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
