Transcripts
1. Introduction to the Coulomb Force (Coulomb's Law - Ep00): this class is a section of the course and the teaching you about electricity from understanding what an electric charges up to. Learning how to solve complex electric circuits in the previous glass. You have learned what the charges. You also have learned that charge into lack by attracting or repelling each other, so there must be a force involved. This force is a cool and force and will be the subject off this class. The first video we discussed the theory. You will learn what this force depends on. All the videos that follow will be training exercises in order for you to get family with a mathematical expression off. Do not buy pasties. Working these exercises will allow you to learn how to manipulate quantities and numbers that are very common in this area of physics. And in addition, I will throw in a bunch off tips on how to solve physics problems. So enjoy the class
2. Understanding Coulomb's Law, the Lesson (Coulomb's Law - Ep01): in the previous class dedicated to the electric charge. We realize that when we had two positive charges goes to each other, they repel if the charges would get if they would also repeal. So two charges on the same sign with power. If the charges are off opposite sign, they attract. So if there's an attraction or repulsion like this, there must be a full slight. And what would this force dependent? Well, it will depend on the magnitude of the charges that called them Q one and Q two. If the charges are large, obviously they will see each other more, so the interaction between them will be stronger. The force will be larger. On the other hand, if the charges are small, while the force will be weak. So we can say that the force is proportional to the magnitude of the charges. I feed it proportional to the product off the charges. What else could influence this force? The distance. Yes. If they are far away from each other, they wouldn't see each other very well, so that would have a weak interaction. Therefore, a week of force between them bigger the distance small. The force their inversely proportional to the distance squared. The reason for wide square is because we live in a three dimensional universe. But I will discuss this in another video and in another class. Okay, so we have this force between them right, which is proportional to product off the charges on the 30 proportional to the square of the distance between them. Proportional means that I could write This like F is equal to a constant multiplied by this number. This constant is called Coolum Constant, and the force here is actually called the colon force. The coolant constant has also other names like the dialectic constant or the electric force constant the Coolum constant effects, the effect of the medium in which charges are located. Let me give you an analogy. Imagine you're with a friend on a sunny day. Your friend is 20 meters away from you. It's a sunny, clear and bright day so you can see your friend of a clearly so you could say that the visual interaction between you and your friend is strong. Imagine the same situation. But on a foggy day there's mist between you and your friend. You can't see your friendly where? So you can say that the visual interaction between you and your friend is weak. It's the same situation here. If I think for the medium. K is equal to 8.99 by 10 to the night. If I take water, que is around 0.11 by 10 to deny if you place for charges in air in a If you put K as he called to this family, if you take the scent situation, the same charges put them in the same distance that place them in the water. You have to put this value and you'll see how the force we have all the facts. The fourth would become much weaker in water than in air. It's like if water was acting like mist shielding partially one charge from the other. Let's look at the units of K. Well, if I rearrange this, I get K equals F D squared of Q one Q two. That means new terms meter square grown men's too You, Times Square. Let's look at the sign off the force. If the two charges over the same sign, their product will be positive, so the force would be positive. But you see that wind hours of same sign the forces repulsive. So if you have a force which is positive, it's repulsive. On the other hand, when the charges off opposite science, the product is negative, so the forces negative and you have a natural difficult. So when the force is negative, it means it's attractive. Another thing is that often you only need to have the magnitude of the force. So in that case, you just put in here the absolute values. If you need the 92 of the force, it's OK. Q. One que two over the square. I would complete this video with the comment. Have you seen this form of equation before? Yes. In my gravity course, the force of gravity between two masses is proportional to the product off the masters and inversely proportional to the square of the distance between them, thes two formulas look furiously like right. The symmetry allows us to realize that mass is a charge off. Gravity, like electric charge, is a charge for electricity. The same symmetry shows us that the force off cool for the electric force and the gravitational force are both inversely proportional to the square of the distance. Yeah, it's still the same story about the three D universe. K and G are also analogous. If you think K as the effect of a medium, you can't think g at the effect off space Time itself. G is universal gravitational constant. The word universal means in reference to our universe. So it's like if you consider the viscosity of space time if you want. So this is 6.67 by 10 10 months, 11 on it's going to be new terms Meter square by Cuban minus two. When doing calculations with gravity, you always use this value. It doesn't change, right? We don't know yet how to change universe. Well, I think you know everything you need to know in order to move on. Creator to check the next videos, which contains exercises the exercises are prepared for. You are focused on the Coolum force and they are off gradual difficulty. For example, the 1st 1 consists in just plug in the numbers in the equation. It's important to do this at this once, you know, for you to get used with the numbers and vote, but then I increase the difficulty or not, where you will need to reflect a little bit and final exercise will be off a level that you could expect at the back of that type of example Now the A levels or the I V. Good luck.
3. Exercise 1 ( Difficulty: easy, Coulomb's Law - Ep02): In this first exercise, we have two point charges which are respectively off plus tube Michael Cool and plus six months on. These charges are positioned 25 centimeters from each other. The question is, find the direction and magnitude off the force between them. This exercise is actually very easy. You just need to plug in the numbers in the equation for the corn Force. But still, I hope you did it right that you tried it because it's important for you to get familiar with these numbers. Okay, so let's write down the Koran force. According force is proportional to the product of two charges anniversary proportional to the square of the distance between them. I realize that the two charges are positive, so the product would be positive. So already know that the forces positive their food impulsive. Now let's plug in the numbers. I can use 9.0, OK, because I have to significant numbers only so zero right into the night on then have the charges. So two times 10 to the minus six by six by tens of my six on the whole thing, divided by the distance in meters to stay aside units. So the 0.25 squared, which will give me on my calculator 1.73 which is about 1.7 near term.
4. Exercise 2 ( Difficulty: moderate, Coulomb's Law - Ep03): in this exercise. I have two charges. Plus for Marco Polo on plus now. And Michael Cool placed vacuum at one meter from one another. I want to place 1/3 charge, a positive one called Little Cube. And I want to place it in between these two point charges, that is, somewhere on this line, I want to place it in a way that if I partition it there, it one move. So it means that place that charge Q here on it would stay at west this question, translate into finding X. What is the position? Well, would place my point Positive charge. Q. Let it lead to the 1st 1 so that the point charge Cube would stay at West. That's with his under Charles Que. Here. Why do I do that? Because I want to feel a free body diagram on it. Yes, there will be moved forces on it. There will be the Force F one due to the repulsion from this charge on a force F two. Due to the repulsion from this one. In order for the charge, Q To stay dressed first wrong, you turn the net force on the trash needs to be the one. The Net force is a some of the charges, so I need to have F one class if to equals zero. If one will be positive that considered this as a positive access after negative. Therefore, F one needs to be equal to have two in manager. Good. Let's write this down under the form of colon forces, so I would have okay Q. One little cute on the distance between Cuban and took you, which is X squint. Same thing for the force between Q two and Q. Okay, cue to cue the distance between Q two and little Q is one minus X. It's this distance, so one minus X squared, he realized that that can cancel things. The value of the charter took U K Let's play in some numbers that would lead to. For Michael Koran, Express equals nine. Michael Colon a one minus X squared. I can't get rid of the Michael. What I noticed now that have an equation with one unknown and this unknown is what I'm nothing. Thanks. That's rearranging today for by one minus X squared equals nine X squared. What I notice is this is an equality between two squares. A squared equals B squared second square root the whole thing giving to buy one minus X equals for the X, which is tu minus two. X equals x 42 equals five x two x, of course to fifth X would be to fifth off a liter 7.4.
5. Exercise 3 ( Difficulty: moderate, Coulomb's Law - Ep04): going to the three points. Hey, de on being I d is probably good to meeting the distance between the points A and E and D and B is one meter, one meter and one meter on all of these points. There was a point charge off one cool. The question is, calculate the fourth that would be exerted on the when I'm in Calgary. The Force, I mean the magnitude of the force and its direction on the charge of point D will be exerted to forces the corn force between A and B. Both charges are positive, so it will be a total force. Call it F a D on the corn force between the nb DnB carry charges which also positive. So it's gonna be reported force. We'll call it f the B. If I want to find the result in force on these two, I need to add these two forces forces are vectors, so I need to add them as vectors. Do you have trouble with vector addition? Do you have trouble with Victor's in general, especially in physics? Well, I recommend that you check my course mathematic for physics. You go to the second section, which is called vectors for physics. And then there you can learn everything they need to know to manipulate vectors in physics . Problems on this includes vector addition like we are going to do now. So the result in force on the charge carried by D, will be the vector addition off these two so we can do it graphically. Tip to tell. I draw this actor at the tip of this one, and then I go from the starting point to the endpoint, and that will be my result in force. So how to calculate its magnitude? Well, I need to find its X component, and it's why component. So for the if I need to Max, it's right. So I'm going to consider positive for the X axis that way and positive for the y axis downwards. So what will be the X component off my force on the charge of the well, it's just have to be. What about the white component? Just f 80 f d. B as a column force between d and B, so I can write it. Okay, cool off the to be divided by the distance between them square, where both charges are one and the distances one. So in the end, I just get K for F y is the same idea. Do a coup D and the distance 80 are all one say it's also okay. So now how to calculate the magnitude where I had a triangle here. This is just the iPod. The news. I just use Peter go F X squared plus four squared, which is actually test too. Okay, squared case, quit the case. Grab and I can take the k out of the square It k square root of two. That's not nine injured. Now I want to find a direction so I could define the night Will say this one. If I can calculate this angle, I would know the direction off my force again. I fear trying with you and I can write tangent. Tita is opposite of adjustment. So it's going to be f a d on FB, which is just k OK, so one So deter is tension minus one most well, which is 45 degrees. So in the end, I can say that the direction of my force will be 45 degrees tools left underneath the X axis soft magnitude. I
6. Exercise 4 ( Difficulty: hard, Coulomb's Law - Ep05): in this exercise, we have two bulls. The first ball is attached to an insulating world itself attached to the ceiling. The water, the bold cannot move. These are fixed at this point. There's also something else attached. I lope on insulating World, to which you have attached a bold so this can move. The two bulls have got a charge to Michael Coolen for the fixed 13 Michael Coolant for the one that can move. So because they are charged, that will repel each other. And there will be a night of 20 degrees between the board and the street. Both bulls have a mass off 100 grand's. The question is, what is the distance that separates the two balls? You can consider the Bulls like point charges a point, my sister. Where to start? What is the unknown? The unknown is the distance between the two Charge two Bulls. Why, that is their distance in the first place because of the re partially due to the cool force . So let's write down the cool C B K Q. One Q two of a D squared wack you and take you to our the respective charges off the books we know Q one and Q two. We don't know D we don't know F but if we knew if we would find the so we need to start thinking on how to find F, I have a problem with forces. A little tape. If you have a problem with full sees, you probably need to find some relationships to do that. Apply the full step technique. One drew a free body diagram on the object you interested in to defined some positive access. Three. Write down an expression for the Net falls full a plan you to his laws. When you do this, you will get some relationships, and for these relationships you will see what to do. Just do that. I'm going to call to do this point like being this ball. What have the forces on this point? Well, there's gravity. France OMG! There will be the cooling force I've seen and they'll be the tension in the string. So I call it T. Now we differential access, so I'm going to choose, for example, positive upwards and positive to the white. That's why she draw them on the field in Iran like this on What other information can I put in my diagram? The angle. Here, you see that attention is like this. So the angle would be 20 degrees, I guess. The vertical we've covered Step one in two. Step three, right down an expression for the next fall's. We have two dimensions, so we're going to define the components of the Net force on the X axis, so f net x component. I have cool force, and I have the X component of the tension. So here it would be minus t sign 20. Let's find now the white component of the Net falls F net. Why will be equal to Okay, so we get Oh, why component of the tension critique costs 20 on. We would have the weight so minus energy. Yes, weight is in the negative direction. That's step three step for applying Newton's laws. Basically, it's just saying that F Net X equals and a X The acceleration in the X axis definite. Why is an A why the acceleration in the Y axis now the bull has been repaired by this one and is now stable is a She in the Caribbean is not moving, so it's for you. The explanation is even so because bold now is addressed. So you can put do you hear? Let's rewrite this FC equals t sign 20 and here and G equals t cost 20. I'm looking for FC, Remember? Maybe I can be raised its oh, even better divide one equation by the other, giving me f c of g attention cancels. And then I got cycle co sign because charge in 20 So my core and force is actually equal to M g tangent. 20. Great. It is also equal to that. Okay, Q one Q two. Poverty squared. Well, if I look at this, I know everything under the mass. Another gravity, the angle of the Chargers. I have only one unknown in the distance. I'm done. I just need to rearrange this equation to find the distance that is. Do that D squared equals K. Q. One Q two over MGI term. 20 d squared. I'll just go with it, and I've got expression for deep, and I just need to plug in the numbers 19 to denying only significant figures. I only have one or two, so I'll just keep this by to 10 months six by 3 10 minus six If I did by the mass is 100 grams in kilograms. You need to stay s I units 1000.1. I use 10. 40. My tangent of 20. Let me grab my calculator. 9/10 of the line by 6 10 to 12. If I did buy, that's one by 10. 20 I'm in degrees good and I think this quick. So the distance is zero point 38 5 which is around dew 50.39 meters.