Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because:

1.  it is difficult to fill the air between the walls of the thermos flask.
2. due to more pressure of air, the thermos can get cracks.
3. by convection, heat can flow through the air.
4. on filling the air, there is no advantage.
Subtopic:  Convection |
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Level 1: 80%+
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While measuring the thermal conductivity of a liquid, we keep the upper part hot and the lower part cool, so that:

1. convection may be stopped
2. radiation may be stopped
3. heat conduction is easier downwards
4. it is easier and more convenient to do so


 

Subtopic:  Convection |
 66%
Level 2: 60%+
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The presence of a gravitational field is required for the heat transfer by:
1. radiation
2. conduction
3. stirring of liquids
4. natural convection
Subtopic:  Convection |
 77%
Level 2: 60%+
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