Friday , August 14 2020
7th Class CBSE Science NCERT Book

Heat: 7th Class NCERT CBSE Science Chapter 04

Heat NCERT 7th Class CBSE Science Chapter 04

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?

Answer: As the temperature increases, then expansion in mercury takes place which leads to the rise in the level of mercury in thermometer.

Heat NCERT – Question: Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.

Answer: As we know that jute sacks is thermal insulators, it helps ice not to be melt immediately. So, shopkeepers used to cover ice blocks with jute sacks.

Question: The radiators in cars are painted black. Explain why.

Answer: Since, the black bodies are better radiators, so due to this reason, radiators in cars are painted black.

Question: To keep her soup warm, Paheli wrapped the container in which it was kept with a woolen clothes. Can she apply the same method to keep a glass of cold drink cool? Give reason for your Answer:

Answer: Yes, she can apply the same method to keep a glass of cold drink cool because wool is a thermal insulator, and it cannot allow to heat pass through it.

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 Swill show a greater rise in temperature? Give reason for your answer:

Laboratory Thermometer and flame

Answer: Thermometer B will show a greater rise in temperature because heated air above the candle rises immediately and increase the temperature of bulb of thermometer B quite greater than A.

Heat NCERT – Question: While constructing a house in a coastal area, in which direction should the windows preferably face and why?

Answer: Window should preferably towards the sea beach so that the sea breeze coming from sea keeps the house cool during the day time.

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

A circular metal loop is heated

  1. In which direction, would heat flow in the loop?
  2. 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?

Answer:

  1. Heat will flow in both the directions from O to P and from O to R.
  2. First of all pin at P and R will fall simultaneously after that pin at Q will fall.

Question: The mercury does not fall or rise in a clinical thermometer when taken out of the mouth. Explain why.

Answer: Because of the kink present in the thermometer, the mercury does not fall or rise.

Question: If a pan is removed from the fire, then why does it cool down?

Answer: When a pan is removed from the fire, it loses heat to the surroundings by radiation and it cools down.

Question: Name the mode of transfer of heat in which medium is not required.

Answer: Radiation is the mode of transfer of heat in which medium is not required.

Question: It is preferred to use two thin blankets rather than one thick blanket. Explain the reason.

Answer: In case of two thin blankets, there is an air gap which does not allow heat pass out from the body and it is not as such as in case of one thick blanket.

Question: Why is it advised not to hold the thermometer by its bulb while reading it?

Answer: It is advised not to hold the thermometer bulb while reading it as the level of mercury increases from the actual reading by our body temperature.

Question: Briefly mention one application of kink in the clinical thermometer.

Answer: The clinical thermometer consists of kink which prevents the mercury level from falling on its own.

Question: Explain how do we find out how hot an object really is.

Answer: By using thermometer, we can measure the degree of hotness of a body.

Question: Is the body temperature of every person 37°C?

Answer: No, the body temperature of every person is not 37°C. It is an average temperature. It could be slightly higher or slightly lower.

Question: How can we measure the temperature of other object rather than human body?

Answer: Temperature of other object is measured with laboratory thermometer because clinical thermometer is not suitable for higher temperature.

Question: How does the heat travel in air?

Answer: Heat travels in air through convection.

Question: We are advised to use an umbrella when you go out in the sun. Comment.

Answer: We use an umbrella to protect us from heat coming from the sun in the form of radiation.

Question: You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?

Answer: Jerks are given to clinical thermometer before using it to settle down the mercury level below normal temperature so that the measurement taken of a body be accurate.

Questions Short Answer Type

Question: The handle of a pressure cooker is covered with the thick plastic. Explain why.

Answer: As we know that plastic is a bad conductor of heat due to which the heat from the cooker does not flow to its handle and we can hold it easily.

So, this is a reason because of which the handle of a pressure cooker is covered with the thick plastic.

Question: Differentiate between two modes of transfer of heat, i.e. convection and conduction.

Answer: Difference between convection and conduction

Conduction Convection
The mode of transfer of heat from the hotter part material to its colder part or from a hot material to a cold material in contact with it without the movement of material as a whole. So, this phenomenon is known as conduction The mode of transfer of heat from the hotter part of a fluid to its colder parts by the movement of liquid itself. So, this phenomenon is known as convection.
In all the solids, heat is transferred by the process of conduction In all the liquids and gases, heat is transferred by the process of convection

Question: While reading a clinical thermometer, what precautions should we take?

Answer: Reading a Clinical Thermometer

There are following steps to read the temperature on a thermometer.

Step I:

Firstly, wash the thermometer with an antiseptic solution and if in case, the antiseptic solution is not available, then wash it with a clean water.

Step II:

Gently, hold the thermometer tube in your hand and give it a jerk in such a way that the mercury thread in the thermometer tube falls below the reading of 35°C.

Step III:

Now, put the bulb of the thermometer under the tongue of the patient for about one minute. Then take out the thermometer from patient’s mouth.

Step IV:

In order to read the temperature, hold the thermometer horizontally in your hand and rotate it slowly. When we see a magnified image of the mercury thread in its tube, then a position will come. Now, read the temperature on thermometer tube in level with the top of the mercury thread.

Precautions while Reading the Thermometer:

A clinical thermometer should not be used for any object other than the human body. There are some following precautions which are to be observed while reading a clinical thermometer.

  1. Wash the clinical thermometer before and after using preferably with an antiseptic solution.
  2. Be ensure that the mercury level before using the clinical thermometer should be below 35°C.
  3. The clinical thermometer should be read by keeping the level of mercury along the line of sight.
  4. While reading the clinical thermometer, it should never be held by the bulb.
  5. The clinical thermometer should be carefully handled.

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

Water is being boiled in a pan of wide base

  1. Which position P or T will feel warmer?
  2. Fill up the boxes P and T to indicate the mode of flow of heat to the hand.

Answer:

  1. As in the given diagram, position P will feel warmer because of heat, air rises.
  2. The flow of heat to P is a convection process and flow of heat to T is radiation.

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.

Answer: For the setting of curd where temperature is below room temperature, the container in which curd is to be made must be kept in a thermally insulated cover or it can be wrapped with wool or jute sacks so that temperature is maintained for the setting of curd.

Question: State the limitation of clinical thermometer.

Answer: The clinical thermometer cannot be used for measuring the temperature of any object more than 42°C (i.e. more than the body temperature). If it kept in the sun or near a flame, then this clinical thermometer can be break.

Question: Explain in brief, why we wear dark colored clothes in winter and light colored clothes in summer.

Answer: We wear dark colored clothes in winter to keep us warm as they absorb all the heat radiation coming from the sun whereas light colored clothes reflect most of the heat radiation coming from the sun that falls on them and therefore we feel more comfortable wearing them in the summer.

Question: Explain the reason for the shiny reflectors of room heaters.

Answer: Since, the shiny surface absorbs very little heat due to which the reflectors reflect all the heat which makes the room heaters more effective. So, due to this reason, room heaters have shiny reflectors.

Question: Describe the effects of heat.

Answer: The effects of heat are

  1. When an object is heated, then it becomes hotter and may get expand. So, in this heat may change the state of the body like ice can also get change into the water.
  2. Heat sometimes acts as a catalyst to speed up the chemical reaction.
  3. Heat can even kill the bacteria as well.

Question: The freezer is located at the top of the refrigerator. Explain why.

Answer: The freezer is located at the top of the refrigerator. It is because the air inside and around the compartments of freezer gets cool and move downwards and on the other side, the lesser cool air moves upward to get cooled.

So, as a result of this, convection of heat are setup in the air and the tower section of the refrigerator also gets cooled faster. .

Question: Instead of water explain the reason, why do the mercury is used in the thermometer?

Answer: Due to following reasons, mercury is used in the thermometers instead of water

  1. When the thermometer breaks, the mercury can be easily collected.
  2. Mercury can has more expansion than the water.
  3. Also, mercury do not stick on the walls of thermometer like water.

Question: Explain the reason for the general fitting of air conditioner at higher level on the wall of the room.

Answer: As a fact that warm air is much lighter than the cold air, so being heavier the cold air from air conditioner moves downwards while the hot air moves upwards at the lower level and gets cooled and come downwards once again. So, due to this convection of heat are set in the air and room gets warmer faster.

Question: Briefly explain, why do the kites and eagles fly without flapping their wings.

Answer: As it is a fact that the hot air during the day time surrounds the land gradually and rises up, due to which a convection air of heat develops. So, kites and eagles start moving along this rising current of heat without flapping the wings to fly high up in the sky.

Question: In the arrangements A and B as 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.

Metal loop and an iron rod

Answer: As in the given figures A and B, in case A P and Q got equal heat, so pin fix at P and Q in case of a fall simultaneously.

But in case of B pin at point P falls first than after pin at Q will fall as because heat received at P is greater than Q at the same time.

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?

Answer: We will prefer white fabric tent in case of summer because it reflects all the radiations from the sun and keeps us cool inside the tent.

But in ease of winter, we should not use white fabric tent, we should use black fabric tent as it absorbs all colors of light from the sun and keeps us warm inside the tent.

Question: Briefly explain the maximum and minimum thermometers.

Answer: Maximum and minimum thermometers are the special thermometers which automatically record the maximum and minimum temperature of the day. The maximum and minimum temperature of the last day reported in weather reports in TV and newspapers are measured by the maximum and minimum thermometers.

Question: Explain the reason for serving of tea in China clay cup and plates.

Answer: Tea is generally served in China clay cup and plates because China clay is a bad conductor of heat which do not allow the heat to pass through it and hence keep the tea hot.

Question: Describe briefly about the direction of the convection current during ventilation.

Answer: Since, with the help of ventilators, warm air being higher get rises up and leaves the room. Even the fresh air enters the room from outside through the doors and windows.

Question: Explain the reason for wrapping of ice in gunny bags.

Answer: Since, we know that gunny bags consist of number of fine pores which are filled with air. As air is a bad conductor of heat, so due to this reason, it does not allow the external heat to move inside and melts the ice.

Question: Mention any two examples each of insulators as well as of conductors.

Answer: Copper and aluminium are the examples of conductors which allow the heat to pass through them. While wood and plastic are the examples of insulators which do not allow the heat to pass through them.

Question: The clinical thermometer is not used to measure high temperature. Why?

Answer: Clinical thermometer has the range 35°C to 42°C. If we use it to measure high temperature, it may break and mercury present in the clinical thermometer is harmful. So, we cannot use clinical thermometer to measure high temperature.

Questions Long Answer Type

Question: Explain the differences between heat and temperature.

Answer: The differences between heat and temperature are given as below

Heat Temperature
It is a form of energy which flows from one body to another when there is a difference in temperature between the objects. It is the thermal condition of a body that indicates whether or not and in which direction, heat will flow from one body to the other.
It is equal to the sum of internal energies of the molecules of a body. It is proportional to average kinetic energy of the molecules of the body.
As a heat between two bodies, total amount of heat of two bodies remains unchanged. As a conclusion of exchange of heat between two bodies, sum of their temperatures before and after exchange is not same.
SI unit of heat is joule (J). SI unit of temperature is kelvin (K).

Question: Explain the reason for the following statement, “When heat is applied at the bottom of the water vessel, then it gets heated more quickly than when it is heated at the top.”

Answer: The particles of water become more lighter and rise up in the middle. Due to this, the cold particles of water which are heavier come down along the sides of the vessel. So, the convection currents of heat are setup and water becomes hot. The moment, when the heat is applied at the top, then the water present in the upper portion becomes hot and these hot water molecules get fixed over the surface. Since, water is a bad conductor of heat, due to which water present at the lower end (or bottom) of the vessel remains.

Question: Prove with the help of an experiment that heat radiation can travel through vacuum.

Answer:
First of all, take a flat bottom flask and fix a rubber stopper having two holes in the mouth of the flask. Shift a thermometer through one hole and a glass tube with stopper through the other hole. Now, with the help of a vacuum pump, remove the air from the flask and try to record the temperature from the thermometer. Put a flask in the bright sunlight and we will observe that the thermometer records a rise in temperature. So, on this basis, we can say that this experiment proves that the heat radiation can travel through vacuum.

Question: On last Monday, Pankaj was dealing immense cough and cold. Besides this, he was suffering from a great headache. So, he decided to take a leave on that day so that he could take some rest at his home. After one hour he thought to check his fever through a clinical thermometer, so before using it, he washed it and started shaking it very fastly in his hand, so that the mercury level of thermometer gets reduced. At the same time, his elder brother Vishwas scold him and suggested him to shake the thermometer slowly.

  1. State some precautionary measures while checking the fever through a clinical thermometer.
  2. Mention the values showed by the elder brother of Pankaj (i.e. Vishwas).

Answer:

  1. The following precautionary measures while checking the fever through a clinical thermometer are
    (i) While reading the clinical thermometer, it should never be held by the bulb.
    (ii) The clinical thermometer should be washed properly (before and after use).
  2. The values showed by the elder brother of Pankaj, Vishwas, are concern towards the way of using the clinical thermometer and the importance of money as well.

Question: Last Sunday, Pooja on her week off entered in her kitchen to cook rice for her family in lunch time. The moment she put fry pan on the gas stove and lighted it through the matchstick, all of sudden her 5 years small daughter entered into the kitchen and tends to touch the hot fry pan. At the same time, Pooja saw her and pushed her away from the hot fry pan. In this way, she saved her child.

  1. Explain why do the fry pan are provided with the plastic handles?
  2. Mention the values represented by Pooja here towards her 5 years small daughter.

Answer:

  1. As we know that plastic is a bad conductor of heat which does not allow the heat to pass through itself. So, heat from the fry pan does not flow to the handle due to which we can hold it easily. So that’s why, the fry pans are provided with the plastic handles.
  2. The value showed by Pooja here is a great concern and careness towards her five years old small daughter.

Heat NCERT – Question: Describe in detail about the occurrence of oceanic currents.

Answer: Since, it is a fact that the water present in the ocean (near the equator) is heated by the sun to a very higher temperature than the water near the poles of the earth. It is due to the falling of the sun rays perpendicularly in the equatorial region. So, in the equatorial region, the ocean water expands and becomes lighter.

While on the other side, the water in the polar region remains cold and heavy. It leads to the flow of convection currents of heat of warm water from the equator towards the poles.

Heat of the cold water flow from the poles towards the equator just below the surface of ocean. So, these types of currents are known as oceanic currents.

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