Good morning.
Let's start with a demonstration.
We take a balloon and rub it against fur which gives the balloon an excess of electrons
which means the balloon has a net negative charge.
Now I put it on the whiteboard...
And it sticks to the whiteboard.
Why?
♪flipping physics♪
- Well, it has to be charging by conduction or induction, right?
- I don't think - I don't think that's right.
- I do not think charges are going to transfer to or from the whiteboard.
- Right.
- Yeah, no.
- Yeah, this is not charging via conduction or induction.
The wall is an insulator and does not have free electrons to move around, so rather than
electrons flowing, this has to do with how the charges in the wall rearrange themselves.
In our previous lesson, the electroscope was made of metal which is a conductor.
Conductors have many electrons which are free to move around, that is why we were able to
charge the electroscope via conduction and induction.
Again, the wall is an insulator and insulators do not have many electrons which are free to move.
In fact, what we are doing here does not change the net charge of the wall, what we're
doing is called "polarization".
How do you think the charges are going to rearrange themselves and how do you think
charges in the wall rearranging themselves will make it so the balloon is attracted to the wall?
- We know the balloon has an excess of electrons so a net negative charge.
- The protons in the wall are not going to move, however, the electrons can move in their
orbits around the nucleus, right?
- Okay, so, the electrons in the wall are going to move away from the electrons in the
balloon because according to the law of charges, the electrons in the wall will be repelled
from electrons in the balloon because they both have negative charges.
- So that means, the protons in the wall will be closer to the electrons in the balloon
and the electrons in the wall will be farther from the electrons in the balloon.
- Well, there it is.
Remember Coulomb's law?
- Yeah.
It is the electric force equals Coulomb constant times charge 1 times charge 2 divided by the
square of the distance between the centers of charge.
- We already talked about this, there is an attractive force between all the positively
charged protons in the wall and the negatively charged electrons in the balloon and there
is a repulsive force between all the negatively charged electrons in the wall and the negatively
charged electrons in the balloon.
But how is there a net attractive force then?
- Because according to Coulomb's law, the smaller the distance between the centers of
charge of the two charges, the larger the electric force.
- Right. Because the electric force is inversely proportional to the square of r, the distance between the
centers of charge.
- Okay, I get it.
Because the opposite charges are closer than the like charges, the attractive electric
force is larger than the repulsive electric force and the net electric force between the
balloon and the wall is an attractive force.
- Very nice everybody.
Again, what happens to the wall is called polarization of charge.
And polarization of charge is why a negatively charged balloon will also pick up some little
pieces of paper.
- Because the paper becomes polarized just like the wall.
- Yes Billy.
The negatively charged balloon picks up little pieces of paper because the balloon polarizes the paper.
Also, look what happens when we place an empty aluminum can on the table.
We can use the negatively charged balloon to move the aluminum can.
- Because the aluminum can you can polarize just like the wall.
The negatively charged balloon repels the electrons in the can and some of the electrons
flow to the opposite side of the can which causes a net attractive electric force on
the aluminum can and causes the can to roll toward the rubber balloon.
- Billy, that is correct.
One quick thing to point out is that each of the electric forces in the polarization
demonstrations is quite small.
The masses of the balloon and little pieces of paper are small, so only a small electric
force is required to hold them up and it only requires a small force to roll the aluminum can.
Notice, the charged balloon is unable to pick up a whole piece of paper...
It can only pick up small pieces of paper.
It can move the whole piece of paper a little bit, but it cannot pick it up off the table here.
And that is because the small electric force caused by polarization is not enough to overcome
the force of gravity of the paper.
- But the electric force caused by polarization will be larger in a conductor than an insulator, right?
- Well, that makes sense.
The wall is an insulator, so the electrons are just pushed to the opposite side of the atom.
However, in the aluminum can, the electrons are free to move about much more and actually
end up on the other side of the can.
- Which is going to produce a larger difference in attractive versus repulsive force and a
larger net attractive electric force.
- That is correct.
Thank you very much for learning with me today, I enjoyed learning with you.