Good morning.
Right here I have an electroscope.
An electroscope is an instrument for demonstrating electric charge.
♪flipping physics♪
This electroscope has a metal ball on top of a vertical metal rod with a hook on the
end of it and two thin foils of aluminum hanging from the hook.
That whole apparatus is electrically insulated from its surroundings via the rubber stopper
and glass flask.
Let's start by charging the electroscope via conduction.
First, I will do the entire demonstration then we will go through it step by step to
make sure you understand what is going on.
So class, are we ready to demonstrate charging via conduction?
- Absolutely.
- Yeah.
- Sure.
- Here we go. First I'm gonna charge this rubber balloon by rubbing it against fur.
So now we have a charge of a balloon. I bring it close to the electroscope and you can see
the foils on the electroscope begin moving apart.
And then when I touch the electroscope, I have now charged the electroscope via conduction.
So this is a charged electroscope via conduction and I can touch the electroscope with my finger... Pik...
And the two foils move back to where they started.
That's it.
- I'm glad to know we're gonna walk through that step by step because I have no idea what
just happened.
- Right that is not a problem because we're gonna go through it step by step.
So let's start with the first step; rubbing the fur on the balloon causes what to happen
to the fur and the balloon?
Bobby...
- Oh, I know that one from before.
Electrons move from the fur to the rubber balloon causing the fur to have an access
positive charge and the rubber balloon to have an excess negative charge.
I believe this is called charging by friction.
- Correct. Although, at this point we really are only concerning ourselves with the net negative
charge on the rubber balloon.
The other object in the demonstration is the electroscope.
Bo, before I bring the balloon near the electroscope, what is the approximate net charge of the electroscope?
- Well, before you start, the electroscope should have a net charge of zero; it should
have a neutral charge, right?
- Right. So, before we bring the two objects near one another, the electroscope has roughly an equal
number of positively charged protons and negatively charged electrons, and the balloon has a larger
number electrons than protons giving it a net negative charge.
Now, watch what happens as I bring the rubber balloon near the electroscope.
Even before I touch the balloon to the electroscope, the two metal foils at the base of the electroscope
are pushed apart from one another.
Bobby, why do you think that is?
- If the two metal foils are pushed apart from one another, according to the law of
charges, they must have the same charge and there must be an electric force pushing them apart.
But I do not know if the net charge on each of the foils is positive or negative.
- We can use the law of charges to figure that out as well.
We know protons do not easily move because they are in the nucleus of the atoms, and
that means the electrons are the ones that move.
So, the electrons in the electroscope which are negatively charged are repelled by the
negative charges in the balloon and float down to the metal foils.
That must mean, when the balloon is brought near the electroscope, the metal foils have
an excess of electrons or a net negative charge and an electric force pushes the metal foils
away from one another.
- That means the top of the electroscope has a net positive charge because there are fewer
electrons than protons in the ball at the top of the electroscope, right?
- Correct. Now we know what happens before the balloon touches the electroscope, but what happens
when the balloon touches the electroscope?
- I have no idea. - I do not know.
- Touching the two objects together allows electrons to transfer between the two objects.
- Oh, that means electrons will transfer in to the electrically neutral electroscope from
the balloon and that is because the electrons will be attracted to the protons near the
top of the electroscope and repelled from one another where they are crowded together,
close together in the balloon.
Because there are balloon transfers some electrons to the electroscope to balance out the net
negative charge of both the rubber balloon and the electroscope end up having some excess electrons.
- But the total number of excess electrons in the balloon electric scope system remains
the same because the total number of electrons will stay the same.
- Conservation of charge; the total electric charge of an isolated system never changes.
- Yeah.
- At this point then, both the balloon and the electroscope have an excess number of
electrons causing a net negative charge, which is why the metal foils of the electroscope
remain repelled from one another.
Again, because of the law of charges; like charges repel.
Again, at this point, the electroscope and the balloon both have an excess number of electrons.
And then I touch the electroscope and that causes the foils to no longer be repelled
from one another.
This is because, compared to the charge on the electroscope, I have a very, very large
number of charges and I act as a ground.
An ideal ground is an infinite well of charge carriers, and this is typically the symbol
used for a ground.
- I'm sorry, what?
- Okay, sorry, I know it can be confusing.
We call it a ground because electrical circuits are literally connected to the earth or the
ground, and the earth has, relatively speaking, an infinite number of electrons which we can
pull from it or we can give to it, again, relatively speaking, an infinite number of charges.
If something goes wrong in a circuit, the ground will serve as a way to balance out the charges.
In this example, when I touch the electroscope, the electroscope is no longer electrically
isolated and the excess electrons on the electroscope flow out of the electroscope into the ground,
into me, and the electroscope is now electrically neutral.
Which is why the foils are no longer repelled from one another.
Does that make sense?
- Yeah. I think it was the "infinite" term that confused me.
What you mean is that you as the ground have so many more charge carriers in your body
than the electroscope such that you can take or give as many charges as necessary to cause
the electroscope to become electrically neutral, right?
- Yes Bo, that is correct.
Okay, so that is charging by conduction.
Two things I want to make sure you realize about charging via conduction; one is that
the two objects have to touch, and two is that the two objects end with the same sign
of net charge.
In this demonstration, both the balloon the electroscope end with a net negative charge.
- You mean before you grounded the electroscope, right?
- Correct Bobby, that's before I ground the electroscope.
Actually, let's watch the whole demonstration through once more, only this time I'm going
to add typical idealized illustrations which go along with this demonstration.
Rather than the electroscope, we have a neutrally charged metal sphere.
Notice the metal sphere has the same number of positively charged protons and negatively
charged electrons.
This is why the metal sphere which represents our electroscope is neutrally charged at the start.
Then, rather than using a rubber balloon, we have a rubber rod with an excess of electrons.
Bringing the negatively charged rubber rod near the top of the metal sphere pushes the
electrons in the metal sphere closer to the bottom of the metal sphere or the bottom of
the electroscope, which is where the metal foils are and this is why the metal foils
are pushed apart.
Touching the rubber rod to the metal sphere transfers some of the electrons from the rubber
rod to the metal sphere.
This decreases the number of electrons on the rubber rod and increases the number of
electrons on the metal sphere.
However, because of conservation of charge, the total number of electrons stays the same.
Now we remove the rubber rod and the electrons in the metal sphere distribute themselves
throughout the metal sphere and the metal sphere has a net negative charge which is
why the metal foils are pushed apart from one another now.
The metal sphere which represents the electroscope has now been charged by conduction.
Then we ground the metal sphere which allows the electrons to flow from the metal sphere
to the ground and leaves the metal sphere with a neutral charge.
Okay, next let's demonstrate charging via induction.
Class, are we ready to demonstrate charging via induction?
-I am ready.
- Yeah.
- Here we go!
Again, I charge the balloon by friction, by rubbing it against the rubber balloon - well,
far... whatever.
And then I bring the rubber balloon close to the electroscope but I do not touch the electroscope.
You can see the two foils move apart.
Then I touch the electroscope, the two foils move down, I bring my finger away from the
electroscope and then I bring the rubber balloon away and poonk...
The two foils move apart from one another and you can see the electroscope is charged via induction.
And then I can again bring my finger in and ground the electroscope, [bink] and the two
foils move down next to one another.
That's it.
- Now we're going to walk through it step by step, right?
Because I really - I need to...
- Yes. Step one was to give the balloon a net negative charge with an excess of electrons.
Step two was to bring the balloon close to the electroscope but to not touch the electroscope.
We determined in the last demonstration that the electroscope now has negatively charged
electrons in the foils because according to the law charges, they are repelled from the
electrons in the balloon.
And we recall that the positively charged protons do not easily move because they are
in the nucleus of the atoms.
At this point, the demonstration is different because we do not touch the balloon to the
electroscope, instead I touched the ball on the electroscope with my finger.
Billy, what happens here?
- When you touch the electroscope, you act as the ground.
That means, electrons in the electroscope are free to flow into the ground.
They do that because according to the law charges, like charges repel one another, so
some of the electrons in the electroscope flow from the electroscope into the ground.
And they do this because, you essentially provide an escape route for electrons to leave
the electroscope, but there are still electrons in the electroscope, just fewer than before
and many are in the metal foils within the electroscope because they are repelled from
the electrons in the balloon.
That gives the metal foils a net neutral charge and the two foils are not repelled from one another.
- And then when you remove the balloon, the electrons in the electroscope are repelled
from one another in the electroscope and get distributed throughout the electroscope.
That leaves the electroscope with an excess of protons and a net positive charge.
That is why the metal foils are repelled from one another because the positive charges in
the foils repel one another.
- And then you touch the electroscope which grounds the electroscope.
In this case, that means electrons are pulled from your body into the electroscope to balance
out the excess protons in the electroscope and leave the electroscope with an equal number
of protons and electrons and a net neutral charge.
- Exactly. That is charging by induction.
Again, there are two things I want to make sure you realize about charging by induction,
and they are the reverse of charging by conduction.
One is that the two objects never touch one another.
- Okay... Conduction, the two objects touch, induction, the two objects do not touch.
They do not come into contact with one another.
- And two is that the two objects in this induction demonstration end with opposite net charges.
In this demonstration, the balloon ends with a net negative charge and the electroscope
ends with a net positive charge.
- Again, this is before you ground the electroscope, right?
- That is correct Bobby.
Again, before I ground the electroscope.
Let's watch the whole demonstration through once more and again, I'm going to add the
idealized illustrations.
The metal sphere which represents the electroscope still starts with the same number of protons
and electrons giving the metal sphere a net neutral charge.
Again, I bring the negatively charged rod near the top of the metal sphere which pushes
the electrons in the metal sphere closer to the bottom of the metal sphere or at the bottom
of the electroscope, which is where the metal foils are and the metal foils are pushed apart.
Now, I ground the metal sphere which causes electrons to flow into the ground and out
of the metal sphere leaving the metal sphere with a net positive charge.
However, the electrons in the metal sphere are, according to the law of charges, still
repelled from the electrons in the rubber rod and therefore concentrated near the bottom
of the metal sphere and causing a net neutral charge in the foils which is why they are
not repelled from one another.
Then I remove the ground which causes no change to the charges in the metal sphere.
But then, when I remove the rubber rod, the electrons distribute themselves throughout
the metal sphere and because the metal sphere has a net positive charge, the foils also
have a net positive charge and because like charges repel one another, the foils push
one another apart.
The metal sphere has been charged via induction.
The rubber rod and metal sphere never came into contact with one another and they have
opposite net charges.
Then I ground the metal sphere causing electrons to flow from the ground into the metal sphere
giving the metal sphere a net neutral charge.
That's it.
Thank you very much for learning with me today, I enjoyed learning with you.