20 past paper MCQs on First Law of Thermodynamics, from the Thermodynamics unit of MDCAT Physics. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.
1.The rapid escape of air from a burst tyre is an example of a process that is: (MDCAT 2020)
Correct answer: (b) adiabatic
The escape is too quick for any heat exchange, so $Q=0$ and the air cools as it does work.
2.When a gas expands isothermally, the product of its pressure and volume during the process is: (MDCAT 2022)
Correct answer: (b) constant
At constant temperature $PV=nRT$ is fixed, which is Boyle's law.
3.The first law of thermodynamics concerns the conservation of: (MDCAT 2023)
Correct answer: (d) energy
$\Delta U=Q-W$ is the conservation of energy written for a thermodynamic system.
4.During an isothermal process the temperature: (MDCAT 2024)
Correct answer: (b) remains constant throughout the process
Isothermal means at one temperature, so the temperature is constant for the whole process.
5.What is the value of the heat energy $Q$ in an adiabatic process? (MDCAT 2024)
Correct answer: (c) 0
No heat enters or leaves the system in an adiabatic process, so $Q=0$ and the work is done at the cost of internal energy.
6.During an adiabatic compression the work done on a gas is \$200\,\mathrm{J}\$. The change in its internal energy is: (UHS MDCAT 2017)
Correct answer: (b) $+200\,\mathrm{J}$
No heat is exchanged in an adiabatic change, so all the work done on the gas goes into its internal energy.
7.If \$100\,\mathrm{J}\$ of heat is taken out of a system while \$400\,\mathrm{J}\$ of work is done on it, the change in internal energy is: (UHS MDCAT 2017)
Correct answer: (b) $300\,\mathrm{J}$
$\Delta U=\Delta Q-W=(-100)-(-400)=+300\,\mathrm{J}$.
8.According to First law of thermodynamics when heat flows into a system and no work is done the internal energy of the system must (UHS MDCAT 2025)
Correct answer: (a) Increase
With no work done, all the heat taken in goes into the internal energy, which therefore rises.
9.What is the value of heat energy (Q) in an adiabatic process? (UHS MDCAT 2024)
Correct answer: (c) 0
An adiabatic change is one in which no heat enters or leaves, so $Q$ is zero.
10.A process in which all heat energy is used for increasing internal energy is: (NUMS MDCAT 2024)
Correct answer: (b) Isochoric
At constant volume no work is done, so by the first law all the heat goes into internal energy.
11.The internal energy of a system during an isothermal process: (SZABMU MDCAT 2023)
Correct answer: (d) Remains constant
Internal energy depends only on temperature, which an isothermal change holds fixed.
12.If 42 J heat is transferred to the system during expansion, what is the change in internal energy when work done is 32 J? (NUMS MDCAT 2023)
Correct answer: (b) 10 J
$\Delta U=Q-W=42-32$, which is $10\,\mathrm{J}$.
13.The 1st law of thermodynamics is the generalization of the law of conservation of: (NUMS MDCAT 2023)
Correct answer: (c) Energy
The first law is the conservation of energy applied to heat and work.
14.First law of thermodynamics concerns with the conservation of (UHS MDCAT 2023)
Correct answer: (d) Energy
The first law is the conservation of energy written for heat and work.
15.First law of thermodynamics is based on: (NUMS MDCAT 2022)
Correct answer: (d) Law of conservation of energy
The first law is the conservation of energy written for heat and work.
16.The first law of thermodynamics can be stated as? (PMC Practice 2021)
Correct answer: (a) Q = ΔU + W
The heat put in goes partly into internal energy and partly into work done by the system.
17.In which process the entire heat supplied to the gas is converted to the internal energy of the gas? (PMC MDCAT 2020)
Correct answer: (a) Isochoric process
At constant volume no work is done, so all the heat goes into the internal energy.
18.During adiabatic compression work done is equal to 200 J. What will be the change in internal energy? (UHS MDCAT 2017)
Correct answer: (b) +200 J
No heat can escape, so all $200\,\mathrm{J}$ of work done on the gas goes into its internal energy.
19.Which relation exactly described the isothermal process? (UHS MDCAT 2016)
Correct answer: (a) Q = W
The temperature is held, so the internal energy does not change and all the heat goes into work.
20.If a turbine is working as a heat engine and takes that from hot body (427 °C) and exhausts into a body at 77 °C then what is the possible efficiency? (UHS MDCAT 2016)
Correct answer: (a) 50%
$1-T_c/T_h=1-350/700$, that is $50\%$.