Subtitles section Play video Print subtitles Smart Learning for All Topic: Electric Charge. How does a plastic comb attract paper? What? You don't believe me? Ok. Let us try. Take a plastic comb and bring it close to some pieces of paper. Hey, Wait. Don't laugh. We need to do something first. Rub the comb on your dry hair and then bring the comb close to the pieces of paper. See, I was correct. The pieces of paper got attracted to the plastic comb. Do you think it is magic? No, the reason behind this is electric charge. Electric charge is the quantity of electricity held in an object. There are 2 types of electric charges, positive and negative. However, there are some objects where the positive and negative charges are equal to one another. In such cases, we say that the object is electrically neutral. So, was the plastic comb initially electrically neutral or electrically charged? Initially, the plastic comb was electrically neutral. That means it had equal number of positive and negative charges. Hence, it did not have the ability to exert a force and attract the pieces of paper. So, after rubbing the plastic comb on our dry hair, why was it able to attract the pieces of paper? I will tell you why. When we rubbed the plastic comb on our dry hair, it gained an electric charge. Once it got electrically charged. It got the ability to exert a force on the pieces of paper and attract them. This charge is called as static electricity. However, do you think, like a plastic comb, a metallic comb will also attract the pieces of paper? No, you are wrong. A metallic comb will not attract the pieces of paper like the plastic comb. Wondering why is that so? It is because plastic is not a good conductor of electricity. It does not allow the electric charges to flow through it onto the earth. As a result, the charges build in the plastic comb. Making it electrically charged and enabling it to attract the pieces of paper. However, metal is a good conductor of electricity. It does not let the charges build in it. It allows the electric charges to flow through it onto the earth. Thus, not allowing the metallic comb to get electrically charged. As a result, the metallic comb does not attract the pieces of paper. Topic: Nuclear fusion. Why is nuclear fusion not used to generate electricity? You really want to know the answer to this, right? But wait. Before answering the question, let us understand what is meant by nuclear fusion. When two lighter nuclei combine to form a heavy nucleus, a large amount of energy is released. This process is called nuclear fusion. Where does this nuclear fusion take place? You think that it takes place in a laboratory? No, you are absolutely wrong. Nuclear fusion takes place in the sun. The nuclei of two hydrogen atoms join together to form a heavy nucleus of helium. With the release of a large amount of energy. How do you think this energy reaches us? No. It does not reach us through power lines. Wait I will tell you. The energy released after nuclear fusion reaches us in the form of sunlight, ultraviolet radiations, heat, etc. Hey. But we are already producing electricity with the help of nuclear fission. So, why do we require nuclear fusion? For this, you need to first understand the difference between nuclear fusion and nuclear fission. As we already know. Nuclear fusion is the fusion of two lighter nuclei with the release of a large amount of energy. The exact opposite process happens in nuclear fission. Here, a heavier nucleus splits into two lighter nuclei, releasing a large amount of energy. This process of fission is used in nuclear power plants. Where a heavy nucleus of uranium is split into lighter nuclei. The energy that is released in this is used to generate electricity. However, there is a major disadvantage of nuclear fission. Wondering what it is? The major disadvantage is that uranium is a radioactive element. When uranium undergoes fission, it generates radioactive waste along with energy. This radioactive waste is very harmful for most lifeforms and the environment. Hence, we need to find a clean and safe source of energy to generate electricity. What source would that be? Would it be nuclear fusion? You are right. Then why are we not harnessing the energy of nuclear fusion to produce electricity? This is because, for nuclear fusion, two conditions are required. They are high pressure and high temperature. Only when these conditions are met, can the two nuclei travel at very high speeds resulting in collision. On earth, it is extremely difficult to create such high pressure and temperature. Even if we are somehow able to create these conditions, the question is how will we control them? As there are many questions unanswered and unsolved. We have not yet succeeded in using nuclear fusion in the production of electricity. Topic: Sound. Why do we hear echoes? Hey. Hold On. Don't go into that room. You don't want to listen to me. Ok. Then go ahead. See, I warned you. Will you listen to me now? Don't worry. The sound you heard was just an echo of your voice. Let me explain what an echo is. When we speak or laugh in a big empty hall, we hear our own sound repeatedly. This is because our sound waves get reflected from the walls of the hall back to us. The reflected sound that we hear is called an echo. Hence, an echo is defined as the repetition of sound caused by the reflection of sound waves. From a hard surface back to the listener. Hey. What are you doing? Are you trying to hear an echo? It is not that easy. There are 2 ideal conditions for an echo to be heard. When we speak or laugh, we hear our original sound at that moment. The sensation of this original sound remains in our brain till 0.1 seconds. This time is called the persistence of hearing. When we utter sounds, some of our sound waves get reflected while some get absorbed. If the reflected sound waves reach our ears before the completion of 0.1 seconds. Then our brain does not perceive the original and reflected sounds as separate sounds. They are interpreted as one sound. Therefore, in order to hear two distinct sounds or an echo. The time gap between the original sound and the reflected sound or echo should be at least 0.1 seconds. This is the first condition for an echo to be heard. Do you know when the time gap will be more than 0.1 seconds? This gets us to the second condition for an echo. The minimum distance between the speaker and the reflecting surface should be at least 17.2 meters. When the distance is 17.2 meters and we start speaking. Assume that the original sound reaches our ears at this time. The reflected sound reaches our ears at this time. Then the time interval between the reflected sound and the original sound will be equal to 0.1 seconds. As a result, we will be able to hear an echo. Alright. Now, let us try and hear an echo in this room? It is even bigger than the previous room. Now, why can't we hear an echo? This is because there are many soft furnishings like sofas, carpets, curtains, etc., in this room. They absorb most of the sound waves. As the sound waves do not get reflected, we are not able to hear an echo in this room. Hence, one more thing to consider, if we want to hear an echo. Is that there should be minimal use of such sound absorbing materials. Topic: Concave Mirror. Why is your reflection upside down on a spoon? Wow. Looks like you are getting ready to go for a party. Why don't you stand here and then look into the spoon? Don't worry. Your image appeared upside down because of the inward curve of the spoon. The surface of the spoon which is curved inwards acts like a concave mirror. You look confused. Let me explain. A concave mirror is a mirror whose reflecting surface is curved inwards. Being curved inwards, it reflects or bounces back the light rays in a different manner. When you see yourself in a spoon which is like a concave mirror. The light rays from your face fall on the top of the spoon and get reflected downwards. While, the light rays from your feet fall on the bottom of the spoon and get reflected upwards. As a result, you see yourself upside down. Now, the point where all these light rays meet is called the focal point. When you stand beyond this point, only then will you be able to see an inverted image of yourself. However, if you stand before the focal point, the image will look upright. Topic: Buoyancy. Can you drown in the dead sea? Your answer must be yes, right? You must be thinking that anybody who cannot swim will obviously drown. Whether it is a swimming pool or the dead sea. However, that is not 100% true. Confused? Let me explain to you. When an object is partly or wholly immersed in a fluid, an upward force is exerted by the fluid on that object. This tendency of the fluid to exert an upward force on the object is called buoyancy or upthrust. This upward force is called buoyant force. So, do you finally get it? Let me give you one more example. Place a piece of wood in water and push it downwards. What do you observe? It seems like something is pushing the piece of wood upwards, right? Water exerts an upward force on the wood. That is why the wood is getting pushed upwards. This force is called buoyant force. The tendency of water to exert that buoyant force is called buoyancy. Dead sea has a huge amount of salt dissolved in it as compared to any other sea or ocean. The presence of this salt increases density of water present in the dead sea. Higher density leads to greater buoyant force. As the dead sea has very high density. It exerts enough amount of buoyant force to make us float on it. So, if we can float on the dead sea, we are definitely not going to drown in it. Topic: Light and Sound. Why do we see lightning before thunder? Don't go out tonight. A huge storm is coming. Look at the weather outside. See, there's a huge lightning strike. Now, very soon, you are going to hear some thunder. I told you. Don't get scared. It's just thunder. Do you know why you saw the lightning before you heard the thunder? I will tell you. An interesting fact is that lightning and thunder occur at the exact same time. Then why do we see lightning first? This is because light travels faster than sound. The speed of light is 300 million meters per second. The speed of sound is only 340 meters per second. Thus, the light from the lightning travels much faster to our eyes. As a result, we first see the lightning, shortly followed by the sound of thunder. Topic: Heat transfer. Why do we use a black umbrella in summer? Hey. Looks like you are heading out today. But it is so hot outside. Why don't you take a dark colored umbrella, preferably black? See, I warned you. At least now use the black umbrella. Now, do you notice the heat? No, right? Do you know why? This is because in summer season, the temperatures are quite high. Now, as compared to other colors, a black colored umbrella, being dark, absorbs most of the light. Ultraviolet radiations coming from the sun and gets heated. It then radiates the heat back into the air. Thus, making a black umbrella, a good choice to use in summers. Topic: Heat transfer. Why are cloudy nights warmer than clear nights? Hey. Look at the weather outside. It is too cloudy. No. Don't go near that bonfire. It is going to be warm tonight. You don't want to listen to me, right? Alright. Go ahead. Look. I told you. At least now will you listen to me. During the day, our earth receives light from the sun and gets heated. Now, during a clear night, that is, when there are no clouds. This heat easily escapes through the atmosphere into space, resulting in cooling of the earth. Now, to understand what happens on a cloudy night, let us heat the earth once again. In this case, the clouds act like a blanket, preventing the heat from escaping into space. Since the heat remains in the atmosphere, cloudy nights are warmer than clear nights. Topic: Heat. Why is a laboratory thermometer not used to check body temperature? You have got fever. Why don't you check your body temperature using a thermometer?