Class 7th, Science Ch. 4 Heat
Q. What do you mean by the heat? How the heat transferred or flows?
Ans. Heat: – It is a form of energy which transferred between one objects to another. It is also referred as heat energy or thermal energy.
Heat is transferred or flows from the high temperature (hot bodies) to the low temperature (cold bodies).
Q. What is the reliable measure of hotness? From which device it is measured?
Ans.
A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer.
Q. Why did the sense of touch is not used to take the measurement of the temperature?
Ans. The sense of touch can be used to understand the degree of hotness or coldness of something. But the sense of touch is not reliable and cannot be always used to say how much hot anything is.
Moreover, using the sense of touch can be risky in case of something very hot. It can burn our skin.
A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called thermometer.
Unit of heat:
Three units are used to measure the temperature: 1. Degree Celsius, 2. Fahrenheit and 3. Kelvin.
1. Degree Celsius: Degree Celsius is written as °C and read as degree Celsius or simply Celsius.
For example 30°C; it is read as thirty degree Celsius.
2. Fahrenheit: Fahrenheit is written as °F and read as degree Fahrenheit. For example 30°F; it is read as thirty five degree Fahrenheit.
3. Kelvin: Kelvin is written as K. For example 30K; it is read as thirty Kelvin.
Thermometer: – Thermometer is a device which is used to measure temperature.
Thermometer is made up of a long narrow glass tube; with a bulb at one end. The narrow tube appears as a continuous silver line; because it is filled with mercury. Mercury is a metal which is in liquid state at room temperature and it readily expands or contracts at the slightest change in temperature. Hence, mercury is used in thermometer. It is important to note that mercury is a highly toxic substance.
Types of thermometer:
Laboratory Thermometer: Laboratory thermometer is used to measure the temperature. The scale of temperature is graduated generally from –10°C to 110°C over the glass tube. Each division of temperature scale is further divided into 10 parts to read fraction of temperature.
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.
Clinicalal Thermometer: The thermometer that measures our body temperature is called a clinical thermometer. The normal temperature of human body is 37°C.
The scale of temperature is graduated in two units from 35°C to 42°C and or from 94°F to 108°F.
The temperature of human body always remains within this range of the clinical thermometer. There is a kink make near the bulb of clinical thermometer which prevents the automatic fall of mercury level.
A clinical thermometer consists of a long, narrow, uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining mercury thread can be seen with a scale on the it.
Precautions to be observed while reading a clinical thermometer
* 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.
* Do not use a clinical thermometer for measuring the temperature of any object other than the human body. Also avoid keeping the thermometer in the sun or near a flame. It may break.
Digital Thermometer: Reading of temperature in digital thermometer, is displayed digitally like a digital watch.
This is safer because no mercury is used in this. It is important to note that mercury is a highly toxic substance.
Maximum-Minimum Thermometer: Maximum – minimum thermometer is used to measure the daily temperature to prepare weather reports.
Reading of thermometer and measuring of temperature:
* Take and hold the clinical thermometer with reading scale towards our eye.
* Do not hold the thermometer from the bulb. It can increase or decrease the reading.
* if you are unable to see the reading, rotate the thermometer slightly clockwise or anticlockwise. By doing this you will be able to see a shiny thin silvery thread.
* The end of the silvery thread shows the reading of temperature. If reading of mercury lining ends at 37, the reading is 37°C.
* Wash the bulb end of thermometer with an antiseptic solution when it was used second time.
* If mercury level would falls to 35°C or below, Give two or three jerks slightly and then put it below the tongue and wait for one minute.
* Take out the thermometer and read the temperature which would be near 37°C because the normal body temperature is 37°C. This can be differ from person to person.
Use of Laboratory Thermometer:
* Take some water in a beaker.
* Take a laboratory thermometer and immerse its bulb end in water; holding it vertically. Ensure to dip whole portion of bulb end. The bulb end should not touch the bottom or side of the beaker.
* Observe the movement of rise of mercury. When it becomes stable, take the reading of the thermometer.
* Repeat this with hot water and take the reading.
Difference between clinical and laboratory thermometer:
Transfer of heat:
The heat flows from a hotter object to a colder object. In fact, in all cases heat flows from a hotter object to a colder object.
This flow of heat is known as transfer of heat. Heat is transferred by conduction, convection and radiation.
Conduction:
The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction. In this process, the transfer of heat takes place through adjacent molecules.
[When transfer of heat moves one particle to the adjacent particle is known as conduction of heat. ]
In solids, heat is transferred by the process of conduction.
Q. Show how the Flow of heat through a metal strip took place. explain it briefly.Ans.
Take a rod or flat strip of aluminium or iron metal. Fix a few small nails by wax pieces on the rod. These pieces should be at nearly equal distances. Clamp it at a stand or in between bricks.
Now, heat the other end of the rod and observe that wax pieces begin to fall. The piece nearest to the flame falls the first and for the best at last. The reason is that heat is transferred from the end nearest to the flame to the other end.
The transfer of heat from the hotter end to the colder end of an object is known as conduction.
Q. Why does it difficult to hold and metal rod when other end of this rod is put over flame?
Ans.
When one end of an iron rod is put over flame started becoming hot and heat gradually transferred to the other end of the rod due to the conduction. Now it difficult to hold a hot metal rod in hand.
In conduction particles of iron rod which are nearer to the flame receive the heat and transfers this to the adjacent particles. Subsequently, the adjacent particles transfer the heat to the next adjacent particles. This process continues and heat reaches to the other end of the rod.
Conductor and Insulator:
Conductor or Good Conductor of heat:– Materials which allow heat to pass through them are called conductor or good conductor of heat. Iron, copper, aluminium, and all metals are good conductors of heat.
Since, mercury is a metal and found in liquid state at room temperature, that’s why it is used in thermometer.
Since metals are the good conductor of heat that’s why kitchen utensils are made of metals or alloys of metals.
Materials which do not allow heat to pass through them are called insulators or bad conductor or poor conductor of heat. Rubber, wood, plastic, etc.
This is the cause that handles of frying pan or other kitchen utensils are made of plastic.
Convection:
When transfer of heat take place by the movement of the molecules of the medium is called convection or convection of heat.
Q. The water and air are poor conductors of heat. Then, how does the heat transfer take place in these substances?
Ans. The water and air are poor
conductors of heat. Water and air are fluids also and Heat transfer in fluids takes place through convection that is why they become hot, in spite of being bad conductors.
Convection in water: When water is heated in a pan, the particles of water which are near the flame heated first. After heating, they become light; and rise up in water. The created gap is filled with cold particles of water from the surrounding area. This forms a convection current between the molecules.
Thus a cyclic movement of particles begins and ends up after heating the whole water of the pan.
The cyclic movement of heating in fluids is called convection current.
Convection in air: Air gets heated because of convection current like water. Air near the source of heat gets heated and rises above. This leaves a gap; which is filled by the cool air particals of surrounding and starts the convection current as a results it heats up the air.
Q. When you place your palm above a flame you will feel the hotness of the flame. Why?
Ans. When we place our palm above a flame, the air towards the top gets heated by convection. Due to convection we will feel the hotness of the flame. Therefore, the hand above the flame feels hot.
When we place our palm below the flame the area due to no convection the air does not feel as hot as at the top.
Q. What do you mean by Sea Breeze and Land Breeze ?
Ans. In coastal areas, the air that moves from sea surface to the land is called sea breeze. in other words The air from the sea is called the sea breeze.
During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up leaving gap. The cooler air from the sea rushes in towards the land to take this gap. The warm air from the land moves towards the sea to complete the cycle. The air that moves from sea surface to the land is called sea breeze.
The windows of the houses in coastal areas are made to face the sea to receive the cooler sea breeze.
Land Breeze:–
In coastal areas, the breeze which moves from land towards the sea is called land breeze.
At night the water cools down more slowly than the land. This makes the air over the water surface warmer than land air. Warm air over the water surface rises up in the air and form gap then the air from the land rushes towards the water surface to fill the gap which creates a flow of air from land to the sea.
So, the cool air from the land moves towards the sea is called the land breeze.
Q. Why does the people living in coastal areas prefer to live in a sea facing house?
Ans. The people living in coastal areas prefer to live in a sea facing house to receive the cooler sea breeze.
Radiation:
Radiation is energy that comes from a source and travels in the form of invisible waves or rays through space at the speed of light.
* All hot bodies emit heat waves by the process of radiation. Radiation of heat waves does not require a medium.
* Sunlight comes to the earth in the form of heat radiation as there is no medium present between the atmosphere of the earth and the sun.
* One can feel the heat or warmth from the room heater due to its radiation.
Reflection and absorption of heat: When heat falls over an object some of the heat is absorbed by the body and some of the heat is reflected. Due to absorption of heat the temperature of the object increase.
In conventional room heater you can see the reflector attached with it. The reflector of the room heater reflects the heat towards the person sitting or standing near the room heater.
* The temperature of an object increases due to absorption of heat.
* The temperature of an object decreases due to releasing of heat.
* We use umbrella as the reflector, to protect us from heat of the sun in summer.
* Dark color absorbs more heat and light color reflects most of the heat. That’s why we preferred wearing light colored clothes in summer and dark colored clothes in winter.
* Dark clothes absorb more heat and keep one comfortable in winter due to warmth of absorbed heat. That’s why we preferred wearing dark colored clothes in summer.
* Light clothes reflect most of the heat and keep one comfortable in summer due to the reflection of heat. That’s why we preferred wearing light colored clothes in summer.
* Now-a-days many kitchen utensils come in black color, since utensils of black color absorb more heat and thus cooking becomes faster.
Q. Why does the wearing of woolen cloth makes one comfortable in winter season?
Ans. Woolen Cloth: Wool is a poor conductor of heat because air gets trapped in woolen fiber which further increase the poor conductivity of wool. That is why woolen clothes are used in winter season. These prevents the radiation of heat of our body to the surrounding and prevents the cold from surrounding to affect our body. Thus, wearing woolen cloth makes one comfortable in winter season.
Important Terms:
Temperature: The reliable measure of hotness or coldness of anything is known as temperature.
Thermometer: Thermometer is a device to measure the temperature.
Degree Celsius: Degree Celsius is the unit of measurement of temperature.
Clinical Thermometer: Clinical Thermometer is a device which is used to measure the temperature of human body.
Laboratory Thermometer: Laboratory Thermometer is a device which is used to measure the temperature in laboratory.
Maximum-Minimum Thermometer: Maximum-Minimum Thermometer is a device which is used to measure the temperature of climate.
Conduction: Mode of transfer of heat through atoms to atom in solids is called conduction.
Convection: Mode of transfer of heat through atoms to atom in in liquids and air is called conduction.
Radiation: Mode of transfer of heat in the form of wave without a medium.
Conductor: Materials that allow the flow of heat through them are called the conductor of heat.
Insulator: Materials that do not allow the flow of heat through them are called the Insulator of heat.
Land breeze: Breeze that flows from land surface to sea surface in summer nights is called Land breeze.
Sea breeze: Breeze that flows from sea surface to water surface in summer days is called Sea breeze.
Class seven science Heat NECRT Exercise Solution
Q. 1 State similarities and differences between the laboratory thermometer and the clinical thermometer.
Ans.
Similarities
1. Both thermometers are used to measure temperature.
2. both of them use mercury.
Differences
1. Clinical thermometer is used to measure human body temperature whereas laboratory thermometer is used to measure temperature higher than human body.
2. The temperature range of clinical thermometers is 35°C to 42°C and that of laboratory thermometers is -10°C to 110°C.
3. The least count of both the thermometers is different.
Q. 2 Give two examples each of conductors and insulators of heat.
Ans.
Examples of conductor:
1. Iron, 2. copper
Examples of insulator:
1. Rubber, 2. plastic
Q. 3 - Fill in the blanks:
(a) The hotness of an object is determined 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 Thermometer
(c) Celsius
(d) Radiation
(e) Conduction.
(f) Dark
Q. 4 Match the following :
(i) d , (ii) c , (iii) b , (iv) a .
Q. 5 Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing?
Ans. Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because between more layers of clothing, trapped more air is between them.
We know that air is insulator of heat and these different layers of insulated air, provides better protection against cold.
On the other hand, wearing just one piece of clothing has just one insulated layer of air and hence is less effective in beating the cold.
Q. 6 Look at figure Mark where the heat is being transferred by conduction, by convection and by radiation.
Ans
Q. 7 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 most of the heat falling on it. This makes the house colder.
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
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