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  • Hi. It's Mr. Andersen and this is AP Physics essentials video 6. It is on electric charge,

  • which we can see in this child's hair right here. When I was a kid the playgrounds we

  • played on were dangerous, but they were also covered in metal which is a good conductor.

  • And so now if you go to a playground you are going start to build up this static charge

  • just because we have this plastic everywhere. And so in any kind of a system, like that

  • slide for example, the net charge of the system is equal to the sum of all of those charges

  • that make up the system itself. Now the charges can be moved. They can be transferred from

  • one place to another. And the movement of a charge through a conductor is called current.

  • And so if that charge continues moving around and then comes back again and keeps moving,

  • we call that a circuit. But the one thing you should understand is it is not like that

  • slide was plugged and that we are applying electric charge to it. The amount of charge

  • is conserved over time. We are just moving those charges from one place to another. And

  • this has been known for hundreds and hundreds of years. In fact the word electron and this

  • charge comes from the latin word amber. Because scientists had discovered that if you rub

  • amber on fur and then bring it up to a light object, like a feather or a piece of paper,

  • it will be attracted. There is some magic kind of a force between the two. And so they

  • called it this charge. Now the key thing you should notice is that I did not introduce

  • electrons here. The total amount of charge before we rubbed the amber on the fur and

  • after is equal. We just moved it to different places. And so if you wanted to try this in

  • physics lab you could use a ruler instead of amber if you want to. But we can use something

  • called a pith ball. A pith ball is just a little bit of styrofoam that we cover is some

  • conducting kind of a surface like graphite. And so if you bring it close to the amber

  • it will do this weird behavior like that. So what did we see? We saw attraction between

  • those two objects. And then we also saw repulsion. So early experimentation on this led scientists

  • like Benjamin Franklin to speculate that there was charge. And there were two types of charge.

  • And we just arbitrarily called those positive and negative. And we will talk more about

  • those in the next video. But the idea that they could move was formulated by people like

  • Benjamin Franklin. Now there is this old story that he and his son went up on a hill and

  • they were flying a kite. And his son was holding on to a dry portion of the string so that

  • he would not get shocked. But Benjamin Franklin was able to transfer some of that energy into

  • a laden jar, which is essentially a big capacitor. Again this is just an artist's depiction.

  • And that he could hold his hand up next to it and he felt some of that electric charge

  • showing that lightning was just the same static charge that he had been playing around with

  • in the lab. This is incredibly dangerous. And if there would have been a lightning strike

  • they both would have died. And so that movement of charge is current. And if we can get that

  • current moving in a loop then we can do something with it. And so in this simple circuit diagram

  • we have a battery right here. And then we have a lamp. And so if we can move that current

  • in one direction we can make that light glow. And what is happening here is that there is

  • still a conservation of charge. It is just that we are moving the charge from one place

  • to another and we are able to do work with the movement of that charge. If you wanted

  • to build a simple electric circuit, we have got a lemon clock. So we have copper and zinc

  • that are serving as the cathode and the anode. We have got electrons that are moving and

  • as they do that we can use them to run this simple clock. We are just moving the charge.

  • But the overall charge is conserved over time. So did you learn to make claims about the

  • natural phenomena based on the conservation of the charge? Again we are just moving it.

  • But the sum total we had has not changed. And then do you understand that we can have

  • charges in different places with different amounts. But even as we move that charge around

  • something like a circuit, the total amount is going to be conserved. I hope so. And I

  • hope that was helpful.

Hi. It's Mr. Andersen and this is AP Physics essentials video 6. It is on electric charge,


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電荷 (Electric Charge)

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    Cheng-Hong Liu に公開 2021 年 01 月 14 日