Heat and Thermodynamics - SS2 Physics Past Questions and Answers - page 4
The efficiency of a heat engine is given by:
The input heat divided by the output work.
The output heat divided by the input work.
The input work is divided by the output heat.
The output work is divided by the input heat.
The Carnot cycle is an idealised thermodynamic cycle that consists of:
Two isothermal and two adiabatic processes.
Two isothermal and two isochoric processes.
Two isochoric and two adiabatic processes.
Two isochoric and two isobaric processes.
The third law of thermodynamics states that:
It is impossible to reach absolute zero temperature.
The entropy of a perfect crystal at absolute zero is zero.
Energy is conserved in all thermodynamic processes.
Heat cannot be completely converted into work.
A heat engine takes in 2000 J of heat from a high-temperature reservoir and produces 800 J of work. How much heat is rejected to the low-temperature reservoir?
According to the first law of thermodynamics, the heat input to a heat engine is equal to the sum of the work done and the heat rejected. Therefore, the heat rejected to the low-temperature reservoir is 2000 J - 800 J = 1200 J.
Users' Answers & CommentsA refrigerator operates between two reservoirs at temperatures of 5°C and 25°C. The refrigerator removes 500 J of heat from the cold reservoir. How much work is required for this process?
In a refrigerator, work is required to transfer heat from a low-temperature reservoir to a high-temperature reservoir. The amount of work required is equal to the heat removed from the cold reservoir. Therefore, the work required is 500 J.
Users' Answers & CommentsA heat engine has an efficiency of 50% and takes in 1000 J of heat. How much work does the engine produce?
The efficiency of a heat engine is defined as the ratio of the work output to the heat input. Therefore, if the efficiency is 50%, the work output is equal to 50% of the heat input. In this case, the work output is 0.5 x 1000 J = 500 J.
Users' Answers & CommentsIn a refrigeration system, which component is responsible for changing the refrigerant from a high-pressure gas to a high-pressure liquid?
Compressor
Condenser
Expansion valve
Evaporator
Explanation: The condenser is the component in a refrigeration system where the high-pressure refrigerant gas is cooled and condensed into a high-pressure liquid.
Users' Answers & CommentsWhich of the following statements is true about the coefficient of performance (COP) of a refrigerator or heat pump?
The COP of a refrigerator is always greater than the COP of a heat pump.
The COP of a heat pump is always greater than the COP of a refrigerator.
The COP of a refrigerator and a heat pump are always equal.
The COP of a refrigerator and a heat pump depends on the temperature difference between the reservoirs.
Explanation: The coefficient of performance (COP) of a heat pump is defined as the ratio of the heat transferred from the warm reservoir to the work input, while the COP of a refrigerator is defined as the ratio of the heat removed from the cold reservoir to the work input. Since the heat transferred from the warm reservoir is greater than the heat removed from the cold reservoir, the COP of a heat pump is always greater than the COP of a refrigerator.
Users' Answers & CommentsIn a heat pump, which component is responsible for absorbing heat from the external environment and transferring it to the indoor space?
Compressor
Condenser
Expansion valve
Evaporator
Explanation: The evaporator is the component in a heat pump where the low-pressure refrigerant liquid absorbs heat from the external environment and evaporates into a low-pressure gas. This heat is then transferred to the indoor space.
Users' Answers & CommentsWhich of the following statements is true about the Carnot refrigeration cycle?
It operates between two temperature reservoirs and has the highest possible efficiency.
It operates between two temperature reservoirs and has the lowest possible efficiency.
It operates between multiple temperature reservoirs and has the highest possible efficiency.
It operates between multiple temperature reservoirs and has the lowest possible efficiency.
Explanation: The Carnot refrigeration cycle is an idealised cycle that operates between two temperature reservoirs. It is based on the Carnot principle, which states that no refrigeration cycle operating between two temperature reservoirs can have a higher efficiency than a Carnot cycle. Therefore, the Carnot refrigeration cycle has the highest possible efficiency.
Users' Answers & Comments