Non-flow processes (A-level only) Flashcards – AQA A-level Physics (7408)

For a closed system, what is the first-law equation when $Q$ is energy transferred to the system by heating and $W$ is work done by the system?

$Q = \Delta U + W$, where $\Delta U$ is the change in internal energy.

Under the convention that $W$ is work done by a system, what sign does $W$ have for work done on the system?

Negative.

In non-flow processes, how does the conservation of energy compare with the amounts transferred as heating and work?

Energy is conserved, while the amounts transferred as heating and work depend on the process.

What remains constant during an isothermal change?

Temperature.

What equation relates the pressure, volume, amount and absolute temperature of an ideal gas?

$pV=nRT$, where $p$ is pressure, $V$ is volume, $n$ is amount of gas, $R$ is the molar gas constant and $T$ is absolute temperature.

How are pressure and volume related during an isothermal change of a fixed amount of ideal gas?

$pV=\text{constant}$: pressure falls as volume rises, and vice versa.

What determines the internal energy of an ideal gas?

Only its temperature.

What is the change in internal energy of an ideal gas during an isothermal change?

$\Delta U=0$

During an isothermal change of an ideal gas, how does energy supplied by heating relate to work done by the gas?

They are equal: $Q=W$, where $W$ is the work done by the gas.

What defines an adiabatic change?

There is no energy transfer by heating between the system and its surroundings: $Q=0$.

What form does the first law take for an adiabatic change if $W$ is the work done by the gas?

$\Delta U=-W$

How does the temperature of an ideal gas change during adiabatic expansion and compression?

It cools during expansion and warms during compression.

What pressure–volume relationship holds for a reversible adiabatic change of an ideal gas with constant heat capacities?

$pV^{\gamma}=\text{constant}$

What is $\gamma$ in the reversible adiabatic relationship for an ideal gas?

$\gamma=\frac{C_p}{C_V}$, the ratio of the heat capacities at constant pressure and constant volume.

What pressure–temperature relationship follows from the ideal gas law for a fixed amount of gas at constant volume?

$p/T = \text{constant}$, where $T$ is measured on the absolute temperature scale.

How much pressure–volume work does a gas do during a constant-volume change?

Zero: $W = 0$, because the volume does not change.

What does the first law give for heat transfer in a constant-volume change of an ideal gas?

$Q = \Delta U$: energy transferred by heating changes the gas’s internal energy.