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question_answer1) Assertion : A body that is a good radiator is also a good absorber of radiation at a given wavelength. Reason : According to Kirchoff?s law the absorptivity of a body is equal to its emissivity at a given wavelength. [AIIMS 2005]
question_answer2) Assertion : For higher temperature, the peak emission wavelength of a black body shifts to lower wavelengths. Reason : Peak emission wavelength of a blackbody is proportional to the fourth power of temperature. [AIIMS 2005]
question_answer3) Assertion : Temperatures near the sea coast are moderate. Reason : Water has a high thermal conductivity. [AIIMS 2003]
question_answer4) Assertion : It is hotter over the top of a fire than at the same distance on the sides. Reason : Air surrounding the fire conducts more heat upwards. [AIIMS 2003]
question_answer5) Assertion : Bodies radiate heat at all temperatures. Reason : Rate of radiation of heat is proportional to the fourth power of absolute temperature. [AIIMS 1999, 2002]
question_answer6) Assertion : Woolen clothes keep the body warm in winter. Reason : Air is a bad conductor of heat. [AIIMS 2002]
question_answer7) Assertion : The equivalent thermal conductivity of two plates of same thickness in contact (series) is less than the smaller value of thermal conductivity. Reason : For two plates of equal thickness in contact (series) the equivalent thermal conductivity is given by [AIIMS 1997] \[\frac{1}{K}=\frac{1}{{{K}_{1}}}+\frac{1}{{{K}_{2}}}\]
question_answer8) Assertion : A hollow metallic closed container maintained at a uniform temperature can act as a source of black body radiation. Reason : All metals acts as a black body [AIIMS 1996]
question_answer9) Assertion : If the temperature of a star is doubled then the rate of loss of heat from it becomes 16 times. Reason : Specific heat varies with temperature. [AIIMS 1996]
question_answer10) Assertion : The radiation from the sun?s surface varies as the fourth power of its absolute temperature. Reason : The sun is not a black body. [AIIMS 1999]
question_answer11) Assertion : Blue star is at high temperature than red star. Reason : Wein?s displacement law states that \[T\propto (1/{{\lambda }_{m}})\]. [AIIMS 2002]
question_answer12) Assertion : The S.I. unit of thermal conductivity is watt m?1 K-1. Reason : Thermal conductivity is a measure of ability of the material to allow the passage of heat through it.
question_answer13) Assertion : A brass tumbler feels much colder than a wooden tray on a chilly day. Reason : The thermal conductivity of brass is less than that of wood.
question_answer14) Assertion : Like light radiations, thermal radiations are also electromagnetic radiation. Reason : The thermal radiations require no medium for propagation.
question_answer15) Assertion : Snow is better insulator than ice. Reason : Snow contain air packet and air is good insulator of heat.
question_answer16) Assertion : Water can be boiled inside satellite by convection. Reason : Convection is the process in which heat is transmitted from a place of higher temperature to a place of lower temperature by means of particles with their migrations from one place to another.
question_answer17) Assertion : The absorbance of a perfect black body is unity. Reason : A perfect black body when heated emits radiations of all possible wavelengths at that temperature.
question_answer18) Assertion : A man would feel iron or wooden balls equally hot at 98.4°F. Reason : At 98.4°F both iron and wood have same thermal conductivity.
question_answer19) Assertion : As temperature of a black body is raised, wavelength corresponding to maximum energy reduces. Reason : Higher temperature would mean higher energy and hence higher wavelength.
question_answer20) Assertion : All black coloured objects are considered black bodies. Reason : Black colour is a good absorber of heat.
question_answer21) Assertion : Greater is the coefficient of thermal conductivity of a material, smaller is the thermal resistance of a rod of that material. Reason : Thermal resistance is the ratio of temperature difference between the ends of the conductor and rate of flow of heat.
question_answer22) Assertion : Radiation is the speediest mode of heat transfer. Reason : Radiation can be transmitted in zig-zag motion.
question_answer23) Assertion : Two thin blankets put together are warmer than a single blanket of double the thickness. Reason : Thickness increases because of air layer enclosed between the two blankets.
question_answer24) Assertion : Animals curl into a ball, when they feel very cold. Reason : Animals by curling their body reduces the surface area.
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