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Thermos flask - class-VIII

Description: Thermos flask
Number of Questions: 16
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Tags: physics transfer of heat heat energy transfers
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A thermos flask is designed to prevent the contents of the flask from losing heat. Which of the following statements concerning the design of thermos flasks is incorrect?

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. The stopper helps reduce heat loss by convection

  2. The glass walls are silver coated to reduce emitted radiation

  3. The flask is designed to prevent heat loss by conduction, convection and radiation

  4. The vacuum between the glass walls prevents heat loss by preventing radiation

Show answer
Correct Option: D
Explanation:
Yes, thermoflask is designed to prevent  the contents of the flask from losing heat.
The stopper, i.e. vacuum between double walls of thermoflask prevents heat transfer through convection.
The glass walls are silver coated because it reduces emitted radiation when they are silver coated i.e. made shiny. And we know that shiny surfaces reflects all the heat so all the heat is being reflected inside the walls and heat remains unaffected. And the thinesss of glass walls will stop heat entering or leaving the flask by conduction.
So, the flask is designed to prevent heat loss by conduction, convection and radiation.

A                is a storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Closed flask

  2. Vacuum flask

  3. Conducting flask

  4. None of these

Show answer
Correct Option: B
Explanation:

A thermoflask is a storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings. As thermoflask is designed to prevent heat loss by conduction, convection and radiation.

Vacuum in a vacuum flask prevents heat transfer through process of-

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Conduction only

  2. Conduction and Convection

  3. Convection only

  4. Radiation only

Show answer
Correct Option: B
Explanation:
Vacuum in a vacuum flask prevents heat transfer through process of conduction and convection.
It is so because, in conduction there needs a direct contact for transfer of heat and there is vaccum between two layers, so conduction can't  transfer heat in this case. And in convection, heat is transferred by molecular motion in fluid (liquid or gas). But due to vaccum's presence between two layers, molecular motion is not possible So convection is also impossible for this case.

The silver coating on the inner bottle of a thermos flask

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. prevents heat transfer by conduction

  2. prevents heat transfer by convection

  3. prevents heat transfer by radiation

  4. prevents heat transfer by absorption

Show answer
Correct Option: C
Explanation:
Due to the silver coating on the inner bottle, a shiny surface is formed. And these shiny surface act as a good reflector of heat 
As a result, they prevent the transfer of heat by radiation.

The walls of thermo-flask are polished because

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. shining bodies are good absorber of heat

  2. shining bodies are good radiator of heat

  3. shining bodies have emisivity as one

  4. none of the above

Show answer
Correct Option: B
Explanation:

The wall of thermoflask are polished because shiny surface are good radiators of heat. Since we intend to keep in the vacuum flask and not have it radiated, Outside the shiny surface is best option. A black wall is bad option as they are good emitter and absorber of heat radiation.

The vacuum between the walls of the flask prevents the transfer of heat by

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Conduction

  2. Convection

  3. Radiation

  4. All

Show answer
Correct Option: A
Explanation:

Thermos flask is a special kind of bottle used for keeping the hot liquid hot and the cold liquid cool for a sufficiently long time. It was deviced by sir James Dewar and do it is also known as Dewar Flask. The vacuum between the walls of the flask prevents the transfer of  heat by conduction.

Thermos flask was deviced by

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Fischer

  2. Joseph Lister

  3. Joseph Fourier

  4. Sir James Dewar

Show answer
Correct Option: D
Explanation:

Thermos flask is a special kind of bottle used for keeping the hot liquid hot and the cold liquid cool for a sufficiently long time. It was devised by sir James Dewar and do it is also known as Dewar Flask.

Thermos flask is a special kind of 

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Bottle

  2. Process

  3. Apparatus

  4. Weighing machine

Show answer
Correct Option: A
Explanation:

Thermos flask is a special kind of bottle used for keeping the hot liquid hot and the cold liquid cool for a sufficiently long time.

A polished silver surface is

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. good absorber of heat

  2. good reflector of heat

  3. poor reflector of heat

  4. none of these

Show answer
Correct Option: B
Explanation:

Heat rays are electromagnetic rays. Light rays are also electromagnetic rays. We know that a polished silver surface is a good reflector of light. this means that it is a good reflector of electromagnetic rays. Hence it is a good reflector of heat

A thermos flask maintains temperature of objects. It

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. containing some kind of heating element to keep hot things hot.

  2. containing some kind of heating element to keep cold things cold.

  3. keeps hot things hot by not allowing heat to escape.

  4. All of the above

Show answer
Correct Option: C
Explanation:
A thermos flask maintains temperature of objects. It keeps hot things hot by not allowing heat to escape.
Thermos flask doesn't allow heat to escape from it via any mode of heat transfer i.e. conduction, convection, radiation. It does so by silver coating or polishing inner surface thin walls and vacuum between two walls.

Thermos flask prevents loss or gain of heat by

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. conduction only

  2. convections only

  3. radiation only

  4. all of the above

Show answer
Correct Option: C

Thermos flask, when closed, is an approximate

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. open system.

  2. closed system.

  3. isolated system.

  4. None of these

Show answer
Correct Option: C
Explanation:

An open system is a one which can exchange matter as well as energy with the surroundings. Eg: a beaker of water. A closed system is a one which can exchange only energy with the surroundings. Eg: a liquid in a closed bottle. An isolated system is a one which cannot exchange anything with the surroundings. Eg: Thermos Flask .

Liquid nitrogen is stored at $77K$ in a vacuum flask. The surrounding atmospheric temperature in the lab is $33K$.

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. After a few days, the temperature of liquid nitrogen will increase.

  2. After a few days, the temperature of the lab will decrease.

  3. After a few days, the temperature of liquid nitrogen will decrease

  4. After a few days, the temperature of liquid nitrogen will remain unchanged.

Show answer
Correct Option: D
Explanation:
Liquid nitrogen is stored at $77K$ in vacuum flask. The surrounding atmosphere temperature of the lab is $33K$.
After a few days, the temperature of liquid nitrogen will remain unchanged. Because, vacuum flask is designed to prevent heat loss by conduction, convection and radiation. So, the temperature remains unchanged.

Which of the following is an insulating component in thermos flask?

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. plastic foam

  2. air

  3. vacuum

  4. All of the above

Show answer
Correct Option: D
Explanation:
Insulating component in thermos flask are plastic foam, air, vacuum.
As plastic foam is insulator, so to prevent heat to radiate plastic foam is used just after the inner layer, plastic foam is being polished to reflect the inner heat inside, as air is bad conductor of heat it does not conduct heat just outside the outer layer. Vacuum is poor conductor of heat. It is situated between two layers, which prevents heat conduction through convection.

The purpose of using vacuum flask is to store

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. hot drinks.

  2. cold drinks.

  3. hot as well as cold drinks.

  4. None of these

Show answer
Correct Option: C
Explanation:
The purpose of using the vacuum flask is to store hot as well as cold drinks.
The basic concept behind using the vacuum flask is to keep the temperature of liquid constant which is being kept in the flask, no heat transfer would take place via conduction, convection or radiation.

A themoflask prevent heat loss from-

thermo flask thermos flask heat energy transfers transfer of heat physics

  1. Conduction

  2. Convection

  3. radiation

  4. all of these

Show answer
Correct Option: D
Explanation:
A thermoflask prevent heat loss from conduction, convection and radiation.
As it has insulator between two layers, it prevents the direct contact with surrounding and also stops the molecular motion for heat transfer. As it is a closed system surrounded by the layer of insulator, radiation can't even take place.
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