Static Electricity

Ajouté : 2026-05-12

[00:00:00]This is Dr. V from Columbia University.

[00:00:02]>> Uh this is Aborno Isaac Barry. I'm a bachelor student in math and physics at NYU.

[00:00:06]>> Electricity is the study of electric charges especially electrons.

[00:00:11]>> Well yeah of course but not just electrons but in general any charged object and in electricity we study two kinds of situations charges. We study static electricity.

[00:00:23]>> We are saying that charge at risk right we can define it. We can give an example for example my head and balloon right balloon wants to have gain electron but head you know it's okay for head to lose electron my head has five electrons five protons completely neutally charged and balloon imagine a balloon five electrons five protons also neutally charged together how many charges we have >> um well wait 20 charges right of course >> okay great now I bring the balloon and rub it with here. Now the head loses let's say four electrons. Now my head is become positively charged called cation and balloon become >> negatively charged. That's an annion.

[00:01:07]>> Annion right now.

[00:01:08]>> Okay. So here technically the charges are moving but the thing is they are moving for a very short time. So we don't study it with the same tools that we would. So let's get back on.

[00:01:19]>> Shon going to draw a balloon. Uh and uh can you draw a balloon and put pepper and salt?

[00:01:24]>> Sure. Okay.

[00:01:25]>> Okay. Put some paper and salt. Okay. And put the balloon. Balloon is negatively charged.

[00:01:35]Okay. Great. Now give me half of the chop. Now balloon is negatively charged.

[00:01:39]Okay. And paper and salt. Paper and salt. There are how many paper and salt?

[00:01:42]We don't care. But they all are neutally charged. Now balloon create something called electric field. And balloon sends a weapon, right? Weapon to capture.

[00:01:53]weapon to capture to pole anything within the vicinity. Yes. Including salt and paper.

[00:01:59]>> So, uh this is attracted by the electric field and in fact everything in its radius is attracted even the person who you know gave it the charge in the first place. But the problem is most of the time with very large objects like a human body, gravity and other forces are weighing it down much more than the balloon can exert. But salt is light enough.

[00:02:23]>> Yeah. Not salt. Pepper. Pepper. Pepper.

[00:02:26]>> Pepper is light enough that it will be sucked up by the balloon because the force of gravity is not the same as the force of electric charge.

[00:02:35]>> So you're going to say for salt FG of salt is bigger than Fe of the On the other hand, can you write it for paper? FG of pepper is less than F. Mhm.

[00:02:50]of the balloon. So it gets sucked upwards. So that is an example of a static electricity. Okay. Now let's go move to electric current. But today our topic is not electric current. Today our topic is static electricity. By the way, what is the SI unit for charges?

[00:03:07]>> Uh the SI unit for charge is a coolum named after uh Andre Marie Kulum.

[00:03:11]>> Uh not Andre Marie Kulum. Um >> Aier Augustus.

[00:03:17]>> Augustus Kum. Augustus Charles Augustus Kulum Charles Augustus Kulum he he's prince I guess I guess he's friends but medium herench >> he's also friends yeah okay very nice so now is speaking of charges right as the big mass create the what is a big mass creates a large gravitational field a big large mass creates a large gravitational field uh a large charge will also create a larger electric field. And even the same way uh this has to be negative by the way. Uh gravity decays with distance the by the inverse square law uh the electric field decays with distance in the exact same way. Now one note is that if the charge is negative then it will suck in similar to gravity. But if the charge is positive then the electric field actually points outward which if another charge near it is positive will actually cause that charge to be pushed away by the electric force. Now we're going to talk about um making connection between mass and charges. Right? Okay.

[00:04:31]So now particles right if you look at the particles you're going to be able to define particles or divide particles in electrons proton and neutrons the mass of the electron >> I'm pretty sure it's about 9 * 10us 31 kg >> and who discovered the electron >> uh I'm pretty sure it was JJ Thompson >> what year was it >> 1879 >> 1897 very close very close and then um uh I mean if you look at the if you look at the uh if you look at the Oh by the way can You can you break the electron?

[00:05:03]>> Uh, not that we know of right now. We don't think it decomposes into quarks or any other subatomic particle. So, we're pretty sure it's a fundamental.

[00:05:12]>> That's right. So, if you take the matter, right, if you break down the matter, what do you get?

[00:05:16]>> Hadrons and >> lepttons.

[00:05:18]>> Lepons you cannot break anymore, but hedrons you can break into >> quarks.

[00:05:22]>> No, sorry. Bzon, >> and me. Speaking of quark, quark do not go alone.

[00:05:30]We don't know how to split them up yet.

[00:05:32]So far, all we can see is that they've always stayed bonded to each other.

[00:05:37]>> That's right. So, they they they work together like three at a time. So, if you look inside the proton, how many you going to see?

[00:05:44]>> Three. A proton is composed of an up quark, an up quark, and a down quark.

[00:05:51]And the neat thing is that with this you can actually calculate. That's >> right. Before you do that, we're going to have to define how many type of quirks there are. There are six. There is the up and down which are the most common ones. There is the top and the bottom. Okay. And there is the strange and the char. And the up one has a charge.

[00:06:11]>> It has a a charge of 2/3 e. Now what is >> and the down one?

[00:06:18]>> Well, oh very good.

[00:06:19]>> The charge of the electron is about 1.6 * 10 -9 kum. But it turns out that we see uh things coming in units of this multiples of this so often that we've actually defined a new unit one e is equal to 1.6 * 10 - 9. Uh this you cannot prove by the way it's experimental value.

[00:06:41]>> It's experimental value and any location you go in the universe and you find any electron whose value would be this >> and nothing more nothing less. So now uh the down part instead has a charge of minus 1/3 E. So here in the proton we get 2/3 E plus 2/3 E minus 1/3 E gives us a charge of exactly plus E.

[00:07:06]Meanwhile for the neutron the neutron is actually composed of one up quark and two downs. So we get plus 2/3 E but minus 1/3 and minus 13 E.

[00:07:19]So the charge cancels out to zero which is neutral particle.

[00:07:22]>> So that's right. So proton has charge of course positive E and neutral has zero.

[00:07:27]And on the other hand electron you cannot you cannot break it down to the smaller particles which has also charged.

[00:07:35]>> The protons discovered well let's talk about it. The protons was discovered 1970 by Ernest Ratherford. He was born in New Zealand but spent most of the time in England. I know that. No, I mean, yeah, sure. I mean, nothing wrong to to know more.

[00:07:49]>> Chadwick in 1932 uh shot array of alpha particles at the atom burillium because he wanted to know what was inside the nucleus. So when he shot the particle the alpha particles, he didn't see what he expected or what any other physicist expected. uh the radiation that came out was actually neutally charged and at first other physicists said it must be gamma rays but the other measurements didn't coincide with that. um it wasn't as high frequency as a gamma ray should be. So they figured out they had found a new particle and because it had no charge they named it the neutral neutron and that's open box was discovered 1897 by JZ Thompson. It is the mass is 9 * 10 -31 and charge is 1.6 * 109 kum and JZ Thompson how did he discover it by cathode ray experiment has a charge of + 1.6 6 * 10 -9 kum or +1 E. Um it was discovered in 1917 by Ernest Ratherford when he was bombarding gold foil with um alpha particles. Uh oh sorry he was bombarding gold foil with electrons.

[00:09:06]>> Yes.

[00:09:06]>> And then um the proton generate uh the proton is found in the nucleus the center of the atom. Um, and you would think because the nucleus is full of protons, they would repel each other, but they're actually being kept together by a nuclear force called the strong force.

[00:09:22]>> That's right. So that's another story we're going to talk in the future.

[00:09:26]Strong nuclear force and weak nuclear force. Alto together there are four fundamental forces in the universe. The gravity, the electromagnetic force, strong and weak nuclear force. But let's move to neutron. Neutron was discovered by as you explained 1932 by James Chadwi whose mass is similar to mass of proton.6 * 10 to 27 kg. but whose charge is zero and which was of course done by famous experiment very famous experiment bombarding bombarding the barelium um uh >> with alpha particles >> with with alpha particles now let's move to the big mass and big the small mass over here let's say 10 m above the ground right so it creates the uniform field and that fg right and g is at the same direction right f and g are always in the same rest for gravitational fields. What you'll notice is that um the formula for gravitational force looks like this. And so far most objects including all we deal with in classical mechanics can't have negative mass. It's either zero or positive. Which means that since um FG is equal to this and also equal to mg when we're close to the ground that means that since g is equal to g over r 2 g and mg always have to have the same sign. So there you go. So always have the same direction fg and g that's why you know this is very easy.

[00:10:52]This one like big mass this one big cube. This one like small mess this one is small cube. Big mass create the field small mass small q follow it. Now Isaac are going to first draw the electric field first. The easy one. Electric field. Draw it here.

[00:11:07]>> Sure. So same thing happened here.

[00:11:10]Different thing happens here. Can you name it?

[00:11:12]>> All right. So can you name it? E E. All right. And then we're going to do F in a few seconds.

[00:11:19]All right. Uh this is this agree with few second ago when Isaac draw positive charge create the field outward and negative charge create field inward and this is equivalent to that is because why this is big Q this is big Q and a small Q like a small mass we ignore not not not because they don't create anything they do but we ignore uh okay now Isaac is going to draw the force here at force here at force here at force at positive. It's positive.

[00:11:54]>> Okay.

[00:11:55]>> Okay. So, it's positive.

[00:11:57]>> Okay.

[00:11:57]>> All right. Let's do this one.

[00:11:58]>> Q is negative that means that E has to point in the exact opposite direction as F. So, it goes down. Now, here since Q is negative, we we notice that oh wait, no, we don't care about that. We care about what this is. And since the test charge is positive, that means since this is positive, E and F point in the same direction. So it goes this way. And finally here Q is negative. So we don't care about what the actual charge is doing. We only care about what uh the one being acted on the test charge. And since the test charge is a negative charge, um F acts in the opposite direction of E. So now what we find is that positive uh gets repulsed from positive positive gets attract negative gets attracted to positive positive gets attracted to negative negative and negative gets repulsed from negative. So what did we learn? Well, opposites attract and like charges repel.

[00:13:06]Okay, that's that's uh amazing. That's awesome. And let's see whether that agrees with this one. EQ. F is equal to EQ. F is equal to EQ. Whether you can do shortcut very quickly. So since E is as said E is this one is positive. So E is of course positive and this is negative.

[00:13:27]So positive negative negative. Is this negative? Is F negative?

[00:13:30]>> Yes.

[00:13:31]>> Okay, great. So E over here is negative.

[00:13:34]And q over here is positive. Positive negative is negative. Is f negative?

[00:13:39]>> Yes.

[00:13:39]>> Okay. Great. So e over here is negative.

[00:13:42]And q over here is >> negative. So negative negative positive.

[00:13:47]Is f positive here?

[00:13:49]>> Yes.

[00:13:49]>> Okay. So that agrees no matter we give you like many options, many uh preferences. Now let's summarize. Okay.

[00:13:57]Isaac you summarize everything.

[00:13:59]>> Okay. So today we learned about the >> static electricity.

[00:14:02]>> Yeah. Three subatomic particles. We learned about the electron, the neutron, and the proton. We learned about how charges in general, not just the subatomic particles, act when they're around each other, like charges repel, opposite charges attract. And we also learned about static electricity >> especially the field created by big charge and the of course the force experienced between big charge and a small charge by of course using kulum's law and the field uh that was discovered by Michael Faraday Right.