## Subtitles section Play video

• 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