Heat - Introduction
Why is it difficult to touch an object which is on fire for sometime? Because it is hot. Why it feels so difficult to keep ice in our hand for some time? Because it is cool, very cool than our hands. So here we get to know that we come across many things in our life, some are very hot and some very cold, Some cool, and some hot.
A hot cup of tea is hot , a soft drink kept in fridge is cool, a Frying pan in which some thing has been fried a while ago is hot, while ice cream is cool. We observe that some objects are hotter than others , while some objects are cooler than others. How can we measure which object is hottest or which object is the coolest? By touching.
But touching can be confusing sometimes. It is because a thing which seems hot to a cool hand, may feel cool to a hot hand. A hand if is cool will give us a feeling of any body to be hotter then the surroundings. Meaning thereby that our skin can sometimes give us wrong indication of a body’s temperature. Hence we need to have a device which can measure the hotness or coldness of a body.
Temperature is the measure. degree of hotness or coldness of a body. We can measure the temperature of a body with the help of a Thermometer. A thermometer is of two types based on the level of temperatures it can measure. The thermometers that we use at home or the doctors use are called CLINICAL THERMOMETERS. While those thermometers which can measure high ranges of temperatures are called LABORATORY THERMOMETERS.
The thermometers of present days use infra red technology and are far better then the older thermometers which used mercury in it. The new age thermometers are digital thermometers and are far more accurate, handy and durable.
Digital and Mercury Thermometer
Modes of Transfer of Heat and their usefulness.
Heat gets transferred from one place to another , from one body to another through 3 different modes. 1. Conduction, 2. Convection 3. Radiation.
Conduction is daily process occurring when the food is being cooked in the kitchen. The rice kept in cooker gets cooked due to heating of aluminum of cooker , which conducts heat and the rice is cooked.
Convection is the process through which we get our room heated with the help of a small heater or angithi. The heat transfers through hot air which travels from near to the heater to the whole room. Sea breezes are also examples of convection.
Radiation is the process in which no medium is required to heat any object. The sun light reaches our earth from very far away. It doesn’t need any medium and heats our Earth.
Types of clothes and seasons in which we wear them
Cotton and light colored clothes are worn in Summers. It is because lighter colors do not absorb, but reflect the light hence are suitable for summers. Cotton clothes allow the sweat to evaporate easily hence cause less heating and itching.
Dark colored and woolen clothes are preferred in winters because woolen clothes trap heat inside the body as they are bad conductors. Also the dark color absorbs heat coming from sunlight causing the hotness in winters.
EXERCISES
1. State the similarities and differences between the laboratory thermometer and the clinical thermometer.
Similarities :
- Both the Laboratory Thermometer and the Clinical Thermometer measure temperatures.
- Both thermometers consist of long narrow uniform glass tubes.
- Both have a bulb at one end.
- Both use Celsius scale on the glass tube.
Differences:-
- A clinical thermometer reads temperature from 35°C to 45°C.
- Clinical thermometer has a kink near the bulb.
- The range of laboratory thermometer is -10°C to 110°C.
- Laboratory thermometer doesn’t have a kink near the bulb.
2. Give two examples each of conductors and insulators of heat.
Answer- Conductors of heat- Metals like Iron, Aluminium.
Insulators of Heat- Wood , Plastic.
3. Fill in the Blanks:
a) The hotness of an object is determined by its Temperature.
b) Temperature of boiling water cannot be measured by a clinical thermometer.
c) Temperature is measured in degree celsius.
d) No medium is required for transfer of heat by the process of radiation.
e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
f) Clothes of dark colors absorb heat better than clothes of light colors.
4. Match the following:
i) Land breeze blows during | d) Night
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ii) Sea breeze blows during | c) Day
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iii) Dark colored clothes are preferred during | b) Winter |
iv) Light colored 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?
Answer- More layers will mean that more free space will be there between the layers. These free space will have air in it which is bad conductor of heat. Hence more layers will ensure more protection from cool temperature outside.
6. Look at the figure , 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.
Answer- In places of hot climate, it is advised that the outer walls of houses be painted white, because white color reflects back the sunlight and hence the walls do not get much heated.
8. One liter of water at 30 degree Celsius is mixed with one liter of water at 50 degree Celsius. The temperature of the mixture will be:
Answer- Between 30 degree Celsius and 50 degree Celsius.
9. An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will:
Answer- 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:
Answer- (d) does not become cold.
11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that:
Answer- (c) Copper is better conductor of heat than the stainless steel
KEY WORDS
- Celsius Scale:- Celsius Scale is is a temperature scale based on 0o for the freezing point of water and 100o for the boiling point of water. The clinical thermometers give readings in Celsius as well as Fahrenheit Scale.
- Insulator (Of Heat)– Those substances which do not allow the heat to pass through them are called Insulators of heat. Eg:- Plastic, Dry wood.
- Sea Breeze– It is the breeze or kind of wind that blows from a large body of water toward or onto a landmas. It flows due to temperature and pressure difference in air.
- Conduction– It is the process of transfer of heat in which the object is in direct contact with the heated object. Eg; A steel spoon dipped in hot tea gets heated.
- Land Breeze-The cool air which flows from land to the sea is called Land breeze.
- Temperature– Temperature is the measurement or degree of hotness or coldness of a body.
- Conductor (of Heat)- The substances whihc allow the heat to pass through them and pass heat to other biodies are called conductors of heat. Eg: all metals are conductors of heat.
- Radiation- Radiation is the process of transfer of heat in which no medium is required for the heat to flow from one place to other. Eg:- Sunrays heat up the Earth’s surface by radiation.
- Thermometer– Thermometer is a device which helps us to read and measure the temperature of a body.
- Convection- Convection is the process of heat transfer in which the heating occurs due to movement of heated fluid or gas. Eg: A room gets heated due to a small heater because of convection through heated air.