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DAV Class-8 Heat Question and Answer
Something To Know
A. Fill in the blanks.
1. Heat is a form of ___________.
2. The range of clinical thermometer is from ___________ °C to ___________ °C.
3. Water is a ___________ conductor of heat.
4. Land and sea breezes help to maintain the ___________ of air.
5. Transfer of heat, from the sun to the earth, takes place mainly through the process of ___________.
Answer: (1) energy (2) 35, 42 (3) poor / bad (4) temperature (5) radiation
B. Write True or False for the following statements.
| Questions | Answers (True/False) |
| 1. Heat cannot be produced by doing work. | False |
| 2. No gaps are left between the rails of railway tracks. | False |
| 3. Poor conductors of heat are also good insulators of heat. | True |
| 4. Ventilators, when provided in rooms, are located near their roofs. | True |
| 5. A material medium is required for transfer of heat by the process of radiation. | False |
Answer: (1) False (2) False (3) True (4) True (5) False
C. Tick the correct option.
1. A copper ball at 30°C is put in a container, containing water at 30°C. In this case-
Ans 1. there would be no transfer of heat between the copper ball and water.
2. The reading of the laboratory thermometer, in the figure shown here is-
Ans 2. 18°C
3. It is not convenient to use the laboratory thermometer to measure our body temperature. This is so because-
Ans 3. as the thermometer is taken out from the mouth, the level of mercury, in it, immediately starts falling.
4. When we hold our hands close to the side of a flame, they get warmed up mainly due to-
Ans 4. conduction as well as convection
5. The freezer compartment in a refrigerator is usually put near its top. This provides good cooling throughout the refrigerator through-
Ans 5. the setting up of ‘convection currents, in the air inside the refrigerator.
D. Answer the following questions in brief.
1. In which direction does the transfer of heat normally take place?
Ans 1: Transfer of heat normally takes place from the hot object to the cold object.
2. What is the cause of heat generation in the following situations:
(a) We apply brakes on our fast-moving car.
(b) People often jump up and down to feel warmer in cold weather.
Ans 2: (a) Frictional force
(b) Blood circulation rate increases
3. State two types of physical changes that may take place when a substance is heated.
Ans 3: Two types of physical changes are as follows:
(a) Change of state: when water is heated over a flame it starts boiling and gets converted into steam.
(b) change in temperature: when heat is added to a body its temperature rises.
4. In what way(s) does a clinical thermometer differ from an ordinary thermometer?
Ans 4: Clinical thermometer
(1) Range of the clinical thermometer is from 35 °C to 42 °C.
(2) It is used to measure the temperature of the human body.
(3) It has a ‘kink’ in its capillary tube.
(4) Jerks are given to lower the mercury level.
Laboratory thermometer
(1) Range of ordinary laboratory thermometers is from -10°C to 110 °C.
(2) Used to measure the temperature in the laboratory.
(3) No ‘kink’ is present in it.
(4) Jerks need not to be given, mercury level falls automatically.
5. State the mode/different modes, of heat transfer, in the following situations.
(a) A paper cup, full of hot soup, lying on a table.
(b) Cooking vegetables in a pan.
(c) Melting of a chocolate bar, in the school bag, on a hot day.
(d) Cooking food in a microwave oven.
Ans 5: (a) Convection
(b) Convection as well as conduction
(c) Conduction
(d) Convection
6. What is meant by sea breeze? When does it occur?
Ans 6: Sea breeze – the breeze that flows, from sea surface to land surface, during daytime is called sea breeze. It occurs during the daytime.
E. Answer the following questions.
Q1. In the arrangement shown in the figure pins A, B, C and D are fixed to a circular metal loop with the help of wax. The circular metal loop is heated at the point A with the help of a candle flame. In which order would the pins fall if AB < AD? Justify your answer.
Ans 1: AB < AD and candle flame is placed at A. Therefore, pin A will fall first and then pin B. After that pin D and at the last pin C will fall. This is because the transfer of heat takes place with a uniform rate on both sides of point A.
Q. 2. Give reasons for the following:
(a) Iron rims are heated red hot before ‘fixing’ them on cartwheels.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
(c) A new quilt is warmer than an old one.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
(e) The bottoms of cooking utensils are often kept black.
Ans 2: (a) Iron rim, to be put on a cartwheel, is designed to have a (slightly) smaller diameter than that of the wheel. When this rim is heated, it becomes ‘red-hot’, expands, and slips onto the wheel easily. When it is cooled, it contracts and grips the wheel firmly.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube because this ‘kink’ helps to maintain the level of mercury that raised due to temperature so that reading can be taken accurately.
(c) A quilt makes use of the bad conductivity of air. It traps a layer of air between themselves and our bodies and does not allow our body heat to escape out easily. Such trapping of air is much better in a new quilt than in an old one because when the quilt gets older, the air spacing of the cotton or wool gets compressed.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day because brass is a good conductor of heat. When we touch it, heat is conducted from the body to the brass tumbler easily and temp. of our body reduces to a lower value. Thus, we feel cooler. on the other hand, wood is a poor conductor of heat and very little heat is transferred to the wooden tray when we touch it. Thus, we don’t feel cooler.
(e) The bottom of cooking utensils is often kept black. This enables the utensils to better absorb the heat of the flame.
3. Akshit visited Rishikesh for river rafting during summer holidays. At a campsite, there were two tents, one made with a black fabric and the other with a white fabric. Which one should Akshit prefer? Give reason for the choice. Should Akshit prefer the same tent during winters?
Ans 3: Akshit should prefer a tent made with a white fabric because it will absorb less amount of heat energy coming from the sun.
No, during winter Akshit should prefer a tent made with a black fabric because it will absorb more heat energy coming from the sun and keep warmer.
Q. 4. Explain briefly how winds are caused.
Ans 4: The wind is caused by differences in the atmospheric pressure. When the air, at a place, gets heated up, it expands and occupies more space.
It, therefore, becomes lighter and moves up. The air pressure, at that place, then gets lowered. Hence, air from surrounding places, rushes in to take the vacated place. This sets up convection currents in air leading to winds and storms.
Q. 5. Supriya, while doing an experiment in the Science Laboratory, kept a laboratory thermometer ‘P’ 10 cm away on one side of the flame of a candle. Her friend Riya kept a similar thermometer ‘Q’ 10 cm above the flame of the candle as shown in the figure.
In which of the thermometers, ‘P’ or ‘Q’, the rise in temperature will be faster? Give reason for your answer.
Ans 5: The temperature in thermometer ‘Q’ will rise faster as it will receive more amount of heat energy because it is placed just above the candle flame. Hot air above the candle flame will raise up consequently temp. so, thermometer Q will rise faster.
Q. 6. In the two ‘set-ups’ X and Y, shown on the next page, the wires AB and PQR are made of the same material and have equal ‘thickness. The length of the wire AB, (in the set-up ‘X’) is equal to the diameter (= PR) of the semi-circle, formed by the wire PQR, (in the set-up ‘Y’). Pins, P1 and P2, are attached, to wires AB and PQR respectively, with the help of wax. Which of the two pins, P1 or P2, will fall off later? Give reason for your answer.
Ans 6: Pin P2 will fall off later because length of the wire PQR is larger than that of wire AB. The required amount of heat to fall off the pins is received by the pin Pl first.
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