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University of California, Berkeley



Electric Charge

In physics, a charge is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge is a characteristic property of many subatomic particles, which determines their electromagnetic interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between a moving charge and the electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces.


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Video Transcript

In this video we begin our discussion of electromagnetism, and for that we start with charge. Ah, fundamental property of particles. Thousands of years ago, the ancient Greeks discovered that by manipulating objects, they could create a repulsive force between them as we see between the blue and green circles here. Or they could also create unattractive force between them as we see between the red and the green circles. This led them to believe that there were two types of charge which we'll call positive and negative and more importantly, the fact that, like charges are those with the same sign repel each other while opposite sign charges attract. Now the important thing to remember is the attraction. And repulsion has nothing to do with the size of the charges on Lee their sign. And so we can see from this picture that since blue and green repel, the blue must also have a positive sign and thus would have an attractive force to the red circle. To better understand the fundamental fundamentals of charge, it's helpful to look at the structure of an atom here. I've drawn an atom with four protons in the nucleus, which makes this beryllium. It also has three neutrons and four electrons on the perimeter. Experiment has shown that the size of the electron charge is equal in magnitude but opposite and sign to the size of the proton charge. Moreover, what we've seen is that this electron charge, which will call E, is the smallest possible charge we can measure. One other thing that experimentation has shown is that any charge must be quanta ized in E. And what we mean by Qantas ized is that any charge Q. Must be equal toe end times e positive or negative where an is simply any integer Ah, positive or negative whole number. And so we cannot break up charges into anything that's less than a fraction less than a holy no fractions, no, have no thirds. Now, looking at our atom, we can rightly assume that the electrons, which are negatively charged by convention, are attracted to the nucleus because of the positively charged protons. But we might ask ourselves, How does the nucleus stay together? All of these protons have positive charge, and as we've shown, that means that they repulsed each other well. This has to do with another fundamental force of physics, which is called the strong nuclear Force. Not to understand this better, we need to know about the size of an atom. Typically, the atom with the electron charge is about an angstrom big or 10 to the negative 10 m. However, the nucleus is only about 10 to the negative 15 m big now, to give you an idea of the scale. Imagine that the entire Adam what the electron cloud was a football field. Then the nucleus would only be about as thick as a blade of grass, so that these tiny, tiny ranges of 10 of the negative 15 m the strong nuclear force is very powerful and is able to hold hold protons together despite the electric repulsive force, charge will always obey to basic principles. The first we've already discussed that all charges are quant sized in e. Any charge that we measure must be an integer multiple of this e the magnitude of the electric charge and it could be positive or negative. The second principle is that charges conserved in any close system. So if we look at our beryllium Adam here, we see that we have four positively charged protons within the nucleus around the perimeter. We have four negatively charged electrons, and the magnitude of their charges is equal to the magnitude of the Proton charges. Just opposite Sign. We also have three neutrons in the nucleus, but those have zero charge, and so we see that our net charge Q is simply equal to zero. This means that when our beryllium interacts with in any system, the total charge within that system must remain the same. So if our burly um, is not interacting, then it's impossible for us to have any charge. Besides a total charge of zero, we cannot simply lose an electron outside of the system without it it interacting with something else. Theoretically, we might be able tohave ah, proton and an electron, interact and say, destroy each other. And we'd still obey the conservation of charge because, as we see, we now have three protons and three electrons. Whether this is possible or not depends on the physical experiment, but it does obey the conservation of charge. We can also demonstrate the conservation of charge with a simple experiment that many of us have probably done in our lives. Here we have a piece of for and we know within this further our atoms. And in those atoms there are electrons. If we then take a plastic rod and rub it against this for what we'll see is those electrons will transfer over the plastic rod. And so, even though we've started with a piece of for where the total charge was zero because within the for the electrons and protons had equal numbers and a plastic rod that was the same way with a total charge of zero, we've ended up with a plastic rod with a charge of Q prime being some negative and e negative. Because Electra it has gained electrons. However, the for has lost those electrons, and so it now has a charge of positive any and we see that our total charge que total, which was originally zero because each was uncharged. It's still zero, even though each individual item has gained or lost positive charge because the total charge within our system is zero and so charge is always conserved. Now we mentioned that we can pass charge from one material to another, but not all materials are the same. To demonstrate this have drawn a battery connected to a light bulb with two wires. But our Blackwater is broken, and we need to fix that so we might try a piece of green rubber. When we do that, we see the light bulb doesn't come on. That's because rubber is known as an insulator. Insulators air Very bad at accepting charge, accepting charge and passing current. So we try again. This time we use a piece of copper and right away the light bulb lights right up. That's because copper and other metals are known as conductors, conductors air great for passing charge. They're great for passing current. They do so very easily compared to other materials. But you probably knew all of this already in your house, you have a lot of wiring. You have wiring to your appliances and to all of your electronic devices. And well, this wiring is probably made out of copper because copper is a good conductor. Meanwhile, all of those wires air probably coated in rubber, and they're coated in rubber because rubber insulate the electricity from passing through. So if you touch a wire or you touch the cord when you're plugging it into the wall, you don't get shocked. In fact, the reason we get shocked is because people are pretty good, pretty good conductors. As for the reason why a shock is painful or even dangerous, well, well. Understand that Maura's. We learn about the properties, conductors and insulators later on. I should also mention here that there is another class of materials known as semiconductors. Semiconductors can act as conductors or as insulators, really, depending on the conditions under which they're placed. Now. Whether a material is a conductor or an insulator is determined by this material structure and the energy structure within that material. On the left, we have a block of wood, and you'll notice that I've drawn electrons in red, but they're very rigidly structured. This'd because in insulators such as wood electrons, air tightly bound to the nuclei of their atoms, they're not very freedom move. Whereas on the right in a piece of copper, there are a lot of loose electrons, meaning they could move around the material as needed. And so if we come in with the charged rod that has, say, a lot of electron excess electrons on it, what will happen is all these electrons that are free to move within. The copper will do so to get away from the repulsion from the electrons in the rod. And so what we end up with is a piece of copper with a lot of excess electrons on one side. And so that side will have a net negative charge, which will call Q minus. And the site closer to the rod will have a net positive charge, which will call Q Plus. Now we all know it's possible to charge a material. Here we have a piece of copper that has no charge on it. So all the electrons equal in number, tow all the protons within this material. But if we bring in a plastic rod with excess negative, charge some of these electrons within the rod. We'll move into the copper because it is a good conductor, and we'll end up with a piece of copper that has a net negative charge Q minus. However, there is another method of induct, of charging called charging by induction, and in this we take the same neutral piece of copper and the same plastic rod. Except this time we don't actually touch the negatively charged plastic round of the copper. We just put it very, very close, as we've previously discussed. There's a lot of loose electrons within the copper and their freedom move, which means they'll want to move away from the plastic rod because it's negatively charged. And there's a repulsive force between the electrons and each material. So we'll end up with a net positive charge closer to the rod and a net negative charge farther from the rod. If we then attached the copper to the earth with a wire thes electrons well feel free to flow away down into the earth, which is a good conductor. And so the net effect without ever touching the piece of copper, is that we remove negative charge and we end up with except positive charge. And this method is called charging by induction. We can charge something without actually touching it with the charge of material. Finally, we conclude our introduction to charge by discussing how we quantify charge the S I unit of charges the Coolum, which we abbreviate us. See, now we've discussed that the electron charge E is the fundamental unit of charge, and it is equal to about one point 602 times 10 to the negative 19 Cool arms. You might ask why we use the Coolum instead of the fundamental charge, since it's the basis of all charge and this will become more apparent as we discuss further concepts within charge.