Physics 9702 · AS & A Level · The first law of thermodynamics

The first law of thermodynamics — practice question

(a(i))[2]

State the meaning of the internal energy of a system.

(a(ii))[2]

Explain why, for an ideal gas, its internal energy is equal to the total kinetic energy of the gas molecules.

(b(i))[2]

The mean kinetic energy $\langle E_K \rangle$ of a molecule of an ideal gas is described by $\langle E_K \rangle = \frac{3}{2}kT$, where $k$ is the Boltzmann constant and $T$ is the thermodynamic temperature of the gas. A cylinder holds $1.0\,\text{mol}$ of an ideal gas. The gas is heated so that its temperature changes from $280\,\text{K}$ to $460\,\text{K}$. Calculate the change in total kinetic energy of the gas molecules.

(b(ii))[3]

While the gas is being heated, it expands and does $1.5 \times 10^3\,\text{J}$ of work. State the first law of thermodynamics. Use the law and your answer in part (i) to determine the total energy supplied to the gas.

(ii)[3]

While the gas is being heated, it expands and does $1.5 \times 10^{3}\,\text{J}$ of work. State the first law of thermodynamics. Use the law and your answer in (i) to determine the total energy supplied to the gas.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: Total of the kinetic and potential energies of the molecules

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