It is hotter for the same distance over the top of fire than it is inside of it, mainly because
(1) Air conducts heat upwards
(2) Heat is radiated upwards
(3) Convection takes more heat upwards
(4) Convection, conduction and radiation all contribute significantly transferring heat upwards
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. |
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. |
Mode of transmission of heat, in which heat is carried by the moving particles, is
(1) Radiation
(2) Conduction
(3) Convection
(4) Wave motion
In heat transfer, which method is based on gravitation ?
(1) Natural convection
(2) Conduction
(3) Radiation
(4) Stirring of liquids
When fluids are heated from the bottom, convection currents are produced because -
(1) Molecular motion of fluid becomes aligned
(2) Molecular collisions take place within the fluid
(3) Heated fluid becomes more dense than the cold fluid above it
(4) Heated fluid becomes less dense than the cold fluid above it
The rate of loss of heat from a body cooling under conditions of forced convection is proportional to its (A) heat capacity (B) surface area (C) absolute temperature (D) excess of temperature over that of surrounding : state if
(1) A, B, C are correct
(2) Only A and C are correct
(3) Only B and D are correct
(4) Only D is correct
Which of the following statements is true/correct ?
(1) During clear nights, the temperature rises steadily upward near the ground level
(2) Newton's law of cooling, an approximate form of Stefan's law, is valid only for natural convection
(3) The total energy emitted by a black body per unit time per unit area is proportional to the square of its temperature in the Kelvin scale
(4) Two spheres of the same material have radii 1m and 4m and temperatures 4000 K and 2000 K respectively. The energy radiated per second by the first sphere is greater than that radiated per second by the second sphere
1. the entire chamber of the refrigerator is cooled quickly due to convection
2. the motor is not heated
3. the heat gained from the environment is high
4. the heat gained from the environment is low
Assertion: It is hotter over the top of a fire than at the same distance of the side.
Reason: Air surrounding the fire conducts, more heat upward.
- If both the assertion and the reason are true and the reason is a correct explanation of the assertion
- If both the assertion and reason are true but the reason is not a correct explanation of the assertion
- If the assertion is true but the reason is false
- If both the assertion and reason are false
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