1. Clothes We Wear -
ü Woollen clothes are obtained from animal fibers. Woollen clothes are generally thick and dark in colour. They are worn in the winter season as they protect us from the cold weather.
ü The dark colour clothes absorb the heat from the environment and keep us warm.
ü Cotton clothes are obtained from the plant fibers. The cotton clothes are generally thin and light in colour. They are worn in summer season so that we can feel cool and sweat can evaporate.
ü The light coloured clothes reflect the heat back into the environment and keep us cool during the summer time.
2. Hot And Cold-
ü The sense of touch help us to identify hot and cool substances. However, it can trick us sometimes. Like touching a hot pan will burn our skin. Therefore, we use a thermometer.
ü The temperature of an object is defined as the degree of hotness or coldness of an object.
ü Thermometer is a device that is used to find out how hot an object is. In other words, we use a thermometer to measure the degree of hotness or coldness of an object.
Figure - Clinical thermometer
ü Clinical thermometer - It is a device that is used to measure the body temperature of a person. It is made up of a glass tube of uniform thickness. The glass tube contains a bulb at one end which is filled with Mercury. The Mercury level in the thermometer rises up in the thread-like portion of the thermometer which therefore indicates the temperature of the body. The level of the Mercury can be measured by reading the scale given on the thermometer. The scale of the thermometer records the temperature in degree Celsius, generally, 35o C to 45o C, which is the range of human body temperature.
ü On an average, the temperature of the human body is around 37o C.
ü The clinical thermometer has a small sharp curve (kink) present near the bulb. This prevents the Mercury level from falling down on its own in the thermometer.
Figure - Laboratory thermometer
ü Laboratory thermometer - The laboratory thermometer is used to find out the temperature of the other objects such as water rather than human body temperature. It can measure the temperature from -10o C to 110o C.
ü Minimum-maximum thermometer: It is a thermometer used to measure the minimum and maximum temperature of the day by weather forecasters.
ü Digital thermometer: It is sometimes difficult to handle a Mercury filled thermometer especially when it breaks and the mercury falls out. However, nowadays digital thermometers are available to use. This type of thermometer does not contain Mercury. It directly displays the correct temperature on a display screen.
3. The Transfer Of Heat -
Figure - Heat transfer
ü The flow of heat always takes place from hot object to a cold object.
ü Conduction - The process of flow of heat from the hot object to the cold object is called conduction. Some objects can conduct heat while others cannot.
ü Conductors- The objects that allow the heat to flow through them from one end to another are called conductors. For Example, metals such as iron, copper and aluminum.
ü Insulators- The objects that do not allow the flow of heat from one end to another through them are called insulators. For Example, wood and plastic.
4. Convection-
Figure - Convection in liquids(water) and gas(air)
ü The transfer of heat in case of liquids and gases is called convection.
ü The molecules of the liquid or gases that are near the source of the heat get heated first. T
ü hey become lighter due to the heat and move upwards.
ü The colder particles being heavier take the place and this process continues until the whole liquid or the gas gets heated.
ü That is why the area above the flame of a candle always feels hot but the area on the sides of the candle does not.
5. Radiation-
ü It is a process of transfer of heat in the form of hot waves.
ü Radiation do not require physical connection between source and receiver.
ü For example- the sun's heat reaches the earth’s surface through radiation.
ü Every hot object radiates some heat into the environment. Hence, many times an object gets heated just by being near to a hot object.
6. Sea Breeze And Land Breeze-
ü The wind blowing from the sea towards the land is called sea breeze.
ü During the daytime, the land in the coastal area gets heated due to the sun's radiation.
ü The sea also gets heated, however it takes more time to get heated up than the land.
ü Hence the air above the land gets heated faster than the air above the sea.
ü The hot air from the land rises above as it is lighter and the cool air from the sea being heavier takes its place. This results in the sea breeze.
ü The wind blowing from the land towards the sea is called the land breeze.
ü During the night time, the land in the coastal areas gets cool down faster than the sea. The air above the sea is hotter than the air above the land.
ü Therefore the air above the sea rises and the air from the land being cool flows towards the sea. This results in the land breeze.
NCERT PRACTICE QUESTIONS
1. State similarities and differences between the laboratory thermometer and the clinical thermometer.
Ans.- Similarities -Both are made of glass and consist of long narrow glass tube. At one end both of them have a bulb. Bulbs of both the thermometers consist of mercury. Celsius scale is present in both the thermometer
Differences
Clinical Thermometer | Laboratory thermometer |
Temperature range if 35C to 42C | Temperature range is 10C to 110C |
Used to measure human body temperature | Used to measure temperature in the laboratory |
It has kink which prevents immediate back flow of mercury | It does not have a kink |
2. Give two examples each of conductors and insulators of heat.
Ans.- Conductors are Iron and Copper while insulators are Plastic and rubber.
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. Heat is transferred to its other end by the process of ______________.
(f ) Clothes of ______________ colours absorb more heat better than clothes of light colours.
Ans.- (a) temperature. (b) clinical (c) Celsius. (d) radiation.(e) conduction. (f ) dark
4. Match the following :
Ans.-
Column-I | Column-II |
(i) The land breeze blows during | (d) night |
(ii) The sea breeze blows during | (c) day |
(iii) Dark coloured clothes are preferred during | (b) winter |
(iv) Light coloured clothes are preferred during | (a) summer |
5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Ans.- Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing as air gets trapped in-between layer when we wear multiple clothes.
6. Look at Fig. 4.13. Mark where the heat is being transferred by conduction, by convection and by radiation.
7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Ans.- In places of hot climate, it is advised that the outer walls of houses to be painted white because white colour reflects back most of the heat that is falls on it as white poorly absorbs heat. This helps in keeping the house cool in warm weather.
8. One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be
(a) 80°C (b) more than 50°C but less than 80°C (c) 20°C (d) between 30°C and 50°C
Ans. - The temperature of the mixture will be between 30°C and 50°C because hot water loses the heat and simultaneously cold water gains heat this keeps the temperature in between high and low.
9. An iron ball at 40°C is dropped in a mug containing water at 40°C. 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.
Ans. - (b) not flow from iron ball to water or from water to iron ball.
10. A wooden spoon is dipped in a cup of ice cream. Its other end
(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.
Ans.- (d) does not become cold because wood is a bad conductor of heat.
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 a better conductor of heat than stainless steel.
(d) copper is easier to clean than stainless steel.
Ans. - (c) copper is a better conductor of heat than the stainless steel.
Junior Science Topic - Heat. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 04)
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