Hot and cold objects :
List some objects you use commonly in table 4.1 . Mark these objects as hot or cold :
|Spoon in a tea cup||√|
|Handle of a frying pan||√|
1) Perform the given activity. Do both the hands get the same feeling?
Ans: No. The left hand feels hot whereas the right hand feels the water is cold.
2) Boojho says, “My left hand tells me that the water in mug C is hot and the right hand tells me that the same water is cold. What should I conclude?
Ans: Both the conclusions are correct relative to each hand. But this shows that we cannot arrive at conclusions just based on our sense of touch.
Measuring Temperature :
Measure the body temperature of some of your friends with a clinical thermometer and record your observations.
Table 4.2 Body temperature of some persons :
Name Temperature (oC)
Q Is the body temperature of every person 37oC?
Ans: No .It can be slightly higher or lower.
Laboratory thermometer :
Q1. Boojho now understands why clinical thermometer cannot be used to measure high temperatures. But still wonders whether a laboratory thermometer can be used to measure his body temperature.
Ans: No. This is because as soon as we remove the laboratory thermometer from our mouth, its mercury level starts falling and we will get wrong value.
Q2. Boojho wonders why the level of mercury should change at all when the bulb of the thermometer is brought in contact with another object?
Ans: The temperature of the other objects may be higher or lower. When the bulb comes in their contact the mercury may expand (hot objects) or contract (cold objects) depending upon their temperature.
Perform the activity :
Q1. Compare the temperature of water recorded by each student in the class. Are there any variations in reading? Discuss the possible reason.
Ans: Yes .The variations in the readings can be due to the differences in the temperature of water over time.
Q1. Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth? (Activity 4.5) .
Ans: Clinical thermometer has a kink near the bulb which prevents the mercury level from falling on its own.
Transfer of heat :
Q1. Paheli asks: “Does it mean that heat will not be transferred if the temperature of two objects is the same?
Ans: Yes. Transfer of heat will not take place.
Activity 4.6 (Perform the activity) :
a) What happens to the wax pieces?
Ans: The wax pieces melt.
b) Do these pieces begin to fall?
c) Which piece falls first?
Ans: The pieces closest to the flame fall first.
d) Do you think that heat is transferred from the end nearest to the flame to the other end?
Q2. Can you lift a hot pan by holding it from the handle without getting hurt?
Ans: Yes. If the handle is made of plastic or wood we can lift it without getting hurt as plastic and wood are bad conductors of heat.
Activity 4.7 (Perform the activity) :
Enter your observations in table 4.3 :
|Article||Material of the article||Does the other end get hot Yes/No|
Activity 4.8 (Perform the activity)
Q1. Write your observation in your notebook and also draw a picture of what you have observed.
Ans: We observe that when water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated.
Activity 4.9 ( Perform the activity )
Q1. Light a candle. Keep one hand above the flame and one hand on the side of the flame Do your hands feel equally hot? If not which hand feds hotter? And why?
Ans: No. The hand above the flame feels hotter. This is because the air gets heated towards the top by convection whereas on the sides there is no convection.
Q2. Why are you advised to use an umbrella when you go out in the sun?
Ans: To protect us from the heat of the sun.
Kinds of cloths we wear in summer and winter (Activity 4.10) :
Q1. Do you find any difference in the temperature?
Q2. In which can is the water warmer?
Ans: The can with black surface.
Activity 4.11 (Perform the given activity) :
Q1. Does the temperature of water in both the cans fall by the same amount .
Ans: No. The water in black can is hotter.
Q2. Suppose you are given the choice in winter of using either one thick blanket or two blankets joined together. What would you choose and why?
Ans: I would choose two thin blankets as there would be a layer of air in between the blankets. Air being the bad conductor of heat prevents heat loss from the body.
TEXT EXERCISES :
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
i) Similarities :
1). Clinical thermometer and laboratory thermometer have mercury in them.
2). At one end both of them have bulb.
ii) Differences :
Clinical thermometer Laboratory thermometer
1) Measures human body temperature 1) Used to measure temperature in the laboratory.
2) Temperature range is 35oC – 42oC 2) Temperature range is -10oC – 110oC.
2. Give two examples each of conductors and insulators of heat .
Conductors – iron, copper .
Insulators – plastic, wood .
3. Fill in the blanks :
a) The hotness of an object is determined by its ___________ .
b) Temperature of boiling water cannot be measured by a _________ thermometer.
c) Temperature is measured in degree __________ .
d) No medium is required for transfer of heat by the process of ___________ .
e) A cold steel spoon is dipped in a cup of hot milk. It transfer heat to its other end by the process of ___________ .
f) Clothes of __________ colours absorb heat better than clothes of light colours.
a) temperature b) clinical c) Celsius d) radiation e) conduction f) dark.
4. Match the following :
i) Land breeze blows during a) summer
ii) Sea breeze blows during b) winter
iii) Dark coloured clothes are preferred during c) day
iv) Light coloured clothes are preferred during d) night
i) d ii) c iii) b iv) a .
5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Ans: Many layers of clothing will have air trapped between its layer. Air being bad conductor of heat will act as an insulator and keep us warm.
6. Look at Fig 4.13 Mark where the heat is being transferred by conduction, by convection and by radiation. (Picture – Fig 4.13)
7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain .
Ans: White colour reflects most of the heat as white is a poor absorber of heat and hence the inside of the house will be cool.
8. One litre of water at 30oC is mixed with one litre of water at 50oC. The temperature of the mixture will be _________ .
a) 80oC b) more than 50C but less than 80oC
c) 20oC d) between 30oC and 50oC .
9. An iron ball at 40oc is dropped in a mug containing water at 40oc. The heat will ______ .
a) flow from iron ball to water.
b) not flow from iron ball to water or from water to iron ball.
c) flow from water to iron ball.
d) increase the temperature of both.
10. A wooden spoon is dipped in a cup of ice-cream. Its other and _________ .
a) becomes cold by the process of conduction.
b) becomes cold by the process of convection.
c) becomes cold by the process of radiation.
d) does not become cold.
11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that _________ .
a) copper bottom makes the pan more durable.
b) such pans appear colourful.
c) copper is better conductor of heat than stainless steel.
d) copper is easier to clean than stainless steel.
EXTENDED LEARNING – ACTIVITIES
1. Go to a doctor of your nearest health centre observe the doctor taking temperature of patients, Enquire.
a) Why she dips the thermometer in a liquid before use.
Ans: The liquid is an antiseptic. It prevents the spread of any infection within patients.
b) Why the thermometer is kept under the tongue.
Ans: The area under the tongue best represents our body temperature.
c) Whether the body temperature can be measured by keeping the thermometer at some place other than the mouth .
Ans: Yes, below the underarm .
d) Whether the temperature of different body parts of the body is the same or different.
Ans: Yes, the temperature varies slightly.
2. Wrap a thin paper strip tightly around an iron rod. Try to burn the paper with candle while rotating the iron rod continuously. Does it burn? Explain your observation does it burn? Explain your observations.
Ans: No, iron being good conductor of heat takes the heat away from paper.
3. Take a sheet of paper. Draw a spiral on it as shown in the fig 4.14. Cut out the paper along the line suspend the paper as shown in fig 4.14 above a lighted candle. Observe what happens, think of an explanations.
Ans: The paper keeps moving. As the air above the flame gets heated up it rises upwards due to convection. This movement of air moves the paper.