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  • 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?