Tag: conversion of heat into work: heat engine and it's efficiency

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Multiple choice physics heat engine: second law of thermodynamics conversion of heat into work: heat engine and it's efficiency engines and cycles heat engines refrigerators and heat pumps

An engine has an efficiency of 0.25 when temperature of sink is reduced by 58C,if its efficiency is doubled, then the temperature of the source is:

  1. 150K

  2. 222K

  3. 242K

  4. 232K
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let us consider $T _1=$ temperature of the source

$T _2=$ temperature of the sink
The efficiency of the engine,
$0.25=1-\dfrac{T _2}{T _1}$
$\dfrac{T _2}{T _1}=0.75$
$T _2=0.75T _1$. . . . . . . . .(1)
When the temperature of the sink is reduced by $58^0C$, then the efficiency is double,
$2\times 0.25=1-\dfrac{T _2-58}{T _1}$
$\dfrac{T _2-58}{T _1}=0.5$
$T _2-58=0.5T _1$
$0.75T _1-58=0.5T _1$ .........(from equation 1)
$0.25T _1-58=0$
$0.25T _1=58$
$T _1=\dfrac{58}{0.25}$
$T _1=232 K$
The correct option is D.

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

Which of the following is true about refrigeration?

  1. Heat is removed from the inside of the refrigerator

  2. Cold air is produced inside the refrigerator

  3. Hot air is removed from the inside of the refrigerator

  4. Hot air is changed to a cold condensate inside the refrigerator

  5. Hot air inside the refrigerator is expanded to remove its heat
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A refrigerator is infact a reverse heat engine which absorbs heat from source (material) at lower temperature, do work on it and then passes the heat to the sink (sorrounding) at higher temperature, thus we can say that heat is removed from inside the refrigerator.

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A 1 h.p. electric motor operates a pump continuously. The work performed by the motor in one day is

  1. 18 kWh

  2. 36 kWh

  3. 54 kWh

  4. 72 kWh
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

1 h.p. is approximately 0.746 kW. In one day (24 hours), the work performed is 0.746 kW * 24 h = 17.904 kWh, which rounds to 18 kWh.

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

Refrigerators and freezers works by:

  1. Converting hot air to cold air

  2. Keeping hot air out with cold air pressure

  3. Removing heat from inside themselves

  4. Blowing cold inside them

  5. Producing cold air
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A gas refrigerant or sometimes air is key to refrigeration in freezers.
The gas refrigerant goes into the compressor and the process of compression makes the gas hot. That hot gas moves through the system getting colder and colder. By the time it reaches the valve – and is pushed through the very small opening – it has turned into a cold mist. 

As the mist goes through the coils under the freezer compartment, it starts to evaporate and turn back into a gas. This evaporation process takes the heat from the freezer compartment with it and as the refrigerant starts to warm up, it is sent back to the compressor to start the process all over again. Eventually the process is completed and there is no heat left in the freezer meaning that all objects in the freezer are frozen.

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

By opening the door of a refrigerator which is inside a room, the temperature of room ____________.

  1. first decreases then increases

  2. remains unchanged

  3. increases

  4. decreases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the door of a refrigerator is opened the temperature of the room increases as the room is acting as the source here where the heat is rejected.

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

A refrigerator is a device which works on the principle that evaporation of a liquid gives a cooling effect.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
A refrigerator works on principle of thermodynamics of extracting heat from a body which means making it drop its temprature. There is nothing like cooling a thing its a by product of removing or extracting heat from a material or a body. This is what a refrigerator does. Refirgerator with help of coolant flowing through coolant tubes and condensor does this job of extracting heat from the all items kept in refrigerator.

A refrigerator is a device which works on the principle that evaporation of a liquid gives a cooling effect.
Option A is correct.
Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

 What function is performed by Condenser in a refrigerator?

  1. To deliver the heat to the heat pump

  2. To deliver the heat to the evaporator

  3. To deliver the heat to the refrigerator

  4. To deliver the heat to the atmosphere
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A condenser is simply a heat exchanger,the purpose of a condenser in the cycle of compression refrigeration is to change the hot gas being discharged from the compressor to a liquid prepared for use in the evaporator. The condenser accomplishes this action by the removal of sufficient heat from the hot gas, to ensure its condensation at the pressure available in the condenser. The heat is shifted to another medium, like water or air, to cool the condenser.

The function is performed by Condenser in a refrigerator to deliver the heat to the atmosphere

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

The heat pump is also called as the :

  1. Reverse motor

  2. Reverse refrigerator

  3. Reverse oscillator

  4. Reverse engine
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Though the working principle of refrigerator and heat pump is same but heat pump is used to warm a room whereas refrigerator is used to cool down the temperature of material inside it , so heat pump is also called a reverse refrigerator .

Multiple choice physics option b: engineering physics conversion of heat into work: heat engine and it's efficiency engines and cycles refrigerators and heat pumps

What function does a Condenser perform,in heat pumps?

  1. Condenser performs the most important function of cooling the room.

  2. Condenser performs the most important function of heating the room.

  3. Condenser performs the most important function of freezing outside the room

  4. Condenser performs the most important function of heating the atmosphere
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A heat pump is a device that transfers heat energy from a source of heat to what is called a "heat sink". Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses a small amount of external power to accomplish the work of transferring energy from the heat source to the heat sink. A condenser is a device or unit used to condense a substance from its gaseous to its liquid state, by cooling it.


The function does a Condenser perform,in heat pumps is condenser performs the most important function of heating the room.