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Thermal Properties of Matter — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 SciencePhysicsThermal Properties of Matter4 Marks
Question
Read the passage given below and answer the following questions from (i) to (v). The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process. For example, if heat is added to a given quantity of ice at –10°C, the temperature of ice increases until it reaches its melting point (0°C). At this temperature, the addition of more heat does not increase the temperature but causes the ice to melt, or changes its state. Once the entire ice melts, adding more heat will cause the temperature of the water to rise. The latent heat for a solid to liquid state change is called the latent heat of fusion (Lf), and that for a liquid-gas state change is called the latent heat of vaporisation (Lv). These are often referred to as the heat of fusion and the heat of vaporisation. We have seen that heat is energy transfer from one system to another or from one part of a system to another part, arising due to temperature difference. What are the different ways by which this energy transfer takes place? There are three distinct modes of heat transfer: conduction, convection and radiation Conduction is the mechanism of transfer of heat between two adjacent parts of a body which are direct in contact because of their temperature difference. Suppose, one end of a metallic rod is put in a flame, the other end of the rod will soon be so hot that you cannot hold it by your bare hands. Here, heat transfer takes place by conduction from the hot end of the rod through its different parts to the other end. Gases are poor thermal conductors, while liquids have conductivities intermediate between solids and gas.Convection is a mode of heat transfer by actual motion of matter. It is possible only in fluids. Convection can be natural or forced. In natural convection, gravity plays an important part. When a fluid is heated from below, the hot part expands and, therefore, becomes less dense. Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated, rises up and is replaced by the relatively colder part of the fluid. The process goes on. This mode of heat transfer is evidently different from conduction. Convection involves bulk transport of different parts of the fluid. Conduction and convection require some material as a transport medium. These modes of heat transfer cannot operate between bodies separated by a distance in vacuum. But the earth does receive heat from the Sun across a huge distance. Similarly, we quickly feel the warmth of the fire nearby even though air conducts poorly and before convection takes some time to set in. The third mechanism for heat transfer needs no medium; it is called radiation and the energy so transferred by electromagnetic waves is called radiant energy.
  1. Which of the following heat transfer mode needs medium?
  1. Conduction
  2. Convection
  3. Both a and b
  4. Radiation
  1. Convection is possible in:
  1. Solids only
  2. Fluid only
  3. Both solid and fluid
  4. None of these
  1. Define latent heat of fusion and latent heat of vapourisation.
  2. Define process of natural convection.
  3. Differentiate between conduction and radiation.
✓

Answer

  1. (c) Both a and b
  2. (b) Fluid only
  3. The heat required for conversion of solid state to liquid state without change in temperature is called the latent heat of fusion (Lf).
  4. The heat required for conversion of liquid state to solid state without change in temperature is called the latent heat of vaporisation (Lv).
  5. Convection is a mode of heat transfer by actual motion of matter. It is possible only in fluids. Convection can be natural or forced. In natural convection, gravity plays an important part. Convection involves bulk transport of different parts of the fluid.
  6. Following are differentiate poin;
Sr No.
Conduction
Radiation
1
It requires medium to transfer heat.
It does not require medium to transfer heat
2
Heat transfer from one part to other part due to direct contact between each other
Heat can be transferred without any contact
3
Temperature difference is cause for transfer of heat
Any body having temperature higher than 0 can radiate energy
4
For example transfer of heat from one hot end of rod to other cold end
For example heat coming from sun to earth

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