Sunday, May 13, 2012

CBSE (VII)PHYSICS: Heat and Temperature for CBSE Science class VII

CLASS VII  Science (Physics) 

Chapter- Heat and Temperature 

Question with solution Part-1
 Q.1. What is heat?
Ans: Heat is form of energy that produces feeling of hotness. It is measured in Joule . Calorie is used to measure energy value of food. 1 cal. =4.2 joule
Q. 2.What is energy?
Ans: Energy is capacity to do work. Energy is neither created nor destroyed it can only changed one form to another.
Q. 3.What are the effects of heat?
Ans: Heat cause many change in living and non living thing. They are
1. Heat increase in temperature 
2. Heat expand a substance   
3. Heat changes the state   
4. Heat bring chemical change
5.Heat effect living thing
Q.4. What is temperature?
Ans: The degree of hotness and coldness of a body is called temperature of body. Thermometer is an instrument used to measure temperature. It is work on principle of expansion in liquid on heating. It is measured in Celsius and Fahrenheit scale but for scientific work Kelvin scale is used.
 0F = (9/5 x 0C) +32  and  oC = (9/5)(F- 32) and K= C + 273o
Q. 5.What is thermometry?
Ans: The science of measurement of temperature is known as thermometry
Q.6. What are different types of thermometer?
Ans: Thermometer: It consist of long narrow glass tube having fine bore.There is a glass bulb filled with liquid mostly mercury at the one end and sealed at other end. There is small bend just above bulb called Kink.  it does not allow the mercury to fall as soon as taken out from our mouth.
(i). Laboratory Thermometer: It ranges from -10oC to 110oC
(ii) Clinical thermometer: It is used to measure body temperature. It ranges from 35oC to 42oC and 94oF to 108oF.  Mercury thermometer can measure temperature from -35oC to 357oC.
(iii) Alcohol thermometer used to measure temp. from -125oC to 50oC.
Q. 7.What is the use of the m a x i m u m - m i n i m u m thermometer?
Ans:  The maximum and minimum temperatures atmosphere reported in weather reports which is measured by a thermometer called the m a x i m u m - m i n i m u m thermometer.
Q.8. What is the normal body temperature?   
Ans: 37oC or 98.6oF.
Q.9. Why mercury preferred in thermometers?
Ans: Mercury used due to following reasons
(i) It expand uniformly  
(ii) It does not stick to wall
 (iii) It is shiny and easy to see
 (iv) It remain liquid at room temperature.
Q.10. Find at what temperature measurement on Celsius equal to Fahrenheit.   
[Ans: - 400]
Q.11. Find at what temperature measurement on Fahrenheit double of Celsius
[Ans: 1600]
Q. 12.What is thermal expansion (enlargement)?
Ans: The expansion produced in substance on heating is called thermal expansion. When we heat a substance its molecules gain energy and start vibrating rapidly and spread out . As a result a substance expands.
Q.13. Why does gas expand more than solid and liquid?
Ans: Gas expands more than solid and liquid because its molecules are bound with very weak force of   attraction.
Q.14. A bimetallic strip made of brass and iron welded together. When it is heated, the strip bend why?
Ans: Brass expand and contract more than iron on heating and cooling. This unequal expansion and contraction bent the strip.
Q. 15.The same quantity of heat supplied to same amount of different substances does not necessarily produce the same increase in temperature. Why?
Ans: Different substances absorb differently that depends on
(a) Mass of body
(b) change in temperature
(c) Nature of substance. For this reason  the same quantity of heat supplied to same amount of different substances does not necessarily produce the same increase in temperature
Q.16. If difference in temperature between A and B is 50C , What is the difference in Kelvin?
Ans: 5K
Q.17. Give some examples expansion causes problems?
Ans (i). Gaps are left between two sections of railway tracks that allow expansion in summer and protect from bending causing accident.
(ii). During summer, iron expands and cause serious accidents. To allow this expansion Space left between two sections of rail tracks.
(ii). When we pour hot liquid, sometime glass tumbler crack because inner wall get heated quickly and expand than outer wall. A Pyrex glass resist from this unequal expansion.
Q. 18.Explain one  good use of expansion:
1. In fire alarm expansion put in good use. The alarm contains two strips of iron and brass bolted together. When it is heated because of fire, brass expands more than iron and bends towards iron. Bell starts ringing. As the fire put out strip gets cooled and straighten again.
Q.19. What is heat capacity?
Ans: The amount of heat required to raise temperature of a substance by 10C . It is different for different substance. It depends on (a) Mass of body (b) change in temperature (c) Nature of substance. µ m    and Q µ t    Þ Q = S m t
Here, S is a constant and named Specific heat of body.
Q. 20.What is specific heat capacity?
Ans. The amount of heat required to raise the temperature of 1 kg  by 10C . Water has highest heat capacity. SI unit is j/kg0C
Q.21. Define melting and boiling point
Ans .The temperature at which solid melts is called Melting point. Melting point of ice is 00C and wax is 630 C. The temperature at which liquid changes into vapour is called boiling point.
Q. 22.What is latent heat of fusion?
Ans: Latent heat or hidden is amount of heat that only change state not temperature.
The amount of heat required to melt 1 kg substance from solid to liquid without change of temperature is called latent heat of fusion.
Q. 23.What is latent heat of vaporization?
Ans: The amount of heat required to change 1 kg substance from liquid to vapour state without change of temperature is called latent heat of vaporization.
Q. 24.Why stem burn more than boiled water?
Ans: Because stem has more latent heat than boiled water[ about 2260j/g]
Q.25. why does water used as coolant in car?
 Ans: Water has high specific capacity this makes water to take long time to heat up by absorbing very large quantity of heat. Therefore water used as coolant in car and factories.
Q.26. What are different mode of transfer of heat?
Ans: Heat flows from higher to lower temperature till both body attend same temperature.
There are three mode of transfer of heat
(a) Conduction: The mode of transfer of heat from molecules to molecules without movement of particles.. Conduction takes place in solid as its molecules ate closely packed. Solids, metals and alloy are good conductor. Non metals, plastic glass are bad conductor of heat.
(b)Convection: The mode of transfer of heat from molecules to molecules with movement of particles. In liquids and gasses heat transferred by convection as molecules are far apart from each other .
(c) Radiation: The mode of transfer of heat that does not require any material medium. Heat of sun reach the earth by radiation.
Q.27. At what factor heat absorbed on radiation by body depends on?
Ans: (a) Distance between body and source of heat
(b) its colour(Black coloured surface absorb more than white surfaces)
Q. 29. A desert is very hot in the day and very cold at night?
Ans: Due to low specific heat capacity air above land heated up quickly in the day and cooled quickly at night. For this reason a desert is very hot in the day and very cold at night.
Q.30. Describe construction and working of thermos flask.
Ans: thermos flask is doubled walled glass vessel. Air inside glass wall prevents conduction and convection. Glass walls are silvered on the inside. Silver is good reflector of heat this prevent heat loss due to radiation. If we remove the silvering thermos flask not keep liquids hot or cold for quite a long time. 

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