RBSE Solutions Class 7 Science Chapter 15 Temperature and Heat

Choose the Correct Answer

Question 1. Which material is used in laboratory thermometer?
(a) Sodium
(b) Mercury
(c) Melted aluminium
(d) Shining water
Answer: (b) Mercury
In simple words: Mercury is put in lab thermometers. It works well because it grows bigger or smaller smoothly when the heat changes, and it stays liquid for many temperatures.

🎯 Exam Tip: Remember that mercury is chosen for thermometers because of its consistent expansion and wide liquid range, which are key properties for accurate temperature measurement.

Question 2. In a heat transfer method, when molecules themselves move from one place to another to transfer heat, it is called.
(a) Conduction
(b) Convection
(c) Radiation
(d) Conduction and radiation both
Answer: (b) Convection
In simple words: Convection is when heat travels because the tiny bits (molecules) of a liquid or gas actually move from one spot to another, carrying the heat with them.

🎯 Exam Tip: Convection is a key method of heat transfer in fluids (liquids and gases), as it relies on the movement of the material itself.

Fill in the Blanks with Suitable Words

Question 1. Degree of hotness or coldness of an object is called..........
Answer: The level of hotness or coldness of an object is known as its temperature. This helps us quantify how warm or chilly something feels.
In simple words: How hot or cold something is called its temperature.

🎯 Exam Tip: A precise definition of temperature emphasizes its role in quantifying the thermal state of an object.

Question 2. A metal spoon is dipped into icecream pan for few moments then it will become............
Answer: If you put a metal spoon into an ice cream pan for a short time, the spoon will become cold. This happens because the heat from the spoon moves to the colder ice cream.
In simple words: A metal spoon put in ice cream will get cold quickly.

🎯 Exam Tip: Remember that heat always transfers from a warmer object to a colder one until their temperatures equalize.

Question 3. A layer of copper is adhered at the bottom of steel pan, because copper is good.
Answer: A layer of copper is often stuck to the bottom of a steel pan because copper is a very good conductor of heat. This allows the pan to heat up evenly and quickly.
In simple words: Copper is put on steel pans because it transfers heat very well, helping food cook faster.

🎯 Exam Tip: Understand that good thermal conductors like copper are used to improve heat distribution in cooking utensils.

Question 4. The heat flows from.............. temperature object to............ temperature object.
Answer: Heat naturally moves from an object with a higher temperature to an object with a lower temperature. This is why a hot cup of tea cools down in a cooler room.
In simple words: Heat always moves from something hot to something cold.

🎯 Exam Tip: This fundamental principle, that heat flows from higher to lower temperature, drives all natural heat transfer processes.

Question 5. Air and water are. of heat.
Answer: Air and water are known as bad conductors of heat, also called insulators. This means they do not transfer heat easily, which is why they are often used in insulation.
In simple words: Air and water do not carry heat well, so they are bad conductors.

🎯 Exam Tip: Recognize that poor conductors (insulators) are valuable for preventing or slowing down heat transfer in various applications.

Long Answer Type Questions

Question 1. Explain the difference between heat conductor and insulator with examples.
Answer: Materials that allow heat to pass through them easily are known as good conductors of heat. Metals such as aluminum, iron, and copper are common examples of good heat conductors. In contrast, materials that do not allow heat to transfer easily are called bad conductors or insulators. Wood, plastics, ebonite, and woollen clothes are examples of such insulators. Insulators are crucial for preventing heat loss or gain, like in thermoses.
In simple words: Good conductors let heat move through them easily, like metals. Bad conductors (insulators) do not let heat move through them easily, like wood or plastic.

🎯 Exam Tip: Always provide clear definitions and relevant examples when explaining scientific concepts to demonstrate a complete understanding.

Question 2. What are the different methods for heat transfer? Explain the difference in each.
Answer: Heat can be transferred through three main methods: conduction, convection, and radiation. Each method works in a unique way to move thermal energy from one place to another.
Here is a comparison of these methods:

🎯 Exam Tip: Clearly distinguishing between the three heat transfer methods, especially regarding the need for a medium and particle movement, is vital for scoring well.

Question 3. When we put objects in sunlight, why do black objects get more heated up as compared to lustrous objects although having same size and shape?
Answer: Black objects absorb heat more effectively, while shiny or lustrous objects are better at reflecting heat. When sunlight falls on them, a black object takes in most of the heat, causing its temperature to rise significantly. In contrast, a shiny object bounces most of the heat away, so it doesn't get as hot. This explains why black items feel much warmer than shiny ones of the same size and shape when left in the sun.
In simple words: Black things get hotter in the sun because they soak up heat, while shiny things stay cooler because they bounce the heat away.

🎯 Exam Tip: Remember that darker colors absorb more light and heat, while lighter and shinier surfaces reflect more, a principle seen in everyday life with clothing choices.

Question 4. Give reasons for formation of hot currents in sea?
Answer: The sun's rays hit the Earth more directly near the equator than at the poles, making the ocean water there much warmer. This hot water stays on the surface and travels towards the colder polar regions, creating what are known as hot water currents. Conversely, the very cold water near the poles is denser and sinks, then flows towards the equator, forming cold water currents. These currents play a major role in distributing heat around the planet.
In simple words: Ocean water near the equator gets very hot from the sun and flows to colder areas, making hot currents. Cold, dense water from the poles flows to the equator, making cold currents.

🎯 Exam Tip: Understanding the role of the sun's angle and water density in creating ocean currents is key to explaining global heat distribution.

Question 6. With the help of a diagram, explain heat conduction procedure.
Answer: To explain heat conduction, we can set up an experiment. Take a thin iron wire, like a bicycle spoke, and attach small pieces of wax along its length at even spaces. Place one end of this wire on a table and secure it with a brick. Then, heat the other end of the wire using a burning candle. You will observe that the wax piece closest to the candle melts and falls first, followed by the next one, and so on. This shows that heat travels along the solid wire from the hot end to the cooler end. This process of heat transfer in solids is called conduction. Conduction is how heat moves through a material without the material itself moving from place to place.
In simple words: To show conduction, heat one end of a metal wire with wax pieces on it. The wax melts one by one, showing heat moves along the wire.

🎯 Exam Tip: When asked to explain a procedure with a diagram, clearly describe each step and the observations, connecting them to the scientific principle being illustrated.

Question 7. How are we protected in winter by wearing woollen clothes. State the reason.
Answer: Woollen clothes protect us in winter because air is a poor conductor of heat. Woollen fabrics have tiny spaces that trap a layer of air. Both wool and the trapped air are bad conductors of heat, meaning they do not let heat pass through easily. This prevents our body heat from escaping into the cold surroundings, keeping us warm. It's like having a warm blanket of air around you.
In simple words: Woollen clothes keep us warm in winter because they trap air, and both wool and trapped air are bad at letting heat escape from our body.

🎯 Exam Tip: Focus on the insulating properties of both wool and trapped air when explaining why woollen clothes provide warmth.

Practical Work

Question 1. Take same sized pots and fill it with same amount of water, oil, sand and salt etc. for similar time period in sun. Measure the temperature of each and tell which one becomes rapidly hot and why?
Answer: If you take pots of the same size and fill them with equal amounts of water, oil, sand, and salt, then place them in the sun for the same length of time, you will find that sand usually heats up the fastest. Sand has a lower specific heat capacity compared to water, meaning it needs less heat energy to increase its temperature significantly. This is why beaches get very hot quickly, but the ocean takes longer to warm up.
In simple words: Sand in the sun will get hot fastest because it needs less heat to warm up compared to water or oil.

🎯 Exam Tip: Remember that different materials have different specific heat capacities, which affects how quickly they heat up or cool down.

Question 1. Make a chart of thermometers having Fahrenhiet and Celsius scales and display it Fahrenhiet.
Answer: To create a chart, one should display a diagram comparing Celsius and Fahrenheit thermometer scales. This diagram should clearly show the different markings and values for common temperatures like freezing point (0°C, 32°F) and boiling point (100°C, 212°F) on both scales. A clear visual representation helps in understanding the relationship between the two temperature systems.
In simple words: Make a chart that shows how Celsius and Fahrenheit thermometers look, with their numbers for hot and cold points side-by-side.

🎯 Exam Tip: When asked to display or create a diagram, ensure it is clearly labeled and accurate to show the required information, such as the major temperature points on different scales.

Question 3. With the help of a thermometer find out the melting point of ice and boiling point of milk.
Answer: To find the melting point of ice, you would gently heat a beaker filled with ice using a spirit lamp on a tripod stand. As the ice completely turns into water, use a thermometer to measure its temperature. This temperature, which is 0°C for ice, is its melting point. Melting is the process where a solid changes into a liquid when heated.
To find the boiling point of milk, place milk in a beaker and heat it on a tripod stand with a spirit lamp. Once the milk starts boiling, use a thermometer to measure its temperature. The temperature at which the milk begins to boil is its boiling point. Boiling is when a liquid changes into a gas, and this process usually occurs at a specific temperature.
In simple words: Melting point is when a solid turns liquid, like ice at 0°C. Boiling point is when a liquid turns to gas, like milk when it starts bubbling and steaming. Use a thermometer to find these exact temperatures.

🎯 Exam Tip: When describing experiments, clearly state the apparatus, procedure, observations, and the conclusion (the specific melting/boiling point) for full marks.

InText Questions

Question 1. We come across a number of objects everyday. Some of them are hot and some others are cold. List the names of some object. Mark these as hot or cold in the table given below. (Page-146)
Answer: In our daily lives, we encounter many objects that are either hot or cold. Here is a list of common objects, categorized by their temperature:

🎯 Exam Tip: When classifying items, ensure your categories are clear and that each item is placed correctly based on its typical temperature.

Activities

Activity-1 (Page-146)

Question 1. What do you feel ?
Answer: In an experiment where both hands are placed into a pot (Pot B) containing normal temperature water, the sensation differs. If one hand was previously in hot water and the other in cold water, the hand from hot water will feel the normal water to be cool, while the hand from cold water will feel it to be warm. This demonstrates that our sense of touch is subjective.
In simple words: When both hands are put in normal water after being in hot and cold water, one hand feels it warm, the other feels it cool. Our feeling of hot or cold can be tricked.

🎯 Exam Tip: Always relate experimental observations back to the underlying scientific principle, such as the relativity of human perception of temperature.

Question 2. Whether the water in pot 'B' is hot or cold?
Answer: The water in pot 'B' is neither strictly hot nor cold; it is at a normal or ambient temperature. Our perception of its temperature (whether it feels hot or cold) depends on the temperature of our hands before they were dipped into the pot. This shows that our sense of touch is not a reliable way to measure exact temperature.
In simple words: The water in pot 'B' is just normal. Whether it feels hot or cold depends on how hot or cold your hands were before.

🎯 Exam Tip: Differentiate between objective measurements (thermometer) and subjective sensory perceptions (touch) when discussing temperature.

Question 3. What does the experiment performed in this activity shows (Proves)?
Answer: The experiment performed in this activity demonstrates that our sense of touch is not a dependable way to determine the exact hotness or coldness of an object. What feels hot to one hand might feel cool to another, depending on its prior state. This highlights the need for precise measuring instruments like thermometers.
In simple words: This experiment proves that we cannot trust our touch to know how hot or cold something truly is.

🎯 Exam Tip: Conclude experiments by stating what they prove, emphasizing the limitations of human senses in scientific measurement.

Question 4. How do we know, how much hot an object really is?
Answer: To accurately determine how hot an object truly is, we must measure its temperature using a thermometer. Unlike our sense of touch, a thermometer provides an objective and precise reading of the object's temperature. It gives a standardized numerical value.
In simple words: We use a thermometer to accurately measure an object's temperature and find out how hot it really is.

🎯 Exam Tip: Emphasize that thermometers provide an objective measurement, which is crucial for scientific accuracy, unlike subjective human senses.

Question 5. What is temperature?
Answer: Temperature is a physical property that tells us the degree of hotness or coldness of an object when compared to other objects. It is a measure of the average kinetic energy of the particles within a substance.
In simple words: Temperature is how hot or cold something is, compared to other things. It tells us its level of heat.

🎯 Exam Tip: Define temperature not just by sensation, but also as a measurable physical property that indicates the thermal state of a body.

Activity-2 (Page-147)

Question 1. Observe carefully laboratory thermometer and fill the maximum and minimum values/ temperature printed on it?
Answer: When observing a laboratory thermometer carefully, you will typically find that the maximum temperature value printed on it is 110°C, and the lowest value is -10°C. This span from -10°C to 110°C represents the total range of temperatures that the thermometer is designed to measure. Understanding this range is important for choosing the correct thermometer for an experiment.
In simple words: A lab thermometer usually shows temperatures from -10°C to 110°C. This is its full range of measurement.

🎯 Exam Tip: When using a thermometer, always check its temperature range to ensure it is suitable for the experiment you are performing.

Question 2. How is temperature measured by thermometer?
Answer: To measure temperature with a thermometer, first dip its bulb into the liquid you want to measure. Keep it there until the mercury or alcohol level inside stops moving and becomes steady. Then, read the mark where the liquid level has settled; this reading indicates the temperature of the liquid. For example, if you measure water in pots A, B, and C, you would find that pot C has the highest temperature, pot B's water is cooler than C, and pot A's water is the coolest, potentially below 0°C.
In simple words: To measure temperature, put the thermometer bulb in the liquid. Wait for the mercury to stop moving, then read the number. That number is the temperature.

🎯 Exam Tip: Always wait for the thermometer reading to stabilize before noting down the temperature to ensure accuracy in your measurements.

Question 2. Clinical thermometer is washed with an antiseptic solution before, it is used, why?
Answer: Before using a clinical thermometer, it is crucial to wash it with an antiseptic solution. This practice ensures that the thermometer is free from any germs or infections, preventing the spread of diseases from one person to another. Sterilization is a vital step for patient safety.
In simple words: We wash clinical thermometers with germ-killing liquid before use to stop the spread of sickness.

🎯 Exam Tip: Always mention the importance of hygiene and preventing infection when discussing the use of medical instruments.

Question 3. Clinical thermometer is given a strong jerk every time before measuring temperature, Why?
Answer: Before taking a temperature reading with a clinical thermometer, it needs to be given a firm shake or jerk. This action helps to bring the mercury level down to its lowest point, usually below 35°C. This ensures that the thermometer is ready to measure the new body temperature accurately from a low starting point.
In simple words: We shake a clinical thermometer before using it to push the mercury back down, so it can measure the new temperature correctly.

🎯 Exam Tip: The 'jerk' is specifically to reset the mercury in a clinical thermometer, which has a kink to prevent mercury from falling on its own.

Activity-4 (Page-150)

Question 1. What is the difference in temperature of both water ?
Answer: When mixing normal water with boiled water, the normal water's temperature increases because it absorbs heat from the boiled water. Conversely, the boiled water's temperature decreases as it loses heat to the normal water. This shows heat transfer from the hotter substance to the colder one.
In simple words: When normal water and boiled water mix, the normal water gets warmer and the boiled water gets cooler.

🎯 Exam Tip: This experiment highlights the principle that heat always flows from a region of higher temperature to a region of lower temperature.

Question 2. What conclusion do you draw from above experiment?
Answer: The key conclusion from this experiment is that heat always moves from an object or substance with a higher temperature to one with a lower temperature. This natural flow continues until both objects reach thermal equilibrium, meaning they are at the same temperature.
In simple words: This experiment shows that heat always travels from a hot thing to a cold thing.

🎯 Exam Tip: State the conclusion clearly and concisely, directly linking it to the observed phenomena in the experiment.

Question 3. What change you noticed in the temperature of hot and cold water?
Answer: After mixing, it was observed that the temperature of the hot water decreased. At the same time, the temperature of the cold water increased. This is because heat energy was transferred from the hotter water to the colder water until they reached a new intermediate temperature.
In simple words: The hot water got colder, and the cold water got warmer.

🎯 Exam Tip: When describing changes, always specify both what happened to the hot component and the cold component in terms of temperature.

Question 4. Why does the temperature of both the containers become equal at the end of experiment?
Answer: The temperature of both containers becomes equal at the end of the experiment because heat energy transfers from the warmer water to the cooler water until a state of thermal equilibrium is reached. At this point, there is no longer a temperature difference to drive further heat flow, so both substances settle at the same final temperature.
In simple words: The temperatures become equal because heat moves from the warmer water to the cooler water until both are at the same heat level.

🎯 Exam Tip: The concept of thermal equilibrium, where heat transfer stops once temperatures are equal, is fundamental to understanding heat exchange.

Question 1. What happens if other end of spoke or wire is heated by burning candle?
Answer: If one end of a wire or spoke with wax pasted on it is heated by a burning candle, the wax on the wire will begin to melt. This occurs as heat travels through the metal wire by conduction, reaching the wax and causing it to change from a solid to a liquid state.
In simple words: When one end of a wire with wax on it gets hot, the wax starts to melt because heat travels through the wire.

🎯 Exam Tip: Observe the sequence of melting to understand the direction of heat flow by conduction along the material.

Question 2. Which piece of wax melts first and fell down?
Answer: The piece of wax that melts and falls off first will be the one closest to the burning candle. This is because heat transfers through the wire via conduction, reaching the nearest wax piece first and causing it to melt before the others further away.
In simple words: The wax closest to the candle melts and drops off first because it gets hot quickest.

🎯 Exam Tip: This observation helps confirm that heat is conducted sequentially along the material, affecting parts closer to the heat source first.

Question 3. What is the transfer of heat in solids called?
Answer: The method by which heat is transferred through solid materials is known as conduction. In conduction, heat energy is passed from one particle to the next without the particles themselves moving from their positions. This is how a metal spoon gets hot when placed in hot soup.
In simple words: When heat moves through solid things, it's called conduction.

🎯 Exam Tip: Clearly define conduction as the primary method of heat transfer in solids, emphasizing the lack of material movement.

Activity-6 (Page-151)

Question 1. Upper end of which of the object has become hot?
Answer: The upper end of objects made from materials like aluminium, a steel spoon, or an iron wire would become hot. This is because these materials are good conductors of heat, allowing thermal energy to transfer easily and quickly from the heated end all the way to the other end.
In simple words: The top part of things made from metal, like aluminum or steel spoons, gets hot because heat travels through these materials easily.

🎯 Exam Tip: Relate the heating of the object's upper end to the material's property of being a good conductor of heat.

Question 2. What are good conductors of heat?
Answer: Good conductors of heat are substances through which thermal energy can be transferred easily and efficiently. Metals like copper and aluminum are excellent examples because their particles are arranged in a way that allows heat to move quickly through them.
In simple words: Materials that let heat pass through them easily are called good heat conductors.

🎯 Exam Tip: Give specific examples of good conductors (metals) and explain why they conduct heat well (particle arrangement for energy transfer).

Question 3. Plastic and wood on heating become less hot. Give reason why?
Answer: Plastic and wood do not get as hot when heated because they are poor conductors of heat, also known as insulators. These materials do not allow thermal energy to transfer through them easily. Consequently, the heat applied to one part does not spread quickly to other parts, so they remain less hot overall.
In simple words: Plastic and wood do not get very hot when heated because they are bad at moving heat.

🎯 Exam Tip: Clearly explain that poor conductors (insulators) resist heat transfer, leading to them heating up slowly or unevenly.

Question 5. Stainless steel pots has a copper base why?
Answer: Stainless steel pots often have a copper base because copper is an excellent conductor of heat. This design helps in cooking food quickly and more evenly, as the copper layer efficiently transfers heat from the stove across the entire bottom of the pan. Steel itself is not as good a conductor as copper.
In simple words: Copper is put on steel pots because it's a very good heat conductor, helping food cook faster and more evenly.

🎯 Exam Tip: Emphasize the role of copper's superior thermal conductivity in improving the cooking efficiency of steel pans.

Question 6. During summers, why is it less troublesome to walk bare foot on mat or carpets as covering floor?
Answer: During summer, walking barefoot on mats or carpets is less uncomfortable because these materials are poor conductors of heat, acting as insulators. They do not readily absorb heat from the hot floor beneath them or transfer it quickly to your feet. This insulating property keeps the surface of the mats and carpets cooler, making them more pleasant to walk on.
In simple words: Mats and carpets don't get very hot easily because they are bad heat conductors. So, it's comfortable to walk on them in summer, even if the floor is hot.

🎯 Exam Tip: Explain that insulators like mats and carpets prevent rapid heat transfer, which is why they feel cooler on hot surfaces.

Activity-7 (Page-152)

Question 1. Why does colourful current set up in water?
Answer: When the bottom of a pot of water is heated, the water molecules at the bottom absorb heat, become less dense, and rise upwards. If colored dye is added, these rising warm water molecules carry the color with them, forming visible "currents." Simultaneously, cooler, denser water molecules from the top sink down to take their place at the bottom, where they then get heated and rise. This continuous circulation creates a pattern of colorful currents, illustrating the process of convection.
In simple words: Colorful currents form in heated water because warm water rises, carrying the color, while cold water sinks. This makes the water move in a cycle.

🎯 Exam Tip: Clearly describe the convection cycle: heating, density change, rising, cooling, sinking, and the continuous circulation.

Question 2. What is called this method of transfer of heat?
Answer: The method of heat transfer described, where heated fluid particles move and carry heat with them, is called convection. This is the primary way heat moves through liquids and gases. For example, the heating of a room by a radiator involves convection currents.
In simple words: This way of moving heat, where the liquid itself moves, is called convection.

🎯 Exam Tip: Define convection concisely and correctly identify it as the process of heat transfer through fluid movement.

Question 3. How is heat transferred in convection method?
Answer: In the convection method, heat is transferred through the actual movement of fluid molecules (in liquids or gases). When a part of the fluid is heated, its molecules gain energy, spread out, and become less dense. These warmer, lighter molecules then rise, carrying the heat with them, while cooler, denser molecules sink to take their place, creating a continuous current that transfers heat.
In simple words: In convection, heat moves because the hot parts of a liquid or gas physically move to a new place, carrying the heat with them.

🎯 Exam Tip: Differentiate convection from conduction (no particle movement) and radiation (no medium needed) by emphasizing the physical movement of particles.

Other Important Questions

Multiple Choice Questions

Question 1. Conductivity of heat is more in:
(a) Wood
(b) Iron
(c) Copper
(d) Soil
Answer: (c) Copper
In simple words: Copper is the best at letting heat pass through it compared to wood, iron, or soil.

🎯 Exam Tip: Remember that metals are generally better conductors of heat than non-metals, with some metals like copper and silver being exceptionally good.

Question 2. Heat is transferred with faster speed in:
(a) Conduction
(b) Convection
(c) Radiation
(d) By its own
Answer: (c) Radiation
In simple words: Radiation moves heat the fastest because it does not need anything to travel through and moves at the speed of light, like sunlight.

🎯 Exam Tip: Understand that radiation is unique because it can transfer heat through a vacuum, making it the fastest method of heat transfer.

Question 3. Heat reaches from the sun to the earth:
(a) By conduction
(b) By convection
(c) By radiation
(d) By conduction and convection's
Answer: (c) By radiation
In simple words: Heat from the sun gets to Earth by radiation, because there is empty space between them where heat cannot travel by touch or moving air/water.

🎯 Exam Tip: Recognize that radiation is the crucial method for heat transfer across space or a vacuum, as it doesn't rely on physical contact or fluid movement.

Question 4. Bad conductor of heat is:
(a) Iron
(b) Wood
(c) Copper
(d) Silver
Answer: (b) Wood
In simple words: Wood does not let heat pass through it easily, so it is a bad conductor of heat. Metals like iron, copper, and silver are good conductors.

🎯 Exam Tip: Distinguish between good conductors (typically metals) and bad conductors (insulators like wood, plastic, air) based on how easily heat transfers through them.

Question 5. By which method, heat is transferred in air?
(a) By conduction
(b) By convection
(c) By radiation
(d) By conduction and convection
Answer: (b) By convection
In simple words: Heat moves in the air mostly by convection, where warm air goes up and cool air comes down, making a flow.

🎯 Exam Tip: Remember that convection is the dominant mode of heat transfer in fluids (liquids and gases), including air.

Fill in the Blanks

Question 1. In weather reports.............. thermometer is used.
Answer: In weather reports, a maximum-minimum thermometer is often used. This special type of thermometer can record both the highest and lowest temperatures reached over a certain period, providing comprehensive climate data.
In simple words: Weather reports use a maximum-minimum thermometer to show the hottest and coldest temperatures.

🎯 Exam Tip: Identify special purpose thermometers like the maximum-minimum thermometer used for recording temperature extremes over time.

Question 2. Exchange of heat between two objects take place till their temperature becomes.
Answer: Heat will continue to transfer between two objects until their temperatures become equal. This state is known as thermal equilibrium, where there is no longer a temperature difference to drive the flow of heat.
In simple words: Heat stops moving between two things when they reach the same temperature.

🎯 Exam Tip: The principle of thermal equilibrium dictates that heat transfer ceases once all interacting bodies reach a uniform temperature.

Question 3. The transfer of heat in solid takes place by ........methods.
Answer: The transfer of heat in solids primarily occurs by the conduction method. In this process, heat energy is passed from one vibrating particle to the next through direct contact, without the overall movement of the material itself.
In simple words: Heat moves in solids through a process called conduction.

🎯 Exam Tip: Conduction is the main way heat travels through solid objects, relying on particle-to-particle energy transfer.

Question 4. In liquid and gas transferred heat takes place by.
Answer: In liquids and gases, heat is transferred mainly by the method of convection. This process involves the movement of the heated fluid itself, as warmer, less dense parts rise and cooler, denser parts sink, creating currents that circulate heat.
In simple words: Heat moves in liquids and gases through convection, where the hot liquid or gas moves around.

🎯 Exam Tip: Remember that convection is the primary mechanism of heat transfer in fluids (liquids and gases) due to the movement of the material.

Very Short Answer Type Questions

Question 1. What is temperature?
Answer: Temperature is a physical property that measures the degree of hotness or coldness of an object when compared to other objects. It reflects the average kinetic energy of the particles within a substance.
In simple words: Temperature tells us how hot or cold something is.

🎯 Exam Tip: A concise and accurate definition of temperature is essential, highlighting its role as a comparative measure of thermal state.

Question 2. Which device (instrument) is used to find temperature of an object?
Answer: The device or instrument used to measure the temperature of an object is called a thermometer. Thermometers work by using a substance (like mercury or alcohol) that expands or contracts predictably with changes in temperature.
In simple words: A thermometer is used to find out how hot or cold an object is.

🎯 Exam Tip: Know the correct instrument for measuring temperature and briefly understand its basic principle.

Question 4. What is the average temperature of human body?
Answer: The average temperature of a healthy human body is approximately 37°C, which is equivalent to 98.6°F. This is a critical value for maintaining normal bodily functions. Small deviations from this temperature can indicate illness.
In simple words: A normal human body temperature is about 37°C or 98.6°F.

🎯 Exam Tip: Remember the standard normal human body temperature in both Celsius and Fahrenheit scales for general knowledge.

Question 5. What precautions should be taken while using thermometer?
Answer: When using a thermometer, important precautions include never placing its bulb directly over a flame, very close to a flame, or in direct sunlight for extended periods. Doing so can cause the thermometer to break or even burst due to rapid expansion of the liquid inside, which could be dangerous. Always handle thermometers carefully.
In simple words: Do not put a thermometer near fire or in the sun for too long, or it might break or explode.

🎯 Exam Tip: Prioritize safety by understanding the risks associated with improper handling of thermometers, especially regarding exposure to extreme heat.

Question 6. By which method heat is transferred in solids?
Answer: In solid materials, heat is transferred primarily by the conduction method. This involves the vibration of particles passing energy to adjacent particles, without any overall movement of the material itself. Metals are good examples of materials where conduction is efficient.
In simple words: Heat moves through solids using a method called conduction.

🎯 Exam Tip: Conduction is the specific term for heat transfer in solids; avoid confusing it with convection or radiation in this context.

Question 7. By which method heat is transferred in liquids and solids?
Answer: In liquids, heat is primarily transferred by the convection method. In this process, warmer liquid moves and carries heat with it. However, in solids, heat is transferred by the conduction method, where heat energy passes from particle to particle through direct contact. Understanding both processes clarifies how heat moves in different states of matter.
In simple words: Heat moves in liquids by convection (liquid moves), and in solids by conduction (heat passes from particle to particle).

🎯 Exam Tip: Remember that convection is for fluids (liquids and gases), while conduction is for solids. Do not mix these methods for different states of matter.

Question 8. By which method heat from the sun reaches to the earth?
Answer: Heat from the sun travels to the Earth through the method of radiation. This process does not require any physical medium, allowing heat to cross the vast vacuum of space via electromagnetic waves. It's the only way heat can travel through empty space.
In simple words: Heat from the sun comes to Earth by radiation, which doesn't need air or water to travel.

🎯 Exam Tip: Recall that radiation is the unique heat transfer mechanism capable of working across the vacuum of space.

Question 9. In which method of heat transfer, medium is not necessary?
Answer: The method of heat transfer where a medium is not necessary is radiation. Radiation involves the emission of electromagnetic waves, which can travel through a vacuum or transparent media, directly carrying thermal energy. This is distinct from conduction and convection, which both require a material medium.
In simple words: In the radiation method of heat transfer, no material like air or water is needed for heat to move.

🎯 Exam Tip: Understand that radiation's ability to transfer heat without a medium is what allows energy from distant sources like the sun to reach us.

Question 10. Why do woollen garments protect us from cold?
Answer: Woollen garments protect us from cold because wool is a bad conductor of heat, and more importantly, they trap a layer of air within their fibers. Since air is also a very poor conductor of heat (an excellent insulator), this trapped air prevents our body heat from escaping into the cold environment, thus keeping us warm.
In simple words: Woollen clothes keep us warm because both wool and the air trapped inside them are bad at letting heat escape from our body.

🎯 Exam Tip: Emphasize the combined insulating effect of both the wool material itself and the air trapped within its structure.

Question 1. What is thermometer? How many types of thermometers are there?
Answer: A thermometer is a device used to measure the temperature of an object. There are different kinds of thermometers. Some common types include the laboratory thermometer, gas thermometer, alcohol thermometer, and digital thermometer. Each type serves a specific purpose for measuring temperature accurately.
In simple words: A thermometer measures how hot or cold something is. There are many types, like the ones used in labs or the digital ones we use at home.

🎯 Exam Tip: When asked about types, always list at least 2-3 examples to show a broad understanding of the concept.

Question 2. Use of digital thermometer is more safe why?
Answer: Digital thermometers are safer to use because traditional mercury thermometers contain mercury, which is a poisonous (toxic) material. If a mercury thermometer breaks, mercury can spread, and if accidentally consumed, it can cause serious harm to the body. For this reason, digital thermometers are now more commonly used instead of mercury thermometers, as they are non-toxic and safer. The absence of mercury makes them ideal for household and clinical use.
In simple words: Digital thermometers are safer because they do not use mercury. Mercury is a harmful substance that can be dangerous if a thermometer breaks.

🎯 Exam Tip: Focus on the safety aspect and the harmful nature of mercury as key points for this answer.

Question 3. How long exchange of heat between two objects takes place?
Answer: Heat continues to be exchanged between two objects until both objects reach the same temperature. Once their temperatures become equal, there will be no further net transfer of heat between them. This is a fundamental principle of thermal equilibrium. In simpler terms, a hot body will transfer heat to a cold body until both are equally warm or cold.
In simple words: Heat moves between two things until they both become the same temperature. Once they are equally hot or cold, heat stops moving.

🎯 Exam Tip: Remember the concept of thermal equilibrium – heat transfer stops when temperatures are equalized.

Question 4. What is importance of ventilation in the homes?
Answer: Ventilation is important in homes because the air we breathe out is warmer and less dense than the normal air. This warmer, lighter air rises upwards towards vents near the roof and escapes from the house. Then, fresh air, which is heavier and denser, enters the room through doors and windows to replace it. This continuous flow helps keep the air inside fresh and regulates indoor temperature, ensuring a healthy environment.
In simple words: Ventilation helps warm, old air leave a house and fresh, cool air come in. This keeps the air inside healthy and pleasant.

🎯 Exam Tip: Highlight the density difference between warm and cool air, and how it drives natural ventilation for full marks.

Question 5. How do air blow?
Answer: Air blows from an area where the air pressure is higher to an area where the air pressure is lower. This movement of air from one place to another is called wind. When the ground in an area heats up more, the air above it also becomes warmer and rises, creating a low-pressure area. Cooler, denser air from surrounding high-pressure areas then moves in to fill this space, causing the air to blow. This natural process is essential for global weather patterns.
In simple words: Air blows because it moves from places with high air pressure to places with low air pressure. This movement is what we call wind.

🎯 Exam Tip: Key concept is air moving from high to low pressure; mentioning temperature differences creating these pressure zones shows deeper understanding.

Question 6. What do you mean by sea breeze and land breeze?
Answer: A sea breeze occurs during the day when land heats up faster than the sea water. As the land gets hotter, the air above it rises, creating a low-pressure area. Cooler air from over the sea then blows towards the land to fill this space, forming the sea breeze. Conversely, a land breeze happens at night. The land cools down faster than the sea, so the air above the sea stays warmer and rises, creating a low-pressure area over the water. Cooler, denser air from the land then blows towards the sea to replace it. Both breezes are important local wind phenomena.
In simple words: Sea breeze is cool air from the sea moving to land during the day. Land breeze is cool air from land moving to the sea at night.

🎯 Exam Tip: Clearly distinguish between day and night, and how land and sea heat/cool at different rates, driving the breezes.

Question 7. Days are very hot and nights are cold in deserts. Why?
Answer: Desert sand is an excellent absorber of heat, meaning it quickly soaks up a lot of heat during the day. This makes desert days extremely hot. However, sand also releases heat very quickly, acting as a good releaser of heat. So, as soon as the sun sets, the sand rapidly loses its absorbed heat, causing desert nights to become very cold. This rapid heating and cooling of sand is why deserts have such extreme temperature changes between day and night. The lack of humidity also contributes to this dramatic temperature swing.
In simple words: Desert sand gets hot very fast during the day because it absorbs a lot of sun. It also cools down very fast at night because it releases heat quickly, making nights cold.

🎯 Exam Tip: The key here is the sand's property of being both a good absorber and a good emitter (releaser) of heat.

Question 8. Why is it comfortable to wear white clothes in summer and coloured clothes in winter?
Answer: In summer, it is comfortable to wear white clothes because white clothes reflect most of the sunlight that falls on them. Only a small amount of heat is absorbed, which helps us feel cooler. In contrast, during winter, wearing dark-colored (or black) clothes is more comfortable. These clothes absorb a lot of heat from sunlight, and they reflect only a small part of it. This absorbed heat helps to keep us warm. This is due to the difference in how colors interact with light and heat.
In simple words: White clothes reflect sunlight and keep us cool in summer. Dark clothes absorb sunlight and keep us warm in winter.

🎯 Exam Tip: Emphasize "reflect heat" for white clothes in summer and "absorb heat" for dark clothes in winter as the core concepts.

Question 9. Why the cloudy nights are warmer than the night with clear sky?
Answer: During the day, the Earth absorbs heat from the sun through radiation. At night, the Earth radiates this heat back into space, causing the temperature to drop. However, on cloudy nights, the clouds act like a blanket. They reflect a significant portion of the heat radiated by the Earth back towards the surface. This reflection traps the heat closer to the Earth, increasing the temperature of the environment and making cloudy nights warmer compared to nights with clear skies. Clear skies allow heat to escape easily into space.
In simple words: Cloudy nights are warmer because clouds trap the heat that the Earth releases. On clear nights, this heat escapes into space, making it colder.

🎯 Exam Tip: The role of clouds as a "blanket" reflecting radiated heat back to Earth is the crucial point for this answer.

Question 10. Mountains are cooler even in summer. Why?
Answer: Mountains are cooler even in summer for several reasons. The surface of mountains is not flat, and many parts remain covered by shadows, which reduces direct sun exposure. Additionally, as altitude increases, the air becomes thinner and contains fewer molecules to trap heat, leading to lower temperatures. Mountains also often have more vegetation and can be covered with snow at higher elevations, both of which contribute to a cooler environment. This combination of factors ensures mountains stay cool even when surrounding plains are hot.
In simple words: Mountains are cooler in summer because they have shadows, thinner air higher up, and sometimes snow, all of which keep temperatures low.

🎯 Exam Tip: Mentioning factors like altitude, thinner air, and shadows provides a comprehensive answer for why mountains are cooler.

Question 1. What do you mean by transmission of heat? Describe different methods of heat transmission.
Answer: Transmission of heat refers to the process where heat energy moves from an object with a higher temperature to an object with a lower temperature when they are in contact or near each other. This flow of heat happens due to temperature differences. The three main methods of heat transmission are conduction, convection, and radiation. Each method describes a distinct way heat travels.
Methods of Heat Transmission
1. Conduction: This is a method of heat transfer where heat moves from one atom to the next. The atoms themselves do not change their positions; they just pass on the energy. This method is common in solids. For example, heat travels through metals and mercury by conduction.
2. Convection: Convection is a method where heat is transferred by the actual movement of the medium's particles (molecules). These heated particles become lighter, move to cooler areas, and colder, denser particles move in to take their place. This creates a current. This method is typical in liquids and gases. For example, heat spreads in water and air through convection.
3. Radiation: Radiation is a method of heat transmission where heat is transferred from a hot body to a cold body without needing any medium between them. It involves electromagnetic waves, like light. This means heat can travel through empty space. For example, heat from the sun reaches Earth through radiation, crossing the vacuum of space.
In simple words: Heat transmission is how heat moves from a warm place to a cooler place. The three ways it moves are conduction (touching), convection (moving particles), and radiation (waves, no touching needed).

🎯 Exam Tip: For each method, define it, explain how it works (particle movement or not), and give a clear example.

Question 2. What are the uses of convection method of heat transmission?
Answer: Convection is a vital method of heat transmission with several practical uses in everyday life and nature.
Uses of Convection:
1. Ventilation: The air we breathe out is warmer and has a lower density than normal air. This warmer, lighter air rises towards roof vents and escapes. Fresh, cooler air, which is denser, enters the room through windows and doors. This natural circulation keeps the air inside fresh and cool. Good ventilation improves air quality by removing stale air.
2. Airflow: Air moves from areas of higher pressure to areas of lower pressure, creating wind. When a place gets very hot, the air above it becomes lighter and rises, causing a low-pressure area. Cooler, denser air from high-pressure areas then rushes in to fill this vacuum. This is how air flows from colder places to hotter places.
3. Sea Breeze: During the day, land heats up faster than the sea. The air over the land rises, creating a low-pressure zone. Cooler air from the sea then blows towards the land, known as a sea breeze. This provides a cooling effect near coastal areas.
4. Land Breeze: At night, land cools faster than the sea. The air over the warmer sea rises, creating a low-pressure zone over the water. Cooler air from the land then blows towards the sea, known as a land breeze. This helps moderate temperatures near the coast.
In simple words: Convection helps move air around, like in house ventilation, creating winds, and causing sea and land breezes near coasts. It's all about warm air rising and cool air sinking.

🎯 Exam Tip: When listing uses, ensure each point explains how the convection principle (hot air rises, cold air sinks) is applied or observed.

Question 3. Which activities take place when heat radiation falls on any object?
Answer: When heat radiation falls on an object, several activities can occur simultaneously, depending on the object's properties. These activities explain how objects interact with heat energy.
The following activities occur when heat radiation falls on an object:
1. Reflection: A portion of the incoming heat radiation bounces back from the object's surface. This is known as reflection of heat. Polished or shiny surfaces, like metals, are excellent reflectors of heat. This property is used in designing reflective materials.
2. Absorption: Another part of the radiant heat is taken in by the object. When an object absorbs heat, its temperature increases. This is why we feel hot when exposed to the sun in summer and why black or dark-colored objects absorb more heat than white or light-colored objects. To stay cool, we might use an umbrella or seek shade.
3. Transmission: Some of the heat radiation can also pass through the object. This is called transmission. Objects like glass allow a certain amount of heat to pass through them. The amount of transmission depends on the material's transparency to thermal radiation.
In simple words: When heat rays hit an object, some bounce off (reflection), some get soaked in (absorption), and some pass through (transmission).

🎯 Exam Tip: Remember the three "R.A.T." outcomes: Reflection, Absorption, and Transmission. Always define each and provide a clear example.