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Air is bad conductor of heat or partly conducts heat, still vacuum is to be placed between the walls of the thermos flask because
(c) No flow of heat by convection in vacuum.
While measuring the thermal conductivity of a liquid, we keep the upper part hot and lower part cool, so that
(a) Heat flows from higher temperature to lower temperature. To avoid movement of heat due to convention, the upper part is kept hot and the lower part is kept cool.
Mode of transmission of heat, in which heat is carried by the moving particles, is
Convection is the mode of heat transfer where heat is carried by the moving particles. In liquids and gases, convection occurs due to the bulk movement of the particles themselves. In solids, convection is not possible as the particles are fixed in position.
In heat transfer, which method is based on gravitation ?
(a) Natural convection arises due to difference of density at two places and is a consequence of gravity.
When fluids are heated from the bottom, convection currents are produced because -
(d) When the heat is transfered to the fluid at bottom, it becomes less dense and moves upwards.
If a liquid is heated in weightlessness, the heat is transmitted through
(a) Convection is not possible in weightlessness. So the liquid will be heated through conduction.
In which of the following process, convection does not take place primarily ?
(c) In warming of glass of bulb, the energy is transfered due to radiation mostly.
On a clear sunny day, an object at temperature T is placed on the top of a high mountain. An identical object at the same temperature is placed at the foot of mountain. If both the objects are exposed to sun-rays for two hours in an identical manner, the object at the top of the mountain will register a temperature
(b) Because of uneven surfaces of mountains, most of it's parts remain under shadow. So, most of the mountains land is not heated up by sun rays. Besides this, sun rays fall slanting on the mountains and are spread over a larger area. So, the heat received by the mountains top per unit area is less and they are less heated compared to planes (Foot).
Heat travels through vacuum by
(c) Heat radiations are electromagnetic waves of high wavelength.
We consider the radiation emitted by the human body. Which of the following statements is true ?
(d) Every body at all time, at all temperatures emits radiation (except at T=0 ). The radiation emitted by the human body is in the infra-red region.
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