Class 7 Science Chapter 3 Question Answers - Heat

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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: The maximum-minimum thermometer is used to measure the highest and lowest temperatures of a place in a day. It is commonly used in weather stations to track daily temperature variations. This thermometer helps in understanding temperature fluctuations, which is essential for weather forecasting and analysis.

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:  Heat from the sun reaches us by radiation, which does not require a medium to transfer heat. Unlike conduction (which occurs in solids) and convection (which occurs in liquids and gases), radiation can transfer heat even through the vacuum of space

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: 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: How does heat transfer take place in solids, liquids, and gases?
Ans: Heat can be transferred in three different ways depending on the medium:

1. Conduction (Solids)

   • In solids, heat travels from one particle to another without movement of the particles.
   • Example: A metal spoon gets hot when placed in a cup of tea.

2. Convection (Liquids & Gases)

   • In liquids and gases, heat moves through rising hot particles and falling cold particles.
   • Example: Boiling water in a pan shows upward movement of hot water and downward movement of cold water.

3. Radiation (Without Medium)

   • Heat can also be transferred without any medium in the form of rays.
   • Example: Heat from the Sun reaches Earth through space by radiation.

Thus, different materials transfer heat in different ways, and understanding these helps in designing better insulators and conductors.

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

Ans: Similarities between laboratory thermometer and clinical thermometer are :

• Both are used to measure temperature.
• Both typically use mercury or digital technology to display readings.
• Both require careful handling to ensure accurate readings.
• Both thermometers have a scale marked in degrees Celsius.

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

Q4: Why do coastal areas experience land breeze and sea breeze?

Ans: In coastal areas, the movement of air between land and sea causes land breeze and sea breeze.

1. Sea Breeze (Daytime)

   • During the day, the land heats up faster than the sea.
   • The air above the land becomes hot and rises, while cooler air from the sea moves in to take its place.
   • This movement of air from the sea to the land is called sea breeze.

2. Land Breeze (Nighttime)

   • At night, the land cools down faster than the sea.
   • The air above the sea remains warmer and rises, and the cooler air from the land moves towards the sea.
   • This movement of air from the land to the sea is called land breeze.

These breezes help moderate temperatures in coastal areas, making them more comfortable.

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.