importance questions of heat transfer for class 8
Answers
Explanation:
A. Objective Questions
1. Write true or false for each statement
(a) Evaporation is rapid on a wet day.
Solution: False.
(b) Evaporation takes place only from the surface of the liquid.
Solution: True.
(c) All molecules of a liquid take part in the process of evaporation.
Solution: False.
(d) Temperature of a liquid rises during boiling or vaporization
Solution: False.
(e) All molecules of a liquid take part in boiling.
Solution: True.
(f) Boiling is a rapid phenomenon.
Solution: True.
(g) All solids expand by the same amount when heated to the same rise in temperature.
Solution: False.
(h) Telephone wires are kept tight between the two poles in winter.
Solution: True.
(i) Equal volumes of different liquids expand by a different amount when they are heated to the same rise in temperature.
Solution: True.
(j) Solids expand the least and gases expand the most on being heated.
Solution: True.
(k) A mercury thermometer makes use of the property of expansion of liquids on heating.
Solution: True.
(l) Kerosene contracts on heating.
Solution: False.
Question 2
Fill in the blanks
(a) Boiling occurs at a fixed temperature.
(b) Evaporation takes place at all temperature.
(c) The molecules of liquid absorb heat from surroundings in evaporation.
(d) Heat is absorbed during boiling.
(e) Cooling is produced in evaporation.
(f) A longer rod expands more than a shorter rod on being heated to the same temperature.
(g) Liquids expand more than the solids.
(h) Gases expand more than the liquids.
(i) Alcohol expands more than water.
(j) Iron expands less than copper.
Question 3
Match the Following
Column A Column B
(a) Blowing air increases (i) increase in inter-molecular separation
(b) Increase in pressure (ii) pendulum of a clock increases
(c) Thermal expansion (iii) cooking utensils
(d) Invar (iv) boiling point
(e) Pyrex glass (v) evaporation
Solution:
Column A Column B
(a) Blowing air increases (v) evaporation
(b) Increase in pressure (iv) boiling point
(c) Thermal expansion (i) increase in inter-molecular separation
(d) Invar (ii) pendulum of a clock increases
(e) Pyrex glass (iii) cooking utensils
Question 4
Select the correct alternative
(a) In evaporation
1. all molecules of liquid begin to escape out
2. only the molecules at the surface escape out
3. the temperature of the liquid rises by absorbing heat from the surroundings.
4. the molecules get attracted within the liquid.
Answer: The correct answer is 2. only the molecules at the surface escape out
(b) The rate of evaporation of a liquid increases when:
1. temperature of liquid falls
2. liquid is poured in a vessel of less surface area
3. air is blown above the surface of liquid
4. humidity increases.
Answer: The correct answer is 3. air is blown above the surface of liquid
(c) During boiling or vaporization
1. all molecules take part
2. temperature rises
3. no heat is absorbed
4. the average kinetic energy of molecules increases.
Answer: The correct answer is 1. all molecules take part
(d) The boiling point of a liquid is increased by
1. increasing the volume of liquid
2. increasing the pressure on the liquid
3. adding ice to the liquid
4. decreasing pressure on liquid.
Answer: The correct answer is 2. increasing the pressure on the liquid
(e) Two rods A and Bof the same metal, but of length 1 m and 2 m respectively, are heated from 0 to 100. Then
1. both the rods A and B elongate the same
2. the rod A elongates more than the rod B
3. the rod B elongates more than the rod A
4. the rod A elongates, but the rod B contracts.
Answer: The correct answer is 3. the rod B elongates more than the rod A
(f) Two rods A and B of the same metal, same length, but one solid and the other hollow, are heated to the same rise in temperature. Then
1. the solid rod A expands more than the hollow rod B
2. the hollow rod B expands more than the solid rod A
3. the hollow rod B contracts, but the solid rod A expands
4. both the rods A and B expand the same.
Answer: The correct answer is 4. both the rods A and B expand the same.
(g) A given volume of alcohol and the same volume of water is heated from the room temperature to the same temperature then.
1. alcohol contracts, but water expands
2. water contracts, but alcohol expands
3. water expands more than alcohol
4. alcohol expands more than water.
Answer: The correct answer is 4. alcohol expands more than water.
(h) The increase in length of a metal rod depends on
1. the initial length of the rod only
2. the rise in temperature only
3. the material of rod only
4. all the above three factors.
Answer: The correct answer is 4. all the above three factors.
(i) The correct statement is
1. Iron rims are cooled before they are placed on the cartwheels.
2. A glass stopper gets tighten on warming the neck of the bottle.
3. Telephone wires sag in winter, but become tight in summer.
4. A little space is left between two rails on a railway track.
Answer: The correct answer is 4. A little space is left between two rails on a railway track.