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Electric Charge Up until now, the entire book has dealt with everyday objects in situations that involve net force, momentum, work, energy, and motion in general. From this point on, the book will now deal with not so everyday objects, although the familiar concepts such as net force and so on will still be addressed.
This chapter will investigate forces between charged particles such as electrons and protons,
or objects that have a net charge. To begin this unit, I like to show a neat demonstration
with a plastic wand and a small strip of Styrofoam that is taped into a circle. If you
take a piece of rabbit's fur (sorry animal rights activists, but it is in the name of science),
and rub it onto the Styrofoam strip and the plastic wand, you give both objects a net
charge. The nature of that net charge we will discuss in a moment. Now, when you flip the
strip into the air over the rod, the strip will actually hover above the wand as shown
below.
 You can walk around the room, and balance that strip above the wand. It seems as though it is magically floating with no strings attached. The reason for this trick is simple: both objects were given the same net charge, since they were rubbed with the same material. As a result, we witness the fact that like charges repel one another, which is why the strip is suspended above the wand. As you can guess, that must mean that opposite charges attract. So, we have two rules for charges: In the case of the wand and floating strip, you can see from the picture above that both objects were given a net negative charge, which is why there are negative signs drawn in the picture. But, how does that work? Question One: How is something given a net negative charge?
A physicist by the name of J.J. Thomson discovered that the electron has a negative charge
in 1897. It is also known that the atom has a concentrated core of positive charge, which
comes from the protons, and this was established in 1911 by James Rutherford. So, the
picture of the atom can be depicted as shown below.
 This model of the atom, which was put forth by Niels Bohr in 1913, is not a 100 percent accurate picture of the atom's true structure. To understand the true picture, one has to turn to modern physics or quantum physics, which will be covered in a section of this website. However, for the purposes of this lesson, this is a good representation, and a handy way to visualize the atom's makeup. The nucleus is located at the center of the atom, and contains the protons and neutrons (not shown), which have a positive and neutral charge respectively. The negative electrons sort of orbit around the nucleus as shown. Notice how there are two electrons and two protons. It turns out that the electron and the proton have the exact same charge, only one is negative and the other is positive.
So, an atom with the same number of each is said
to be electrically neutral, or have zero net charge. As soon as an atom loses an electron,
it has a net positive charge since the protons out number the electrons, and when the atom
gains an extra electron, it has a net negative charge because the electrons now out number
the protons.
Since electrons are out in space around the nucleus, they are easily transfered from one atom
to another. They move around all the time. They can be literally ripped right out of their
orbits due to friction from the rubbing of two different surfaces. In the example described
above, the two surfaces rubbed together were either a plastic wand or Styrofoam strip and rabbit fur. During
the rubbing process, electrons were ripped from the rabbit fur and were transfered to the atoms
of the wand and strip. Thus, since their atoms had an overall net negative charge, the objects
themselves had a net negative charge.
 Van de Graaf Generator Most people are familiar with the Van de Graaf generator because they are used in most high school physics classes. It consists of usually a large base with a shaft that extends two to three feet above and has a large metal sphere at the top.   Electroscope Experiments We will now use the electroscope and Van de Graaf generator to see how charges interact. Question Two: What will happen to the hanging metal strips when the metal sphere from the electroscope is brought near the generator?
When the electroscope is brought near to the generator's metal sphere, the hanging strips
actually move apart as shown below.
 
It is not correct to say that the positive charges are pulled up from the bottom, and the negative charges are pushed down. The sphere is positively charged because there are more protons than electrons, because the protons are left behind. Likewise, the strips are negatively charged because there are more electrons than protons. This is shown in the picture above with the atoms.
the electroscope has now become what is called an induced dipole.
Dipole means two different poles, and in this case one pole is negatively charged, and the other
is positively charged.
  Question Three: What will happen to the hanging metal strips when the metal sphere from the electroscope touches the generator and is then take away
When the metal sphere of the electroscope is allowed to touch the generator, the electrons from
the generator are attracted to the protons on the scope, so they jump across to join them.
However, now the entire scope has a net negative charge, so when it is removed from the
generator, the electrons in the strips have nowhere to go now. They can't go back to the
top because it is now full of electrons, so they stay put, which means the strips stay apart!
 You now have an object that has a total net negative charge. Now the question becomes, how do you get the strips to go back to normal? It turns out that if you just touch the metal sphere, the strips will magically go back together. Of course it isn't magic. Here is what happens.
The electrons are not very happy being that close to each other. They are constantly pushing
against one another. when you touch the scope's sphere, and you are touching the ground
at the same time, you give the electrons a place to go. Therefore, the electrons in the strip
push the electrons ahead of them through your body into the ground, which is always electrically
neutral, and always acts as an electron dumping ground. Once the scope is back to being
electrically neutral, the electrons stop moving, and the strips go back to normal. This is
why there is plastic preventing the metal rod from touching the metal box. If the rod was
touching the metal box, the strips wouldn't stay apart because the extra electrons would be
able to travel to the ground by means of the metal box.
 Given the results of our previous experiments, there is a good question to consider. Question Four: When will electrons move from one object to another?
We have already established the fact that electrons can be removed from one object and given to
another because of friction due to the rubbing of two surfaces. In fact, this is exactly
why you shock someone after walking across a carpeted floor. The rubber from your shoes
rips electrons off of the carpet so you have an excess amount. When you touch a doorknob
or another person, both of which are touching the ground, the excess electrons jump
from you to the other object. So, next time someone shocks you, politely tell them
no thanks, they can keep those electrons to themselves.
 O.K. It is obvious that electrons will move to another object when that object is touching the ground. It is said to be grounded. They will also move if the other object has a net positive charge, which means it is lacking electrons. Lastly, electrons will move to another object if that object is electrically neutral, which was shown with the electroscope touching the generator. The three scenarios are summarized below. Electrons will move from one object to another if:
1. The object is grounded
2. The object has a net positive charge
3. The object is electrically neutral
Question Five: Could someone get shocked if they were hovering in the air and not touching the ground?
Athough it seems that the person couldn't be shocked since he/she isn't grounded, the fact still remains that the person is probably electrically neutral, which means that he would get shocked.
In fact, this was demonstrated once in class when a student jumped into the air and touched the
generator at the same time.
So, using e for the electron, p for a proton, and n for the neutron, we can write:  Interestingly, it takes quite a few electrons (or protons) to actually equal one Coulomb of charge. In fact, it is easy to see how many. Just take the charge of the electron times the number needed, and set that equal to one. You end up taking one divided by the charge value, which is, That is 6.25 followed by 16 zeros! The actual name for that number is 6.25 quintillion! Although possible, everyday objects never reach this staggering number of electrons, and have a net charge of one Coulomb. This is a good thing since if objects did commonly reach one coulomb of charge, touching them would be lethal! For the most part, objects only reach around a millionth of a Coulomb, leaving just an unpleasant shock to the touch.
Which means the proton is 1,836 times more massive than the electron! As far as the neutron goes, it has almost the same mass as the proton at 1.675(10^-27) kg. Writting them all together, we have,  The Atom The approximate diameter of an atom is on the order of 10^-10 meters, which is in the realm of nanometers. If an object were the size of the period at the end of this sentence (say 0.20 mm, it would require roughly 2 million atoms!
It is fun to create analogies to truly appreciate the size of the atom, and how the nucleus
and electron size compare. If the nucleus of the atom was made to be the size of a period
(.), the atomic diameter would stretch nearly 5 meters, and the electrons would still be
invisible to the naked eye.
 We can take this even further. If we expand the nucleus to the size of a basketball, the diameter would be 4000 meters, (4 km), and the electron would be the size of a period (.)! It is pretty obvious by now that the atom is mostly empty space. In fact, the volume of the nucleus of the atom only makes up one hundred thousandth of the volume of the entire atom. Imagine the empty space in our atom with the basketball sized nucleus, 4 km diameter, and period sized electrons. It is truly remarkable. Here are some other interesting facts to note:
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