Grade 7 Q&A: Chapter 9: Heat

Concept Questions

Q1: Differentiate between heat and temperature.

Answer:

• Heat: Heat is a form of energy that flows from a hotter object to a colder object. It is the total kinetic energy of the molecules in a substance. Its SI unit is Joule (J).
• Temperature: Temperature is a measure of the degree of hotness or coldness of an object. It indicates the average kinetic energy of the molecules in a substance. Its SI unit is Kelvin (K), with Celsius (°C) and Fahrenheit (°F) being common units.

Q2: Name the three modes of heat transfer. Explain each with an example.

Answer: The three modes of heat transfer are:

1. Conduction: The transfer of heat in solids, from a hotter part to a colder part, without any actual movement of the particles of the medium. Example: When one end of a metal spoon is heated, the other end also becomes hot.
2. Convection: The transfer of heat in liquids and gases (fluids) from a hotter region to a colder region due to the actual movement of the fluid particles. Example: Boiling water in a pot where hot water rises and colder water sinks.
3. Radiation: The transfer of heat without any medium, through electromagnetic waves. It can occur even in a vacuum. Example: Heat from the sun reaching the Earth.

Q3: What are conductors and insulators of heat? Give two examples of each.

Answer:

• Conductors of Heat: Materials that allow heat to pass through them easily. Examples: Copper, Iron.
• Insulators (Bad Conductors) of Heat: Materials that do not allow heat to pass through them easily. Examples: Wood, Plastic.

Q4: Describe the construction and working of a clinical thermometer.

Answer: A clinical thermometer is a glass tube with a bulb at one end containing mercury. It has a narrow and uniform bore. A small bend or 'kink' is present near the bulb. When placed under the tongue or armpit, the mercury in the bulb expands due to body heat, rising in the tube. The kink prevents the mercury from falling back once removed, allowing the temperature to be read. Its range is typically 35°C to 42°C.

Q5: Explain the phenomenon of land breeze and sea breeze.

Answer:

• Sea Breeze (during the day): During the day, land heats up faster than the sea. The hot air over the land rises, creating a low-pressure area. Cooler, denser air from over the sea moves towards the land to fill this low-pressure area, resulting in a sea breeze.
• Land Breeze (during the night): During the night, land cools down faster than the sea. The sea water retains heat longer, so the air over the sea remains relatively warmer and rises, creating a low-pressure area over the sea. Cooler, denser air from over the land moves towards the sea, creating a land breeze.

Q6: What is specific heat capacity? How does it affect the climate of coastal areas?

Answer: Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree Celsius (or one Kelvin). Water has a very high specific heat capacity. Due to this high specific heat, large water bodies like oceans take a long time to heat up during the day and cool down during the night. This moderates the temperature of coastal areas, making them experience less extreme temperature variations compared to inland areas.

Q7: Explain thermal expansion with an example from daily life.

Answer: Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. Most substances expand (increase in size) when heated and contract (decrease in size) when cooled. An example is the gaps left between railway tracks. These gaps allow the metal tracks to expand safely during hot summer days without buckling or deforming.

Q8: What is a bimetallic strip, and how does it work? Give its application.

Answer: A bimetallic strip is made of two different metals (e.g., iron and brass) bonded together. Different metals expand at different rates when heated. When a bimetallic strip is heated, the metal with a higher rate of expansion expands more than the other, causing the strip to bend. When cooled, it bends in the opposite direction. This property is used in thermostats to control temperature, in fire alarms to detect heat, and in automatic switches.

Q9: List different scales used for measuring temperature.

Answer: The main scales used for measuring temperature are:

• Celsius scale (°C)
• Fahrenheit scale (°F)
• Kelvin scale (K)

Q10: Why is mercury preferred in thermometers?

Answer: Mercury is preferred in thermometers because it is a good conductor of heat, expands uniformly over a wide range of temperatures, does not stick to the glass, and is visible. It also has a high boiling point and a low freezing point, making it suitable for various temperature measurements.

Textbook Exercise Solutions

A. Fill in the blanks:

1. Heat is a form of energy.
2. Temperature is measured with a thermometer.
3. A clinical thermometer has a range of 35°C to 42°C.
4. The transfer of heat by vibration of molecules is called conduction.
5. Dark coloured surfaces are good absorbers of heat.

B. State true or false:

1. Heat flows from a colder object to a hotter object. False
2. The normal temperature of a human body is 37°C. True
3. A laboratory thermometer has a kink. False
4. Wood is a good conductor of heat. False
5. Light coloured clothes are preferred in winter. False

C. Match the columns:

Column A

1. Land breeze
2. Sea breeze
3. Radiation
4. Conduction
5. Convection

Column B

1. At night
2. In solids
3. No medium needed
4. At day
5. In liquids and gases

Answers:

1. Land breeze - At night
2. Sea breeze - At day
3. Radiation - No medium needed
4. Conduction - In solids
5. Convection - In liquids and gases

D. Answer the following questions briefly:

Q1: What is heat?

Answer: Heat is a form of energy that flows from a hotter object to a colder object. It represents the total kinetic energy of the molecules in a substance.

Q2: What is temperature?

Answer: Temperature is a measure of the degree of hotness or coldness of an object. It indicates the average kinetic energy of the molecules in a substance.

Q3: What are the precautions to be taken while using a clinical thermometer?

Answer: Precautions while using a clinical thermometer include:

• Wash the thermometer before and after use, preferably with an antiseptic solution.
• Ensure the mercury level is below 35°C (or 94°F) before use by giving it a few jerks.
• Do not hold the thermometer by the bulb while reading the temperature.
• Do not place it in hot flames or direct sunlight as it might break.

Q4: Name three materials which are good conductors of heat.

Answer: Three materials which are good conductors of heat are: Iron, Copper, and Aluminum.

Q5: Name three materials which are bad conductors of heat.

Answer: Three materials which are bad conductors of heat (insulators) are: Wood, Plastic, and Air.

Q6: What is a bimetallic strip?

Answer: A bimetallic strip is a strip made of two different metals (e.g., iron and brass) firmly joined together. It bends when heated or cooled because the two metals expand or contract at different rates.

Q7: What is thermal expansion?

Answer: Thermal expansion is the tendency of matter to change in volume (expand) in response to a change in temperature (heating). Most substances expand when heated and contract when cooled.

Q8: Why are railway tracks laid with small gaps in between them?

Answer: Railway tracks are laid with small gaps in between them to allow for thermal expansion during hot summer months. Metals expand on heating, and these gaps prevent the tracks from buckling or deforming due to expansion, which could otherwise lead to accidents.

E. Answer the following questions in detail:

Q1: Differentiate between a clinical thermometer and a laboratory thermometer.

Answer:

• Clinical Thermometer:
  • Purpose: Used to measure human body temperature.
  • Range: Typically 35°C to 42°C (or 94°F to 108°F).
  • Kink/Constriction: Has a kink near the bulb to prevent mercury from falling back on its own, allowing for accurate reading after removal.
  • Reading: Can be read after removing it from the body.
• Laboratory Thermometer:
  • Purpose: Used to measure temperatures in laboratories for experiments, such as liquids, chemicals, etc.
  • Range: Typically -10°C to 110°C.
  • Kink/Constriction: Does not have a kink.
  • Reading: Must be read while the bulb is still in contact with the substance being measured, as the mercury level drops quickly upon removal.

Q2: Explain the processes of conduction, convection, and radiation with diagrams. (Note: Diagrams are not provided here but conceptual explanations are below).

Answer:

1. Conduction: This is the transfer of heat in solids. When one part of a solid is heated, its particles gain kinetic energy and vibrate more vigorously. These vibrating particles collide with their less energetic neighbors, transferring energy to them. This process continues, and heat is transferred throughout the solid without the actual movement of the particles themselves.
   (Imagine a metal rod being heated at one end. The heat gradually spreads to the other end.)
2. Convection: This is the transfer of heat in liquids and gases (fluids). When a fluid is heated from below, the particles at the bottom gain energy, become less dense, and rise. Colder, denser fluid particles from the top then sink to take their place, get heated, and rise. This continuous cycle of rising hot fluid and sinking cold fluid creates convection currents, transferring heat throughout the fluid.
   (Imagine boiling water in a pot. Water at the bottom heats up, rises, and cooler water from the top moves down.)
3. Radiation: This is the transfer of heat without any material medium. Heat is transferred in the form of electromagnetic waves (like infrared rays). This process does not require any contact between objects or any medium for heat to travel through. It can occur even in a vacuum.
   (Imagine feeling the warmth of the sun or a campfire. The heat reaches you through radiation.)

Q3: Why do we wear woollen clothes in winter?

Answer: We wear woolen clothes in winter for two main reasons:

1. Insulation: Wool is a poor conductor of heat (an insulator). This means it does not allow body heat to escape easily into the colder surroundings.
2. Trapped Air: Woolen fibers have tiny air spaces trapped within them. Air is an excellent insulator. The trapped air prevents the movement of heat by convection from our bodies to the colder outside environment. Both these properties help to keep our body warm in winter.

Q4: Explain the importance of specific heat capacity in our daily life.

Answer: The high specific heat capacity of water is very important in daily life and natural phenomena:

• Climate Moderation: Large water bodies like oceans and seas absorb vast amounts of heat during the day and summer, and release it slowly during the night and winter. This high specific heat capacity prevents rapid changes in temperature, moderating the climate of coastal areas and making them less extreme than inland regions.
• Coolant: Water is used as a coolant in various applications, such as in car radiators and industrial plants, because it can absorb a large amount of heat without a significant rise in its own temperature.
• Body Temperature Regulation: The high water content in our bodies helps to regulate our body temperature, preventing rapid fluctuations even with changes in external temperature.
• Cooking: Water takes a long time to heat up but also retains heat for a long time, which is useful in cooking.

F. Give reasons:

Q1: White clothes are preferred in summer.

Answer: White clothes are preferred in summer because light colors are poor absorbers and good reflectors of heat radiation. They reflect most of the sunlight falling on them, preventing the absorption of heat by our bodies and thus keeping us cool.

Q2: Cooking utensils are made of metals.

Answer: Cooking utensils are made of metals (like aluminum or steel) because metals are excellent conductors of heat. This allows heat from the stove to transfer quickly and efficiently through the utensil to the food, ensuring that the food cooks uniformly and faster.

Q3: Handles of cooking utensils are made of wood or plastic.

Answer: Handles of cooking utensils are made of wood or plastic because wood and plastic are bad conductors (insulators) of heat. This prevents the heat from the hot metal utensil from transferring to the user's hand, making the handles safe to touch while cooking.

Q4: We use a quilt or blanket in winter.

Answer: We use a quilt or blanket in winter because they are made of materials like wool or cotton, which trap a large amount of air within their fibers. Air is a very poor conductor of heat (an excellent insulator). The trapped air prevents our body heat from escaping into the cold surroundings by convection, thus keeping us warm.

Q5: Mercury is used in thermometers.

Answer: Mercury is used in thermometers for several reasons:

• It is a good conductor of heat and expands uniformly with temperature.
• It is easily visible.
• It does not stick to the glass walls of the thermometer.
• It has a relatively high boiling point and a low freezing point, allowing it to measure a wide range of temperatures.

Q6: A laboratory thermometer cannot be used to measure human body temperature.

Answer: A laboratory thermometer cannot be used to measure human body temperature accurately because it does not have a kink or constriction near the bulb. As soon as it is removed from the patient's armpit or mouth, the mercury level falls rapidly, making it difficult to get an accurate reading of the maximum temperature reached by the body.

Q7: A clinical thermometer cannot be used to measure the temperature of boiling water.

Answer: A clinical thermometer cannot be used to measure the temperature of boiling water because its temperature range is very small (35°C to 42°C). Boiling water has a temperature of 100°C, which is far beyond the clinical thermometer's maximum limit. Attempting to measure such a high temperature would cause the mercury to expand beyond the thermometer's scale, potentially breaking the glass and making it dangerous.

References

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