Conductors and Insulators

by Jared Rovny

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    00:01 Now that we've gone through the electric charge, it's definition, and how to describe it as well as the forces arising from these charges as they interact with each other.

    00:09 We're ready to do one last brief topic which is to look at the difference between conductors and insulators.

    00:14 In a conductor, charge is free to flow completely freely.

    00:19 It can move throughout an object like a metal without anything impeding its flow in any significant way.

    00:26 There is some resistance which we'll talk about but if I brought some positive charge, for example, near this metal, what would happen since the charges can flow freely, is that all the negative charges in our object would move towards the positive charge because again, opposite charges attract.

    00:43 Similarly, the like charges, the two positive charges would all repel, and so all of the positive charges would get pulled off to the side.

    00:50 What would also happen is suppose we connected this to the ground.

    00:55 This is a very common sort of question when it comes to electrical systems which is what happens if you say, connect some object, some conductor to the ground.

    01:03 The reason for this is that the ground, the earth, is so big that it access sort of an infinite source or sink.

    01:10 It can supply as many charges as you're asking it to supply if you're pulling charges out by an attractive force and it will take as many charges as you give it again until every system is in equilibrium and so what happens if we have a positive charge brought near this metal object in which objects, charges rather are free to flow, all the positive charges will flee, if you will, from this positive charge we've brought near and go into the earth while on the other hand, electrons might flow into your metal because they're attracted to the positive charge that you brought near your metal object.

    01:42 What we could then do is allow this to equilibrate after holding our positive charge near the metal for a while and once we've done that, we might disconnect our metal object, whatever it is, from the earth.

    01:55 The important thing here is now that the connection with the earth has broken, it's no longer touching the ground, whatever charges are left on our metal conductor are stuck on our metal conductor. They can flow around in the conductor but they have no way to leave or be added to the conductor anymore.

    02:11 What this means is that now I can take that positive charge away and I found a way to add many electrons to some metal object.

    02:19 So, even when I've removed my positive charge, again the charges because they're no longer in contact with anything else, are stuck on my metal conductor.

    02:27 So, these kinds of procedural questions are in fact very common.

    02:30 You'll be asked, what happens if I add this and then subtract this, and then connect it to the earth, and then detach it from the earth again.

    02:37 So, be aware of a very procedural way of thinking about how a problem like this would act out using the idea of charge flowing away from similar charges and towards opposite charges as we've discussed.

    02:51 We could also talk about an insulator.

    02:54 In an insulator, the charges cannot move freely and flow around.

    02:58 So, if I brought a positive charge near, they couldn't flow to the opposite side for example of the entire insulator.

    03:04 On the other hand it is true that while they're stuck, each particle or each molecule subunit in your object is stuck in a particular position, it is free usually to polarize. It could flip or orient, or stretch a particular object, a particular molecule in our system.

    03:20 One side may be the negative side towards our positive charge and one side may be the positive side away from our positive charge.

    03:28 So, this would be a polarizing effect where again you can see that each unit here, each negative positive unit which might be coming from a molecule or like a water molecule which can be polarized, stays where it is but orients itself in such a way that it is polarized and all of these different units will be polarized in the same direction.

    03:47 That summarizes our definitions for both a conductor and an insulator and also wraps up our introduction to electrical charges.

    03:56 As we go forward, we'll introduce some more ideas about electricity as well as how it relates to magnetism and how to use electricity as it's flowing in circuits.

    04:04 Thanks for listening.

    About the Lecture

    The lecture Conductors and Insulators by Jared Rovny is from the course Electrostatics.

    Included Quiz Questions

    1. The metal object becomes positively charged.
    2. The metal object becomes negatively charged.
    3. The negatively charged object becomes positively charged.
    4. Protons flow out of the metal object.
    5. No change occurs, since the negative charge does not touch the metal.
    1. The metal object will be positively charged.
    2. The metal object will be negatively charged.
    3. The ball will still be neutral.
    4. The ball will initially be neutral, but then become negatively charged.
    5. No charge can flow if the negative object does not touch the metal ball.
    1. The object will still be neutral.
    2. The object will be negatively charged.
    3. The object will be positively charged.
    4. The object will initially be neutral, but then become negatively charged.
    5. No charge can flow if the negative object does not touch the metal object.

    Author of lecture Conductors and Insulators

     Jared Rovny

    Jared Rovny

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