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.
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