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