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Q1. One thermometer is placed half metre above the tip of the fire and one thermometer is placed half metre at the side of the fire. Would the readings of the two thermometers be same or different and why?

Solution

The thermometer above the fire would show higher temperature as compared to the thermometer placed at the side of the fire because above the fire, heat is received due to radiation as well as convection whereas at the side of fire, heat is received only due to radiation.

Q2. Define triple point of water. Why is it unique?

Solution

It is the temperature at which the three phases of water, namely, ice, liquid water and water vapour are equally stable and coexistent. The triple point is suitable because it is unique, i.e., it occurs at one single temperature = 273.15 K and one single pressure of about 0.46 cm of the Hg column. This point is unique because any temperature or pressure higher or lower will change the phase of one or more states of water.

Q3. What is thermal conductivity of a perfect heat conductor and a perfect heat insulator?

Solution

The thermal conductivity K is infinity for a perfect heat conductor and it is Zero for a perfect heat insulator.

Q4. Is C_p greater than C_v? Why?

Solution

Yes, C_p is greater than C_v because when a gas is heated at constant volume, the work is done only to increase the internal energy of the system. Whereas, when the gas is heated at constant pressure work is done to overcome this pressure and expand in volume and also to increase the internal energy of the system. Hence, C_p greater than C_v.

Q5. What is calorimetry?

Solution

Calorimetry is method used to measure or determine the specific heat/latent heat of substance. In calorimetry we use calorimeter to find the specific heat/latent heat of a substance.

Q6. By what mode of heat transfer is mercury heated?

Solution

Mercury is available in liquid form but it gets heated by conduction and not by convection method as in most of the liquids.

Q7. If a liquid is heated from top, then by which mode the transfer of heat would take place?

Solution

If a liquid is heated from top, then the heat transfer would be by conduction process as heated layer of liquid would not move down and so convection could not take place.

Q8. What do you mean by phase change of matter?

Solution

A phase change is a process accompanied by the emission or absorption of heat without any temperature change.

Q9. What is heat? What is the unit of heat?

Solution

Heat is the type of energy which is transferred from one body to another on account of temperature difference between the two bodies. Calorie is the unit of heat energy. But the SI unit off heat energy is Joules.

Q10. What is thermal capacity?

Solution

Thermal capacity is the amount of heat required to raise the temperature of whole body through 1 degree Celsius or 1 K.

Q11. What are the various changes that takes place on heating a substance?

Solution

When we heat a substance it causes: a.     Expansion of the matter b.     Change in its temperature c.     State of the matter changes d.     Chemical properties of the matter changes.  

Q12. State some applications of thermal expansion.

Solution

Applications of Thermal expansion are: a.     Some space is always left between the girders used for supporting bridge. b.     To remove a stuck glass stopper the neck of the bottle is heated. c.     Clock pendulum is made up of invar which has very low coefficient of linear expansion which helps in keeping the correct time.

Q13. Why does the inner side of the window pane in a room sweat in cold weather?

Solution

At any given temperature, the amount of water vapour that can be present in air is fixed.  When air is cooled it sweats out its water content.  So window panes sweat in winter.

Q14. If a drop of water falls on a hot plate it takes long time to evaporate.  Why?

Solution

The vapour formed at the instant of landing of the drop acts as an insulator and prevents the heat being passed on to the water above.  So it takes time to evaporate.

Q15. What is Joule's Mechanical Equivalent of Heat?

Solution

Joule proved that mechanical work is converted into heat and the ratio of the work done to the heat is always a constant, represented by J. It is also known as Joule's Mechanical Equivalent of Heat. J = 4.186 joules/calorie

Q16. What is triple point? What is the triple point of water?

Solution

Triple point is a point in the phase diagram, which represents a particular pressure and temperature at which the solid, liquid and vapour phases of a substance can co-exist. The triple point temperature of water is 273.16K and triple point pressure of water of 0.46 cm of mercury column.  

Q17. What are thermal radiations? Give some of its basic characteristics.

Solution

Thermal radiations are radiations emitted by a body (that has temperature above 0 Kelvin) on account of its temperature. Thermal radiations are also called infrared radiations as their wavelength ranges from 8 x 10^-7 m to 4 x 10^-4 m. Basic characteristics of thermal radiations are: i. They require no medium to propagate. They can travel through vacuum. ii. They travel in straight lines with the speed of light. iii. They do not heat the intervening medium through which they pass. iv. Their intensity varies inversely as the square of the distance from the source. v. They show the phenomena of interference, diffraction, reflection, refraction and polarization like light radiations.

Q18. What are the two principal specific heats of a gas?

Solution

The two principal specific heats of a gas are: a.     Specific heat of a gas at constant volume (c_v) b.     Specific heat of a gas at constant pressure (c_p)   Specific heat of a gas at constant volume (c_v): is defined as the amount of heat energy required to raise the temperature of one gram of gas through 1 degree Celsius or 1 K, when the volume of the gas is kept constant.   Specific heat of a gas at constant pressure (c_p): is defined as the amount of heat energy required to raise the temperature of one gram of gas through 1 degree Celsius or 1 K, when the pressuree of the gas is kept constant.

Q19. Explain the working of room heater. How can we increase the efficiency of convections in radiator of a room heater?

Solution

The coil or radiator in room heaters transfer heat to the nearby air. The air near the radiator on heating expands and its density decreases and it rises up. As the hot air rises up it pushes down the cold air which further gets in contact with heater and gets heated. This process goes on and makes the room warm. As the hot air touches the roof or walls of the room, it loses its heat by collisions and cools down. The Efficiency of the radiator of room heaters can be improved by placing a shiny metal foil behind it.

Q20. Which of the three modes of transfer of heat is the fastest?

Solution

Radiation is the fastest mode of heat transfer its velocity being 3 x 10⁸ m/s in vacuum. For example: the heat from the Sun comes to Earth in the form of radiation and it takes approximately 8 minutes for the solar rays to reach Earth.