Notes
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Q. Rub your palms against each other and put them on your cheeks. What do you feel?
Ans. Heat is produced due to friction.
Heat : Heat is a form of energy. It can flow from one place to another which is called transfer of heat.
Heat is transferred by modes called conduction, convection and radiation.
Conduction : Transfer of heat from a hot part of a solid substance to its cold part is Called conduction. For conduction of heat a medium is required. It means heat cannot be conducted in vacuum. Conduction of heat takes place in solid substances.
Good and bad conductors of heat :
- Substances which easily transfer heat are called good conductors of Examples: copper, gold, aluminum etc.
- Substances which do not transfer heat are called bad conductors of heat. Examples: wood, plastic, paper, glass.
Q. Why does the halwai wrap up cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai?
Ans. When the slotted spoon comes in contact with the boiling milk, heat is transferred to the spoon because metal is good conductor of heat. The halwai may get a burn if he touches the hot spoon. To avoid burning of hand by holding the spoon while stirring, he wraps up a cloth around the end of the spoon. The cloth being a bad conductor of heat, does not transfer the heat to the hands of halwai. Hence, the halwai wraps up cloth around the end of his slotted spoon while stirring the boiling milk.
Q. Why do we hold a steel glass in a handkerchief while drinking hot milk from it? What are other examples of this kind? Make a note of them.
Ans. The handkerchief being a bad conductor of heat, does not transfer the heat to our hands from the steel glass. Therefore we hold a steel glass of hot milk in a handkerchief.
Some other examples are:
Cooking utensils are made with plastic coated handles.
We wear hand gloves while taking out hot pot from an oven.
Convection: In liquids, heat is transferred by means of convection. When heat is supplied at the bottom of a container, water at the bottom warms up first. As warm water has lower density it moves upwards and it is replaced by the surrounding cold water.
Owing to difference in density currents are formed and heat is transferred. Such transfer of heat by means of currents is called convection.
Convection of heat also requires a medium which may be in liquid or gaseous form. In liquids and gases heat is transferred by convection.
Q. Why do we use white clothes in summer and dark or black clothes in winter?
Ans. Absorption and reflection of heat radiations depends on the colour of the object.
White and light colours reflect most of the heat falling on them and dark or black colours absorb most of the heat.
Hence, to help in maintaining our normal body temperature we use white clothes in summer and dark clothes in winter.
Q. Why do we wear woollen clothes in winter?
Ans. Wool is a bad conductor of heat. The woollen clothes prevent the flow of heat from our body to the surroundings. Hence, we wear woollen clothes to keep our body warm during winter.
Radiation : Transfer of heat by means of electromagnetic waves is called radiation.
- It does not require solid or liquid medium.
- In space and vacuum heat is transferred by radiation.
- Radiation may occur even in the presence of solid or liquid medium. When heat waves fall on an object a part of it is absorbed and the rest is reflected.
Remember :
- Metals expand when heated and contract when heat is taken away.
- Generally, liquids expand when heated and contract when cold.
- Heat causes gases to expand and their volume reduces when heat is taken away.
All solids, liquids and gases expand on heating and contract on cooling.
Q. Why is there a gap at the joints of rails and of cement concrete bridges?
Ans. All solids, liquids and gases expand on heating and contract on cooling.
Rails made from metals and bridges contain some metals which may expand during summer due to temperature rise.
If there is no gap provided at the joints of these metals, the expansion may destroy the bridge or rail structure or may cause serious accidents.
Hence, to provide room for expansion of solids, there is a gap at the joints of rails and cement concrete bridges.
Q. Why is mercury or alcohol used in thermometer?
Ans. Mercury is a good conductor of heat, Mercury and alcohol need very little heat to expand. As a result, they can measure slight changes in temperature easily. Hence, mercury or alcohol is used in a thermometer.
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I appreciate your work brooda… 😊
Excellent…💯