MP Board Class 7th Science Solutions Chapter 4 Heat
Heat Intext Questions
Boojho says, My left hand tells me that the water in mug C is hot and the right hand tells me that the same water is cold. What should I conclude?
Both of the conclusions are true relative to each hand. But a single conclusion can not be drawn from the given information.
Boojho wondered which of the two scales shown in fig. he should read. Paheli told him that India has adopted the Celsius scale and we should read that scale. The other scale with the range 94-108 degrees is the Fahrenheit scales (°F). It was in use earlier.
Boojho got a naughty idea. He wanted to measure the temperture of hot milk using a clinical thermometer. Paheli stopped him from doing so?
Yes, do not use a clinical thermometer for measuring the temperature of any object other than he human body because it may break.
Boojho now understand why clinical thermometer can not be used to measure high temperatures. But still wonders whether a laboratory thermometer can be used to measure his body temperature?
Yes, laboratory thermometer can also be used for measuring body temperature.
Boojho wonders why the level of mercury should change at all when the bulb of the thermometer is brought in contact with another object?
The temperature of other object may not be the same as that of the bulb of the thermometer. When bulb is brought in contact with that object, the temperature of the bulb changes. Thus, the level of mercury also changes.
specific heat Calculator of liquid water at constant volume or constant pressure at temperatures from 0 to 360 °C.
Paheli asks: “Does it mean that heat will not be transferred if the temperature of two objects is the same?”
Lost some objects you use commonly in given table. Mark these objects as cold or hot?
Measure the body temperature of some of your friends (at least 10) with a clinical thermometer.
Observation for body temperature of some persons.
Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax pieces on the rod. These pieces should be at nearly equal distances (fig.). Clamp the rod to a stand. If you do not find a stand, you can put one end of the rod in between bricks. Now, heat the other end of the rod and observe.
What happens to the wax pieces? Do these pieces begin to fall? Which piece fall the first? Do you think that heat is transferred from the end nearest to the flame to the other end?
The wax pieces start to melt and fall down.
The piece nearest to the flame tall first. Yes, heat is transferred from the end nearest to the flame to the other end.
Heat water in a small pan or a beaker. Collect some articles such as a steel spoon, plastic scale, pencil and divider. Dip one end of each of these articles in hot water (fig.). Wait for a few minutes. Touch the other end. Enter your observation in Table.
Explain with activity heat transfer by conduction method?
To understand, ‘heat transfer by conduction’.
2. Required material:
Iron stick, some nails, candle.
Attach the nails on thin iron rod at some intervals with help of candle-wax as shown in figure. Now heat one end of iron rod with candle.
When we Start heating one end of iron rod, nails start falling one by one. The nail which is adjacent to hot end of iron – rod, falls first and then other nails 1 fall in sequence. Falling of nails in sequence indicate heat flow by conduction. Some time later, heat reaches to other end of rod and entire rod becomes hot.
Heat transfer in iron – rod takes place by conduction.
Take a round bottom flask (if flask is not available, a beaker can be used). Fill it two – thirds with water. Place it on a tripod, or make some arrangement to place the flask in such a way that you can heat it by placing a candle below it. Wait till the water in the flask in still. Place a crystal of potassium permanganate at the bottom of the flask gently using a straw. Now, heat the water by placing the candle just below the crystal. Write your observation in your notebook and also draw a picture of what you observe.
When water is heated, the water near the flame gets hot. Hot water rises up. The cold water from the sides moves
down towards the source of heat. This water also gets hot and rises and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is knwon as convection.
Heat Text book exercises
State similarities and differences between the laboratory thermometer and the clinical thermometer?
- Both use mercury.
- Both measure temperature.
1. The range of a mercury thermometer is 100°C. A mercury thermometer having graduations from 0°C to 100°C while the temperature interval marked on the clinical thermometers ranges from 35°C to 43°C.
2. The temperature on laboratory thermometer falls by itself, but not so in clinical thermometer. In clinical thermometers, a jerk is given so that it is set again for measuring human body temperature.
Give two emamples each of conductors and insulators of heat?
Conductors: Copper, Aluminium
Insulators: Wood, Plastic
Fill in the blanks:
- The hotness of an object is determined by its ………………
- Temperature of boiling water cannot be measured by a ………………. thermometer.
- Temperature is measured in degree …………………
- No medium is required for tansfer of heat by the process of ………………..
- A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of …………………..
- Clothes of ………………. dolours absorb heat better than clothes of light colours.
Match the following:
Discusss why wearing more layers of clothing during winter keeps us warmer than waering just one thick piece of clothing?
More layers of clothing keep us warm in winters as they have a lot of space between them. Air is a poor conductor of heat. This increases the insulation and thus, comfortability warm of the clothes as a whole.
Look at fig Mark where the heat is being transferred by conduction, by convection and by radiation?
In places of hot climate it is advised that the outer walls of houses be painted white. Explain?
In places of hot climate it is advised that the outer wall of houses be painted white because white colour do not radiate heat easily.
One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be
- More than 50°C but less than 80°C
- Between 30°C and 50°C
4. Between 30°C and 50°C.
An iron ball at 40°C is dropped in a mug containing water at 40°C. The heat will
- Flows from iron ball to water.
- Not flow from iron ball to water or from water to iron ball.
- Flows from water to iron ball.
- Increase the temperature of both.
2. Not flow from iron ball to water or from water to iron ball.
A wooden spoon is dipped in a cup of ice cream. Its other end
- Becomes cold by the process of conduction.
- Becomes cold by the process of convection.
- Becomes cold by the process of radiation.
- Does not become cold.
4. Does not become cold.
Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
- Copper bottom makes the pan more durable
- Such pans appear colourful
- Copper is a better conductor of heat than the stainless steel
- Copper is easier to clean than the stainless steel.
3. Copper is a better conductor of heat than the stainless steel
Extended Learning – Activities and Projects
Go to a doctor or your nearest health centre. Observe the doctor taking temperature of patients Enquire?
- Why she dips the thermometer in a liquid before use.
- Why the thermomter is kept under the tongue.
- Whether the body temperature can be measured by keeping the thermomter at some place other than the mouth.
- Whether the temperature of different parts of the body is the same or different. You can add more questions which come to your mind.
- Because to prevent infection. This liquid is an antiseptic.
- Because the temperature below the tongue represents actual temperature of body.
- Yes, but not so desirable.
- Temperature of different parts of the body are different.
Go to a veterinary doctor (a doctor who treats animals)?
Discuss and find out the normal temperature of domesitc animals and birds.
Wrap a thin paper strip tightly around an iron rod. Try to burn the paper with candle while rotating the iron rod continuously. Does it burn? Explain your observation?
No, the paper strip does not burn. Iron it a good conductor of heat. It takes away .the heat from paper and saves it from burning.
Take a sheet of paper. Draw a spiral on it as shown in the fig. Cut out the paper along the line. Suspend the paper as shown in figure, above a lighted candle. Observe what happens. Think of an explanation?
The paper spiral keeps moving. This is so because the air above the flame moves up due to convection.
Take two similar transparent glass bottles having wide mouths. Put a few crystals of potassium permanganate or pour a few drops of ink in one bottle. Fill this bottle with hot water. Fill the other bottle with cold water. Cover the cold water bottle with a thick piece of paper such as a postcard. Press the postcard firmly with one hand and hold the bottle with the other hand. Invert the bottle and place it on top of the hot water bottle. Hold both the bottles firmly. Ask some other person to pull the postcard. Observe what happens. Explain.
Do with the help of your subject teacher.
Heat Additional Important Questions
Objective Type Questions
Choose the correct alternative:
By which method heat transfer takes place in solids –
(d) All three.
When can feel the hotness of some hot substance like electric iron from a little distance by which method?
(d) All three methods.
By which method sun’s heat reaches to earth –
(d) Conduction and convection.
The handle of cooker is make up of bakelite because it is –
(b) Looks beautiful
(c) Bad conductor of heat
(d) Good conductor of heat.
(c) Bad conductor of heat
By which of the following processes the water circulates in radiator of the bus engine and keeps the engine cool –
(b) Conduction and radiation.
Ice is kept in four boxes made of the following materials. If the same quantity of ice is kept for same time in the boxes, in which box the ice will melt least –
In radiation, heat energy travel in –
(a) Curred lines
(b) Zig – zag lines
(c) Straight lines
(d) None of these.
(c) Straight lines
A thermos flask prevents loss or gain of heat by –
(a) Radiation only
(b) Conduction only
(c) Convection only
(d) None of these.
(b) Conduction only
Fill in the blanks:
- The measure of …………… energy is temperature.
- ……………. is the apparatus to measure temperature.
- The temperature at which a substance changes its state from solid to liquid is called its …………….
- In convection, water travels from …………. region to the ……………. region.
- The black bodies are good ………….. of heat.
- Water is a …………… conductor of heat.
- Transfer of energy between different part of a body is called ……………
- Shining bodies are poor ……………. of heat energy.
- Dark objects absorbs …………… heat then light coloured object.
- Melting point
- Hotter, colder
Which of the following statements are True (T) or False(F):
- Heat is a form of energy.
- Heat flows from a lower temperature to a higher temperature.
- The temperature of a healthy adult is 98.4°C.
- Iron is better conductor of heat than aluminium.
- The temperature at which a substance changes its state from solid to liquid is called its melting point.
- In thermometer highest points is 100°C.
- In thermometer’s the lowest point is 273°K Radiation of heat can take place without a medium. See
- breeze is the blowing of air from sea toward the land.
- Air is a good conductor of heat.
- Ture (T)
- False (F)
- False (F)
- False (F)
- True (T)
- Ture (T)
- True (T)
- True (T)
- True (T)
- False (F).
Match the items in Column A with Column B:
Heat Very Short Answer Type Questions
Define clinical thermometer?
The thermometer that is used to measures our body temperature is called a clinical thermometer.
What is the range of a clinical thermometer?
35°C to 42°C.
Why is range of a clinical thermometer is chosen to be 35°C to 42°C?
The clinical thermometer is designed to measure the temperature of human body only. The temperature of human body normally does not go below 35°C or above 42°C. That is the reason why this thermometer has the range 35°C to 42°C.
What is the use of kink in thermometer?
Kink prevents mercury level from falling on its own.
Why dose the mercury not fall or rise in a clinical thermometer when taken out of the month?
The kink prevents the mercury level from falling down.
Name two substances which contract on heating.
Which substances has the highest heat capacity?
Which gets hotter soon, land or water?
What is thermos flask?
Thermos flask is a device in which heat losses due to conduction, convention and radiation are minimised.
How does the heat travel in air? In which direction does the smoke go?
Heat moves in air by convection. The smoke moves in the upward direction.
How does the heat from the sun reaches?
Why are you advised to use an umbrella when you go out in the Sun?
The umbrella protects us from heat rays of the Sun.
What is temperature?
Temperature of an object is the degree of its hotnees. It tells us how hot or cold the object is.
Name the instrument we use to measure temperature.
We use thermometers to measure the temperature.
Write the S.I. unit of heat?
Write the names of the methods of heat transfer?
The names of methods of heat transfer are conduction, convection and radiation.
The iron nails from aluminium rods falls faster than from iron rod. What do you conclude from this statement?
It means that aluminium is a better conductor of heat as compared to iron. So it passes heat much quickly to the iron nails and they fall down due to melting of wax.
Why in summer days does the fan give hot air first in a closed room when it is switched on?
In a closed room the hot air remains at top because it is lighter. So when fan is turned on, it circulates the upper air first and it gives a current of hot air.
Why are the cooking utensile made up of metals?
Cooking utensils are made up of metals as they are good conductor of heat. They transfer the heat to the food inside quickly and the food is cooked easily.
What are good conductors of heat? Give examples.
Substances which allow the heat to pass through them are called good conductors of heat. Examples : all metal like iron, aluminium, copper and alloys like steel etc.
What are bad conductor of heat? Give examples.
Substance which do not allow the heat to pass through them are called bad conductor of heat.
Examples: wood, paper, asbestos, rubber etc.
Why do fire workers wear bright clothes?
Fire worker wears bright clothes because bright clothes are good reflector of heat. They send back the heat falling on them. Thus fire workers do not catch fire and heat easily.
On a cold winter night, why do we feel colder when the sky clear than when it is cloudy?
As clouds are bad conductor of heat, they do not allow the warm air around us to flow up. So heat remains near the ground and we feel warmer.
When does the conduction of heat stop in two bodies in contact?
The conduction of heat stops in two bodies in contact when the two bodies attain equal temperature. That is the two bodies have no temperature difference.
How do you note the temperature of the body with thermometer?
When bulb of thermometer comes in contact with our body. The mercury in the thin tube rises up. When the temperature of thermometer and body becomes equal, the level of the mercury does not rise further. This is the temperature of the body.
Heat Short Answer Type Questions
What is the difference between heat and temperature?
- Heat is a form of energy. It flows from a body at higher temperature to a body at lower temperature.
- Heat is measured in Joules or Calorie.
- Temperature is a measure of energy in the form of heat. Generally the temperature of a body increases when it is heated.
- Temperature is measured in Celcius.
On basis of what properties, mercury is used in thermometer?
Mercury is used in thermometer because of the following properties:
- Thermal expansion is uniform in mercury.
- As it is shining silvery white, its thermal expansion is seen easily. This helps in noting the reading of thermometer easily.
- It does not stick to the surface of glass.
- Mercury does not freeze at 0°C because it’s freezing point is 39°C.
It does not boil at 100°C because it’s boiling point is 357°C. Hence, mercury has wide range of temperature between its freezing point and boiling point.
Distinguish between conduction and convection.
- In conduction the molecules do not leave their position.
- It generally takes place in solid.
- In convection the molecules of the substance actually move from hotter region to colder region.
- It generally takes place in gases and liquids.
Why should we wear dark coloured clothes in winter and light coloured clothes in summer?
Bright clothes objects tend to reflect back the heat more, than to absorb the heat. Therefore, it is advisable to wear them in summer. On the other hand, dark coloured clothes have a greater tendency to absored and hence “retain” the heat. Hence they are worn in winter.
Why rise in temperature in two substances of equal quantity is not same even through equal amount of heat is given to them?
The same quantity of heat supplied to the same amount of different substances, then the rise in teperature would not be same bacause the rise in temperature of the substance would depend on their specific heat. The temperaute of the substance which has less specific heat would rise more.
What are the precautions to be observed while reading a clinical thermometer?
Precautions to be observed while reading a clinical thermometer are:
- Thermometer should be washed before and after use, preferably with an antiseptic solution.
- Ensure that before use, the mercury level is below 35°C.
- Read the thermometer keeping the level of mercury along the line of sight.
- Handle the thermometer with care. If it hits against some hard object, it can break.
- Don’t hold the thermometer by the bulb while reading it.
What are the precautions to be observed while reading a laboratory thermometers?
In addition to the precautions needed while reading a clinical thermometer, the laboratory thermometer
- Should be kept upright not tilted.
- Bulb should be surrounded from all sides by the substance of which the temperature is to be measured. The bulb should not touch the surface of the container.
Define mamixum – minimum thermometer.
Different types of thermoters are used for different puroposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum – minimum thermometer.
Draw diagram of the laboratory thermometer. Mark the lower and upper fixed temperatures on a Celsius scale and indicate the values in Kelvin scale aslo.
Name three different types of thermometers. Which of these is most sensitive?
The three different types of thermometers are:
- Mercury thermometer
- Alcohol thermometer
- Gases thermometer.
Mercury thermometer is used to measure nominal temperature. Alcohol thermometer is used to measure very low temperature. The gaseous thermometer is the most sensitive of all, as the gases are very sensitive to the slightest temperature change. The gaseous thermometers are rarely used because of the difficulty they pose in handling.
Define digital thermometers with diagram?
There is a lot of concern over the use of mercury in thermometers. Mercury is a toxic substance and is very difficult to dispose of if a thermometer breaks. These days, digital thermometers are available which do not use mercury.
Heat Long Answer Type Questions
Explain with experiment the method of heat transfer by radiation?
To know that the amount of heat absorbed or radiated by a substance depends upon colour of a substance.
Two containers of same metal, two thermometer, black and white colour, two cardboard pieces.
1. Colour the outer surface of one container with black and other with white.
2. Add equal quantity of tap water in both and insert thermometer by making a hole at the centre of card board as shown in picture.
3. Keep both containers for about 30 minutes under the sun. Then note down the temperature of both the containers.
4. Now add same quantity of hot water heated upto 50 to 60°C in both containers. Arrange the card board and thermometer properly and keep both of them is shadow for about 10 minutes. Then note down temperature of water from both containers.
After keeping containers for 30 minutes under the sun, you will see that, there is more increase in temperature of water in black container in comparision with white one. It is because black or dark colours are good absorbers of heat while white (light) colour is not.
After keeping the containers for 10 minutes in shadow, you will see that water in black container has cooled more than the water in white container. It is because black or dark colours are good emitters of heat, while white (light) colours are not.
- Black or dark colour are good absorbers or emitters of heat.
- White or light colurs are good reflectors of heat. It means they are not good absorbers or emitters of heat.
- The amount of heat emitted or absorbed by a substance depends upon colour of a substance.
Write some facts which indicates that sun light reaches to us by method of radiation?
Medium is required for transfer of heat by conduction and convection process. When you stand under the sun, you experience heat. Heat reaches upto us from the sun. It is necessary to know following facts:
1. Air is poor conductor of heat hence heat is not transfered by conduction method in air.
2. Sun’s heat does not reach upto us by convection process also. Beacuse, you know that, convection will start only after earth is heated.
3. There is vacuum between sun and errth i.e. there is no medium. Hence sun’s heat can not reach upto us by conduction or convection method because medium is required for both methods. It means that sun’s heat reached us by some other method called radiation.
How does the highest and lowest constant point of thermometer marked?
Two constant points i.e., lowest and highest are required to be marked while graduating the thermometer.
The lowest constant point:
The thermometer bulb is kept in a melting ice as shown in figure. Mercury descends in tube and remains constant after some time. This point is marked. This is lowest constant point of thermometer.
Highest constant point:
The thermometer bulb is kept in vapour of pure boiling water. Mercury ascends in tube and remains constant after some time. This point is marked. This is the highest constant point of thermometer.
Lowest constant point (temperature of melting ice) is indicated by °C and highest constant point (temperature of boiling water) is indicated by 100°C in this scale. The length between these two point is divided into 100 equal parts. The value of each parts is 1°C.
How does sea breeze flow?
Convection current has got an important role in change of seasons. Sea breeze is the best example of this principle. In fact during day time, seashore land gets hotter earlier than water. Air in contact with it, becomes hot and rises. (Density of hot air is less and hence it is lighter) and hence to take its place cold air above sea flows towards seashore. Because of it, during day time sea breeze flows.
During night time, this process gets reversed. See water becomes cool later than seashore during night time. (As the specific heat capacity of water is more than the seashore land, it takes longer time to become hot and also takes longer time to become cool). Hence during night time, temperature of sea is more than seashore. As a result surface air of sea gets hotter and rises and air above seashore takes its place. In this way during night time cool air flows towards sea.
What is thermometer? Describe its construction?
A thermometer is a device which is used to measure temperature accurately. The two commonly used thermometers are clinical thermometer and laboratory thermometer. The clinical thermometer is used by doctors to measure the body temperature. The laboratory thermometer is used in laboratories to measure melting point and boiling points.
A thermometer consists of a very fine capillary glass tube attached to a bulb containing mercury. In laboratory thermometer the mercury is at zero mark when placed in ice and is at hundred mark when placed in boiling water. Therefore, zero, mark is considered as lower fixed point and 100 mark is considered as upper fixed point (see fig.).
Explain the structure or thermos flask with neat diagram?
Thermos Flask was prepared first of all by Sir James Debar. It is used for storing cold or hot liquids for hours together. It is designed in such a way that heat transfer by conduction, convection and radiation should be minimum.