Most Repeated MDCAT Physics: Heat and Thermodynamics MCQs

Explanation

This is the correct answer. Internal energy is a thermodynamic property that represents the total energy of a system, including the kinetic energy of its molecules and the potential energy stored in their bonds. It encompasses all forms of microscopic energy within the system.

Explanation

Boyle's law: states that the volume of a given amount of gas held at constant temperature varies inversely with the applied pressure when the temperature and mass are constant.

Explanation

The temperature coefficient of resistance represents how the resistance of a conductor changes with temperature. It is defined as the fractional change in resistance per Kelvin.

Explanation

PV= NkTV= NkT/PThe mass of one molecule of gas is ‘m’So total mass= N*mThe density of the gas is = total mass/ total volumeDensity= N*m/ (NkT/P)Which simplifies to mP/kT That is consistent with option D.

Explanation

Boyle's law gives the relation between pressure and volume at constant temperature (isotherm). Hence, the graph of Boyle's law is similar to that of isothermal process. The graph of Charles law is similar to graph of isobaric process. Hence, option A is correct.

Explanation

In adiabatic process, heat remains constant. Q =constant ΔQ = O Hence, option D is correct.

Explanation

The internal energy of an ideal gas is determined by its temperature because it is directly related to the kinetic energy of its molecules. According to the kinetic theory of gases, the average kinetic energy of gas molecules is proportional to the absolute temperature of the gas. Pressure and volume do not directly affect the internal energy in an ideal gas scenario. Therefore, the correct answer is temperature, as the internal energy is a function of temperature alone in ideal gases.

Explanation

This is correct. Because the gas expands into a vacuum with no heat exchange and no work done, internal energy remains unchanged. This is a key characteristic of free expansion for ideal gases.

Explanation

First law of thermodynamics states “ energy can neither be created nor destroyed, it can only be altered/changed into different forms”. Δθ= Δu + Δw is one of the mathematical expression for first law of thermodynamics which explains that heat supplied to the system(Δθ) is equal to change in internal energy of the system(Δu) plus work done by the system on the surrounding(Δw).

Explanation

The ratio of heat accepted to the heat rejected by a Carnot engine gives the efficiency of the working substance. A Carnot engine is a theoretical heat engine that operates on the reversible Carnot cycle. It consists of two isothermal and two adiabatic processes. The efficiency of a Carnot engine depends only on the temperatures of the hot and cold reservoirs, and is given by the Carnot efficiency formula: Efficiency = 1 - (Temperature of the cold reservoir / Temperature of the hot reservoir) The heat accepted is the heat energy absorbed by the engine from the hot reservoir, and the heat rejected is the heat energy expelled by the engine to the cold reservoir.

Explanation

A heat engine operating in reverse is known as a refrigerator. In a heat engine, work is done on the system to extract heat from a colder reservoir and deliver it to a hotter reservoir. In a refrigerator, the process is reversed, where work is done on the system to transfer heat from a colder space (usually the interior of the refrigerator) to a hotter external environment. Therefore, option b is correct.

Explanation

In an ideal gas the inter-molecular collisions are assumed to be absent and the collisions are perfectly elastic, thus, the gas only possesses kinetic energy and hence the internal energy of the ideal gas only depends on temperature,hence the answer will be B.

Explanation

The correct answer is 0 Kelvin. The efficiency of a heat engine is given by the equation:η = 1 - T2/T1 where η is the efficiency, T2 is the exhaust temperature, and T1 is the input temperature. For a heat engine to be 100% efficient, the exhaust temperature (T2) must be 0 Kelvin. This is because the efficiency equation approaches 1 (or 100%) as T2 approaches 0. None of the other options reach 100% efficiency because their exhaust temperatures are not low enough to maximize the efficiency ratio.

Explanation

To calculate the average translational kinetic energy of molecules in a gas at a given temperature, we can use the equation:K.E. = (3/2) kTWhere K.E. is the average translational kinetic energy, k is the Boltzmann constant (approximately 1.38 x 10^-23 J/K), and T is the temperature in Kelvin.Given that the temperature is 27°C, we need to convert it to Kelvin by adding 273.15:T = 27°C + 273.15 = 300.15 KSubstituting the values into the equation:K.E. = (3/2) * (1.38 x 10^-23 J/K) * (300.15 K)= 6.21 x 10-21 JTherefore, the average translational kinetic energy of molecules in the gas at a temperature of 27°C is approximately 6.21 x 10-21 J.

Explanation

The maximum theoretical efficiency of a system operating as a heat engine is given by the Carnot efficiency, which is determined by the temperatures of the hot and cold reservoirs. The formula is η=1−Tc/Th, where Tc is the cold reservoir temperature and Th is the hot reservoir temperature, both in Kelvin. For this problem, convert the given temperatures from Celsius to Kelvin: Tc = 77°C + 273 = 350K and Th = 427°C + 273 = 700K. Substituting these into the formula gives η=1−350/700 = 0.50, or 50% when expressed as a percentage.