negative, as work is done *by* the system (meaning the work done *on* the system is negative).
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What does a negative \(\Delta U\) indicate?
The internal energy of the system has decreased.
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What is the formula for work done by a gas expanding at constant pressure?
Work done = \(p\Delta V\), where \(p\) is the constant pressure and \(\Delta V\) is the change in volume.
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In the equation \(\Delta U = Q + W\), how is \(W\) related to \(p\Delta V\) for an expanding gas?
\(W = -p\Delta V\), because \(p\Delta V\) is work done *by* the gas, while \(W\) in the equation is work done *on* the gas.
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What happens during an adiabatic process?
No heat is transferred to or from the system (\(Q = 0\)). Therefore, the First Law simplifies to \(\Delta U = W\).
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What is an isothermal process?
A process that occurs at a constant temperature. For an ideal gas, this means \(\Delta U = 0\), so the First Law becomes \(Q = -W\).
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What is an isovolumetric (isochoric) process?
A process that occurs at a constant volume (\(\Delta V = 0\)). No work is done (\(W = 0\)), so the First Law becomes \(\Delta U = Q\).
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When is \(W\) positive in \(\Delta U = Q + W\)?
When work is done *on* the system, for example, when a gas is compressed.
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Is internal energy a state function?
Yes, its value depends only on the current physical state of the system, not on how it got there.
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Name two ways to increase a system's internal energy.
By transferring heat *into* it (positive \(Q\)) or by doing work *on* it (positive \(W\)).