Heat class 7 SDI

 

Date : May 26,2025                             HEAT                           Class work

Presentation

Sea breeze and land breeze

 

 

 

 

Concept 1 “ Hot and Cold”

 

Hot: Degree of measurement of having a high temperature than surroundings

Cold:  Degree of measurement of having a low temperature than surroundings

Concept 2 “ Measuring temperature”

Celsius scale:- A type of Scale use to measure temperature .

Temperature:- A reliable measure of the hotness of object is called temperature of that object.

Thermometer:- Device used to measure temperature is called thermometer.

It is of two types:-

Clinical thermometer

Laboratory thermometer

Question: State similarities and differences between the laboratory thermometer and the clinical thermometer.
Ans. Similarities:
(i) Both thermometers consist of long narrow uniform glass tubes.
(ii) Both have a bulb at one end.
(iii) Both contain mercury in bulb.
(iv) Both use Celsius scale on the glass tube.
Differences:
(i) A clinical thermometer reads temperature 35°C to 42°C while the range of laboratory thermometer is -10°C to 110°C.
(ii) Clinical thermometer has a kink near the bulb while there is no kink in the laboratory thermometer.
Due to kink mercury does not fall down on its own in clinical thermometer.

Question :In a mercury thermometer, the level of mercury rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury?

Solution:It is a property of mercury to expand when heated. Hence, after expansion it rises in the capillary tube.

Question : For setting curd, a small amount of curd is added to warm milk. The microbes present in the curd help in setting if the temperature of the mixture remains approximately between 35°C to 40°C. At places, where room temperature remains much below the range, setting of curd becomes difficult. Suggest a way to set curd in such a situation.
Solution:(i) In order to maintain the desired temperature of the mixture, the container can be wrapped either by woollen material or any other poor conductor of heat. Alternatively, the mixture can be kept in a heat resistant container.
(ii) The container can be kept in the sun or near the gas stove while cooking.

Question :To keep her soup warm Paheli wrapped the container in which it was kept with a woollen cloth. Can she apply the same method to keep a glass of cold drink cool? Give reason for your answer.
Solution:Yes, she can apply the same method to keep a glass of cold drink cool because wool is a poor conductor of heat.

Question : You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?
Solution:A clinical thermometer has a kink which prevents immediate backflow of the mercury from the tube to the bulb, thus allowing us to read the temperature conveniently. This is the reason why we give a jerk to the thermometer before taking a new reading. The jerk positions the mercury below 35 degree .

Question: Why is it advised not to hold the thermometer by its bulb while reading it?
Solution:It is advised not to hold the thermometer by its bulb while reading it because on holding a thermometer by its bulb, the mercury in the bulb expands due to our body temperature and gives false reading

Concept 3 “ Transfer of heat”

Conduction:-The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.

Conductor:-The materials which allows heat to pass through them easily are called conductors.

Example:- Aluminium, Iron and copper.

Insulator:- The material which donot allow heat to pass through them easily are called insulators.

Example:- Plastic and wood.

Convection:- The process of transfer of heat by actual motion of matter is called convection.

It is possible only in fluids.

Radiation:- The transfer of heat between separated by a distance even in vaccum is called radiation. Here no medium is required.

Land breeze:- At night cool air blows from land to sea in a cyclic manner. This is called land breeze.

Sea breeze:-At day cool air blows from sea to land in a cyclic manner. This is called Sea breeze.

Question: Give two examples each of conductors and insulators of heat.
Ans. 

Conductors—aluminium, iron

Insulators—plastic, wood.

Question : A laboratory thermometer A is kept 7 cm away on the side of the flame while a similar thermometer B is kept 7 cm above the flame of a candle as shown in figure.

Which of the thermometers, A or B, will show a greater rise in temperature? Give reason for your answer.

Solution: Thermometer B will show a greater rise in temperature because hot air rises up or air on the top of the candle flame is getting heated by convection

 

 

 

 

 

Question :A circular metal loop is heated at point O as shown in figure.

(i) In which direction would heat flow in the loop?
(ii) In which order the pins at points P, Q and R fixed with the help of wax fall if points O, P, Q and R are equidistant from each other?

Solution:
(i) The heat will flow in both the directions i.e. from O to P and O to R.
(ii) At first the pins at R and P will fall simultaneously followed by the pin at Q.

Question :In the arrangements A and B shown in figure, pins P and Q are fixed to a metal loop and an iron rod with the help of wax. In which case are both the pins likely to fall at different times? Explain.

Solution:
In case B, the pin P will fall before the pin Q because the heat will reach pin P first. In case A, the heat travels in both the directions and pins P and Q will fall simultaneously.

Question : While constructing a house in a coastal area, in which direction should the windows preferably face and why?
Solution: The windows of houses in coastal areas should preferably face towards the sea as sea breeze will keep it cool during day time.

Q.6. Look at figure 4.6. Mark where the heat is being transferred by conduction, by convection and by radiation
Ans.

 

Question : Observe the picture given in figure. Water is being boiled in a pan of wide base.

(i) Which position P or T will feel warmer?
(ii) Fill up the boxes P and T to indicate the mode of flow of heat to the hand.
Solution:(i) Position P will feel warmer than T because when the air gets heated, it rises upwards.
(ii) P → Convection
T → Radiation

Question : Look at figure.

The length of wire PQ in case of A is equal to the diameter of the semicircle formed by the wire CDE, in case B. One pin is attached to each wire with the help of wax as shown in figure. Which pin will fall first? Explain.
Solution:The pin on the wire in case A will fall first as heat will reach to it before it reaches the pin in case B (as CDE > PQ). In both the cases, heat is transferred to the pins mainly through conduction.

Concept 4 “ Kinds of clothes we wear in summer and winter”

Ques :Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.
Solution:
Shopkeepers selling ice blocks usually cover them with jute sacks because jute sack is an insulating material so it prevents ice from melting at a faster rate.

Question : At a camp site there are tents of two shades – one made with black fabric and the other with white fabric. Which one will you prefer for resting on a hot summer afternoon? Give reason for your choice. Would you like to prefer the same tent during winter?
Solution:(i) On a hot summer afternoon, the tent made up of white fabric will be preferred as white colour is a bad absorber and good reflector of heat.
(ii) No, the black fabric tent will be preferred during winter.

Question : Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing?
Ans: More layers of clothing keep us warm in winters as they have a lot of space between them. This space gets filled up with air. Air is a bad conductor, it does not allow the body heat to escape out.

Question: 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 wall of houses be painted white because white colour reflects heat and the houses do not heat up too much .

Back exercise

Q.3.Fill in the blanks
The hotness of an object is detetmined 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 transfers heat to its other end by the process of ____________
(f) Clothes of ___________ colours absorb heat better than clothes of light colours.

Ans. (a) temperature (b) clinical (c) Celsius (d) radiation (e) conduction (f) dark

Q.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.(d) Between 30°C and 50°C.

Q.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

Q.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.

Q.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 the stainless steel
(d) copper is easier to clean than the stainless steel
Ans.(c) copper is better conductor of heat than the stainless steel