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.