Class 7 Science Chapter 3 Question Answers - Heat

Short Questions and Answers

Q1: Define heat.
Ans: Heat is a form of energy which produces a sensation of warmth and flows when there is a temperature difference between two bodies.   It is measured in joules or calories.

Q2: Define temperature. What is its unit?
Ans:  A reliable measure of the hotness of an object is its temperature. Temperature is measured by a device called a thermometer. The unit of temperature is degree Celsius (°C), Fahrenheit, and Kelvin (SI unit).

Q3: What is the use of the maximum-minimum thermometer?
Ans:  Different types of thermometers are used for different purposes. The maximum and minimum temperatures of the previous day, reported in weather reports, are measured by a thermometer called the maximum-minimum thermometer.

Max-min Thermometer

Q4: Why does the mercury not fall or rise in a clinical thermometer when taken out of the mouth
Ans: Kink prevents mercury levels from falling on its own.

Q5: Why does a clinical thermometer range from 35^oC to 42^oC.?
Ans:  The average temperature of the human body is 37°C. The temperature of the human body normally does not go below 35^oC or above 42^oC. That is the reason that a clinical thermometer has a range of 35^oC to 42^oC.

Q6: What is conduction?
Ans: Heat flows from a hotter object to a colder object. The process by which heat is transferred from the hotter end to the colder end of an object is known as conduction.

Q7: Why conduction is only possible in solids?
Ans: In solids, generally, the heat is transferred by the process of conduction because particles of solids are closely packed, and heat is transferred from the hotter end to the colder end of an object.

Q8: How does the heat from the sun reach us?
Ans: It cannot reach us by conduction or convection as there is no medium such as air in most parts of the space between the earth and the sun. From the sun, the heat comes to us by a different process known as radiation, which does not require any medium for heat transfer.

Q9: Explain how water is heated by convection.

Ans: The water is a poor conductor of heat, so do not heat it by conduction. When water is heated, the water becomes lighter. Hot water rises up. The cold water from the sides moves down towards the source of heat. This water also gets hot and rises upward, and water from the sides moves down. This process continues till the whole water gets heated. This mode of heat transfer is known as convection.

Q10: Differentiate between conductor and insulators?
Ans: The materials which allow heat to pass through them easily are conductors of heat. For example, aluminum, iron, and copper The materials that do not allow heat to pass through them easily are poor conductors of heat, known as insulators, such as plastic and wood.

Q11: Explain land breeze.
Ans: The water cools down more slowly at night than the land. So, the cool air from the land moves towards the sea. This is called the land breeze.

Q12: Explain sea breeze.
Ans: During the day, the land gets heated faster than the water. The air over the land becomes hotter and rises up. The sea's cooler air rushes towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. The air from the sea is called the sea breeze.

Q13: Why is one thick blanket less warm than two thin blankets joined together?
Ans: There is a layer of air in between the blankets. Since air is a bad conductor of heat and prevents body heat from escaping out, two thin blankets provide more heat than one thick blanket.

Q14: How do woollen clothes keep us warm in winter?
Ans: Woollen clothes keep us warm during winter. It is so because wool is a poor conductor of heat, and it has air trapped in between the fibres. This air prevents the flow of heat from our body to the cold surroundings. So, we feel warm.

Q15: What is the unit of heat?
Ans:  Units of Heat

1. C.G.S. unit of heat is Calorie.
2. The M.K.S. or S.I. unit of heat is Joule

"1 calorie equals 4.18 or 4.2 joules approximately."

Q16: In hot climates, it is advised that the outer walls of houses be painted white. Explain.
Ans: In places with hot climates, it is advised that the outer walls of houses be painted white because a light colour absorbs very less radiant heat, and we feel comfortable inside such houses due to the lower temperature inside the house.

Long Questions and Answers

Q1: What is the Relation between Celsius, Fahrenheit, and Kelvin scales?
Ans: Since the range of temperature from ice-point to steam-point is equal in all three scales, 100 centigrade degrees = (212 - 32) or 180 Fahrenheit degrees = (373 - 273) or 100 absolute degrees. We consider three thermometers in the above three scales dipped simultaneously in a liquid of a certain temperature. Let the temperatures recorded in the Celsius, Fahrenheit, and Kelvin thermometers, respectively, be C, F, and K. Now it can be proved that C / 5 = F- 32 / 9 = K- 273 /5.

Q2: State similarities between the laboratory thermometer and the clinical thermometer.

Ans : 

• The laboratory thermometer and the clinical thermometer consist of a long, narrow, uniform glass tube.
• The laboratory thermometer and the clinical thermometer have a bulb containing mercury at the end of the tube.
• The laboratory thermometer and the clinical thermometer are marked with a Celsius scale on the glass tube.

Q3: State differences between laboratory thermometers and clinical thermometers.
Ans : 

Q4: Explain different thermometer scales?
Ans: There are three temperature scales in use today, Fahrenheit, Celsius, and Kelvin:

Fahrenheit temperature scale is based on 32 for the freezing point of water and 212 for the boiling point of water, the interval between the two being divided into 180 parts. The 18th-century German physicist Daniel Gabriel Fahrenheit originally took as the zero of his scale the temperature of an equal ice-salt mixture and selected the values of 30 and 90 for the freezing point of water and normal body temperature, respectively; these later were revised to 32 and 96, but the final scale required an adjustment to 98.6 for the latter value.

Until the 1970s, the Fahrenheit temperature scale was in general common use in English-speaking countries; the Celsius, or centigrade, scale, was employed in most other countries and for scientific purposes worldwide. Since that time, however, most English-speaking countries have officially adopted the Celsius scale. The conversion formula for a temperature that is expressed on the Celsius (C) scale to its Fahrenheit (F) representation is F = 9/5C + 32.

The celsius temperature scale, also called the centigrade temperature scale, is the scale based on 0 for the freezing point of water and 100 for the boiling point of water.  Invented in 1742 by the Swedish astronomer Anders Celsius, it is sometimes called the centigrade scale because of the 100-degree interval between the defined points. The following formula can be used to convert a temperature from its representation on the Fahrenheit ( F) scale to the Celsius (C) value: C = 5/9(F - 32). The Celsius scale is in general use wherever metric units have become accepted, and it is used in scientific work everywhere.  

The Kelvin temperature scale is the base unit of thermodynamic temperature measurement in the International System (SI) of measurement. It is defined as 1/ 273.16 of the triple point (equilibrium among the solid, liquid, and gaseous phases) of pure water. The kelvin (symbol K without the degree sign []) is also the fundamental unit of the Kelvin scale, an absolute temperature scale named for the British physicist William Thomson, Baron Kelvin. Such a scale has, as its zero point absolute zero, the theoretical temperature at which the molecules of a substance have the lowest energy. Many physical laws and formulas can be expressed more simply when an absolute temperature scale is used; accordingly, the Kelvin scale has been adopted as the international standard for scientific temperature measurement. The Kelvin scale is related to the Celsius scale. The difference between the freezing and boiling points of water is 100 degrees in each so that the kelvin has the same magnitude as the degree Celsius.

Q5: Explain laboratory and clinical thermometers along with their function and uses.
Ans: Difference between Lab Thermometers and Clinical Thermometers

1. Temperature Range in Laboratory and Clinical Thermometer- A clinical thermometer’s range is 35 degrees Celsius to 42 degrees Celsius, while a laboratory thermometer’s range is wider at -10 degrees Celsius to 110 degrees Celsius.
2. Accuracy of Laboratory and Clinical Thermometer- As a laboratory thermometer is more complex, it yields more accurate results as compared to a clinical thermometer.
3. Location- Clinical thermometers are often used in various locations like houses, clinics, and hospitals, while lab thermometers are mainly located in laboratories.
4. Application of Laboratory and Clinical Thermometer- Lab thermometers may be partially or fully immersed in liquids. On the other hand, clinical thermometers may be placed in the armpit, mouth, or anus.
5. Kink- Regarding mercury-filled devices, clinical thermometers have kinks to prevent the instant backflow of mercury. On the other hand, laboratory thermometers do not usually need to actively stop such backflow.
6. Users- Due to its simplicity, almost anybody can be taught to use a clinical thermometer. On the contrary, the lab thermometer is mainly used by individuals in the science field.
7. Purpose of Laboratory and Clinical Thermometer- While laboratory thermometers are intended to support a wide array of research activities, clinical thermometers are made for health care purposes.
8. Mercury in Laboratory and Clinical Thermometer- The utilization of mercury is typically less disadvantageous among laboratory thermometer users as compared to clinical thermometer users as the latter is more commonly used by the masses and has fewer restrictions.
9. Factors to Consider- Regarding clinical thermometers, the nature of suspected illness and the developmental stage of the individual are typically taken into consideration. As for laboratory thermometers, the nature of the research method is mainly factored in when choosing which device to use.
10. Accessibility of Laboratory and Clinical Thermometers- Since there are more individuals who need clinical thermometers, these devices are more accessible as compared to laboratory thermometers.