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VIII-SCIENCE-HEAT-EVALUATION QUESTION AND ANSWERS
HEAT
TEXTBOOK EXERCISES
I. Choose the best answer.
1. Heat is a form of ________.
a) electrical energy b) gravitational energy
c) thermal energy d) None of these
Ans: c) thermal energy
2. If you apply some heat energy to a substance, which of the following can take place in it?
a) Expansion b) Increase in temperature
c) Change of state d) All the above.
Ans: d) All the above
3. Which of the following substances will absorb more heat energy?
a) Solid b) Liquid
c) Gas d) All the above
Ans: a) Solid
4. If you apply equal amount of heat to a solid, liquid and gas individually, which of the following will have more expansion?
a) Solid b) Liquid
c) Gas d) All of them
Ans: c) Gas
5. The process of converting a liquid into a solid is called_________.
a) sublimation b) condensation
c) freezing d) deposition
Ans: c) freezing
6. Conduction is the way of heat transfer which takes place in a____________.
a) solid b) liquid
c) gas d) All of them
Ans: a) solid
II. Fill in the blanks.
1. A calorimeter is a device used to measure the________.
Ans: heat capacity of water
2. _______ is defined as the amount of heat required to raise the temperature of 1kg of a substance by 1°C.
Ans: specific heat capacity
3. A thermostat is a device which maintains _________.
Ans: sensing
4. The process of converting a substance from gaseous state to solid state is called _________.
Ans: deposition
5. If you apply heat energy, the temperature of a system will ________.
Ans: increase
6. If the temperature of a liquid in a container is decreased, then the interatomic distance will ___________.
Ans: decrease
III. State True or False. If false, correct the statement.
1. The applied heat energy can be realised as an increase in the average kinetic energy of the molecules.
Ans:True
2. The dimensions of a substance are increased if the temperature of the
substance is decreased.
Ans:False
Correct Statement: The dimensions of a substance are increased if the temperature of the substance is increased.
3. The process of converting a substance from solid state to gaseous state is called condensation.
Ans:False
Correct Statement: The process of converting a substance from solid to gas is called sublimation.
4. Convection is the process by which the thermal energy flows in solids.
Ans:False
Correct Statement: Convection is the process by which the thermal energy flows in liquids and gases.
5. The amount of heat gained by a substance is equal to the product of its mass and latent heat.
Ans:True
6. In a thermos flask, the silvered walls reflect and radiate the heat outside.
Ans:False
Correct Statement: In a thermos flask, the silvered walls reflect and radiate the heat back to the liquid in the bottle.
IV. Match the following.
Ans:
V. Consider the statements given below and choose the correct option.
1. Assertion: Radiation is a form of heat transfer which takes place only in
vacuum.
Reason: The thermal energy is transferred from one part of a substance to another part without the actual movement of the atoms or molecules.
2. Assertion: A system can be converted from one state to another state.
Reason: It takes place when the temperature of the system is constant.
a. Both assertion and reason are true and reason is the correct explanation of assertion.
b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
c. Assertion is true, but the reason is false.
d. Assertion is false, but the reason is true.
Ans:
1.b. Both assertion and reason are true, but reason is not the correct explanation of assertion.
2.a. Both assertion and reason are true and reason is the correct explanation of assertion.
VI. Answer briefly.
1. What are the applications of conduction in our daily life?
We cook food in vessels made up of metals. When the vessel is heated, heat is transferred from the metal to the food.
When we iron dresses, heat is transferred from the iron to the cloth.
2. What are the effects of heat?
When heat energy is supplied to any substance, it brings about many changes.
• Expansion
• Increase in temperature
• Change in state
3. Name three types of heat transfer.
If heat energy is supplied to any substance, it will be transferred from one part of the substance to another part in different ways depending on the state of the substance. Three ways of heat transfer are:
• Conduction
• Convection
• Radiation
4. What is conduction?
Conduction is the process of heat transfer in solids from the region of higher temperature to the region of lower temperature without the actual movement of atoms or molecules.
5. Write a note on convection.
The form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases.
6. Define specific heat capacity.
Heat capacity is defined as the amount of heat energy required by a substance to raise its temperature by 1°C or 1 K. It is denoted by the symbol C'.
C' = Q / ΔT
7. Define one calorie.
One calorie is the amount of heat energy required to raise the temperature of 1 gram of water through 1°C.
VII. Answer in detail.
1. With the help of a neat diagram, explain the working of a calorimeter.
A calorimeter is a device used to measure the amount of heat gained or lost by a substance.
It consists of a vessel made up of metals like copper or aluminium which are good conductors of heat and electricity.
The metallic vessel is kept in an insulating jacket to prevent heat loss to the environment.
There are two holes in it. Through one hole a thermometer is inserted to measure the temperature of the contents.
A stirrer is inserted through another hole for stirring the content in the vessel.
The vessel is filled with liquid which is heated by passing current through the heating element.
Using this device we can measure the heat capacity of the liquid in the container.
2. Write a note on thermostat.
A thermostat is a device which maintains the temperature of a place or an object constant.
The word thermostat is derived from two Greek words, ‘thermo’ meaning heat and ‘static’ meaning staying the same.
Thermostats are used in any device or system that gets heated or cools down to a pre-set temperature.
It turns an appliance or a circuit on or off when a particular temperature is reached.
Devices which use thermostat include building heater, central heater in a room, air conditioner, water heater, as well as kitchen equipment including oven and refrigerators.
Sometimes, a thermostat functions both as the sensor and the controller of a thermal system.
3. Explain the working of thermos flask.
The thermos flask (Vacuum flask) is an insulating storage vessel that keeps its content hotter or cooler than the surroundings for a longer time.
It is primarily meant to enhance the storage period of a liquid by maintaining a uniform temperature and avoiding the possibilities of getting a bad taste.
A thermos flask has double walls, which are evacuated. It is silvered on the inside. The vacuum between the two walls prevents heat being transferred from the inside to the outside by conduction and convection.
VIII. Higher Order Thinking Questions.
1. Why does the bottom of a lake not freeze in severe winter though the surface is all frozen?
Air changes temperature faster than water.The water at the very top of the lake is in direct contact with the cold air, so it freezes first.The sheet of ice at the top of the lake helps the rest of the water from becoming ice.The amazing fact that water freezes on the surface of a lake but not allows it below to plants and fish to live through the winter in lakes and ponds.So it is a good thing that H20 has the very strange property that its solid phase is less dense than its liquid phase.
2. Which one of the following statements about thermal conductivity is correct?
Give reason.
a) Steel > Wood > Water
b) Steel > Water > Wood
c) Water > Steel > Wood
d) Water > Wood > Steel
Ans:
b) Steel > Water > Wood
The conductivity of steel is more than wood.So,steel can absorb heat faster than water and wood.
Wood has the least absorption of heat.
Thus the order thermal conductivity is steel > water > wood.
IX. Numerical Problems.
1. An iron ball requires 1000 J of heat to raise its temperature by 20°C. Calculate the heat capacity of the ball.
Amount of heat energy supplied (Q) = 1000 J
Rise in temperature Δt = 20°C
Q
Therefore, heat capacity ‘C’ = -----
DT
1000 J
= ---------
20 °C
= 50 J °C–1
2. The heat capacity of the vessel of mass 100 kg is 8000 J/°K. Find its specific heat capacity.
Mass, m = 100 Kg
Heat CapacityC’ = 8000J/K
Specific heat capacity C = Q / m x ΔT
Heat capacity C’ = Q / ΔT
Specific heat capacity C = C’ / m
8000J/K
= ----------= 80
100kg
Specific heat capacity C = 80JK-1kg-1
