Constructing Physics

Elementary Physics Course Notes

Adam Johnston
Department of Physics
Weber State University

 

"Weekends sure seem longer than two days since there isn't any physics class," a woman was overheard telling her friend.  "Thank goodness I had my research project to work on -- I don't know how else I would have made it through."
Adam welcomed everyone back to physics class.  Everyone cheered.
"You'll really get a 'charge' out of today's discussion," he noted.  No one got the pun, but they wrote it down in their notes anyway.  They were sure that it would come in handy at some party, once they understood what this thing "charge" was and why it made for such a funny joke.
We began by playing with pieces of transparent tape.  When tape is stripped off of its roll or off of another piece of tape, it has some seemingly magical properties.  Two pieces of tape, when one is peeled from the other, will attract one another.  However, if you have two pieces of tape, each of which was peeled off the top of other pieces, these two will repel one another.  It seems that the process of "peeling" has given the tape some sort of property which creates forces, either attractive or repulsive.  Similarly, Adam could rub a rubber wand with some fur, and this wand was then able to attract a ping pong ball or pick up pieces of puffed wheat.  It could also be used to attract or repel various pieces of the peeled tape.
Obviously, there is some kind of force.  But where is it coming from?  We are reminded of gravity, which depends on how much mass two bodies have and how much distance is between them.  In the very same way, Coulomb's Law describes how the amount of charge in two bodies and the distance between them create electric force.  Amazingly and beautifully, the expression for electric force and the one for gravity's force look exactly the same, except that electric forces use "charge" instead of mass.
But what on Earth is "charge"?
Recall back to when we were talking about matter.  Matter is made of atoms, which are composed of clouds of electrons whirling about a massive nucleus.  Something must be holding the electrons to their respective atoms of matter.  This is due to the electric force.  Electrons are defined to have a "negative" charge, while the protons within the nucleus of an atom are defined to have "positive" charge.  These opposite charges attract one another.  If the electron had a positive charge (or if the proton had a negative charge), the charges would be the same variety and would repel.  This would not be good for the existence of matter.
So, unlike gravity, electric forces can either attract or repel, since there are two kinds of charge.  There is only one kind of mass, so it is only attractive.  Again, this is probably a nice property of the universe.  If gravity could have repulsive properties, maybe it would be harder to form stars and planets and (as a result) kittens.  Everyone loves kittens.
Speaking of kittens, what happens when you rub a balloon on a furry little kitten?  The kitten's fur stands on end, and the balloon has the ability to stick to walls and stuff.  What's going on here?  Similarly to things done in class, the balloon is stealing extra electrons away from the kitten.  The kitten, left with a lack of electrons, is positively charged.  The balloon, now with an excess of electrons, is then negatively charged.  If you had two kitten-rubbed-balloons (or two balloon-rubbed-kittens), they would repel one another.  The kitten and the balloon attract one another.  You could test what the charges of your two pieces of tape were by seeing what force they experience next to the negatively charged balloon.
Charge can exist in conductors or insulators.  In insulators (like rubber, glass, wood, etc.), charge sticks in one place.  In a conductor (like metals such as copper, aluminum, steel, etc.), the charge is able to move around easily.  This means that when a negatively charged rod touches a metal apparatus with a needle inside it (called an "electroscope"), the negative charges move all over the metal and all repel one another.  This means that the needle will jump away from its metal post.  If more negative charge is pushed into the needle (due to a negatively charged rod coming close to the electroscope), then the needle will repel from the post even more.  If negative charge is attracted out of the needle (due to a positively charged rod coming close to the electroscope), then the needle will "relax" back towards the post.  (This description might require some more visual aids than what is provided on this set of notes.)
If we put a whole bunch of charges in one place, they would all want to repel one another.  Adam volunteered a student (who naively sat in the front row) for a demonstration.  She signed some kind of a waiver and then stood atop a bench while placing her hands on a giant, metal sphere.  As the sphere was given negative charge, she, too, got negative charge, and this made her hair stand on end (literally).  Then, everyone left to go eat lunch.  For all we know, she's still standing there, since Adam told her not to let go, no matter what.
. . . 
The poor volunteer who was attached to a giant sphere of charge last time was still standing there 48 hours later.  Her hair was on end, as were her wits.  Yet, she had not let go, just like Adam instructed.  Just before class started, the student had accumulated so much extra charge that she attracted a giant lightning bolt from out of an electrical socket.  Everyone cheered, and then the student, charred and tired but still in good spirits, decided to sit in the back row where she wouldn't get volunteered ever again.
We open class with a joke:
Two atoms meet one day while walking down the street. One asks the other how he's doing. The atom responds: "Oh, just awful. I've lost an electron!"
Shocked and stunned by such a horrific idea, the atom asks her friend, "Really?! Are you sure?"
"Yes, I'm positive."
Hee hee.  Get it?  As you can see, physics is good not only for its ability to explain the natural world but also for providing us with good humor.  A good reason to take a physics course (or any science course, for that matter) is so that you can understand the jokes.
Since we've been playing with charge so much lately, Adam summarizes the ways by which charges can get moved around: Friction or rubbing (as with fur and a balloon), direct contact (as with the rod that contacts the electroscope), and induction (as happens when a rod is moved next to the electroscope).  In all of these cases, net charges are going one place or another not because negatives and positives are being created out of then air, but because the negatively charged electrons from neutral atoms are either in abundance or lacking.  Some examples of how to manipulate charge are demonstrated.
It can also be demonstrated that charged objects can attract neutral ones.  They do this by separating (by induction) the negatives and positives of the neutral object.  Like charges in the neutral material will push away from a charged rod, while opposite charges get pulled towards a charged rod.  With all the opposite charges of the rod on the near side, the object is more attracted than repelled by the charged rod. 
Wow!  With all these forces being produced, it seems that you could really start getting electricity to do something useful.  After all, when gravity pulls on a bowling ball, the bowling ball can be made to move faster and faster until it does something useful (like smash a physics professor's toe).  If electric forces pull and push on charges, what can we make them do?  Adam demonstrated some strong electric forces building up on the big metal sphere called a Van de Graaf generator.  With all of that repulsive charge in one place (in other words, a very strong electric field), eventually the charges are going to get pushed so much that they should be required to find somewhere else to go.  This causes lightning bolts to strike from one sphere to another.  Adam mentioned that there were tens of thousands of "volts" required to produce these lightning bolts. 
With all of this potential to move charge, Adam began to wonder what he could do with it.  He grabbed a long fluorescent light tube and held it up next to the big sphere of charge.  The charge not only went through the lightbulb, it also went through the very brave (idiotic?) physics instructor, lighting up another light tube held in his other hand.  So, it is evident that moving some charge from one place to another (which probably requires some force through a distance, or work) can light lightbulbs and/or zap physicists.  Both possibilities seem promising.
*****

Magnetism is the kind of force felt by compass needles.  We can show that these kinds of forces are different from the forces of static electricity.  But exactly how to you produce them?  Adam produced electric currents, and from these currents (moving charges), magnetic forces would be produced.  These are summarized as magnetic fields, and we note that the magnetic fields and the electric currents always point in perpendicular directions to one another.  Either the magnetic fields are going around in circles about the currents or vice-a-versa.  These forces are not felt by all objects.  It was shown that even metals generally aren't all susceptible to magnetism.  Steel or iron will attract to a magnetic field, though copper, brass, aluminum, etc. will not.
Why?  What produces magnetic forces?  We just argued that the magnetic force resulted from moving charges; but where are the moving charges in a permanent magnet?  Certainly, electrons are moving about in the magnet, but their translational motion is mostly random, and not aligned in the same way.  Physicists describe electron "spin" (as though the charged particles are spinning around on an axis, like the Earth or a top).  Certain materials are very good at getting many electrons all spinning in the same directions, creating magnetic domains.  These materials, such as iron and nickel, tend to make good magnets or at least are attracted to magnetic fields.
If a moving charge creates a magnetic force, then what does a magnetic force do to a moving charge?  (A static charge and a magnetic force simply ignore one another.)  We witness (with various demos both today and the next lecture) how a magnetic force will deflect a moving charge to move in circles.  The magnetic field and the force that the moving charge experiences are perpendicular to one another.  This is unlike anything we've yet seen.  But it begins to makes sense if you consider the fact that the moving electric charge produces its own magnetic force, and this magnetic force interacts with the external magnetic force.

****
Notes: Faraday's Law of Induction
Is there any kind of symmetry to nature?  If moving charge produces magnetic force, then can magnetic force produce electric current?
Of course, otherwise we wouldn't have brought it up.  This is known as Faraday's Law of Induction.
Demonstrated with the magnet in the coil . . .
Lenz's law is simply conservation of energy applied to a particular situation.  The energy in the current isn't free.  You have to do work to get it.  . . .
Magnet falling down tube . . .  sliding down copper ramp?  . . .

****

It's a beautiful, symmetric kind of nature that we live in.  Electricity and magnetism dance around one another, keeping their separate identities but remaining intertwined and interdependent.  If we picture electric and magnetic fields and how they change in space, we can begin to see how light is constructed.
Imagine that you get a research grant for $0.07.  You're upset, but given the current national budget situation and the threat of cutbacks in science research, you are grateful that you got anything.  You go to the electricity and magnetism store and are delighted to find that they're having a sale: But one electron, get one free!  Better yet, an electron costs only 7 cents -- exactly what you had in your budget!  So, you get two electrons.  You only need one, but the other one will come in handy in a few days.

An electron bobbing up and down produces a changing electric field, which in turn produces a changing magnetic field . . .
This travels as a wave of E&M radiation, or "light."
Demo: Radio transmitter and lightbulb antenna.
Light is all the same, just of various wavelengths: radiowaves, microwaves, infrared, visible (red, orange, yellow, green, blue, violet), ultraviolet, x-rays, gamma rays.
These are all the same and can all do the same things.  Your eyes are just sensitive to one narrow band of wavelengths; and (thankfully) the Sun happens to be especially abundant with these particular wavelengths.
Demo: Construct and use a "radio" station/receiver that uses visible light.

So, what is light?  We started to hit at that the other day.  But, what are the rest of the details?
Speed:  How to measure?  How fast?
How to produce light?  It always comes down to conservation of energy. 
Light is a wave, so it should do wavelike things.  Examples of diffraction and interference (in bathtub and also shown for laser through slit/grating).

***
We didn't do the following, but they're worth looking into if you're interested:
As waves, we should see other examples of interference.  Thin film interference.  (Bubbles, oil slicks, etc.)  Specific thickness of a film will destructively and constructively interfere with specific wavelengths of light, producing a specific blend of visible color.
Polarization shows that the waves of light are oriented in all different ways.  Polarized light is oriented in only one direction (e.g., up & down).  Scattered light from the sky (or a cloudy fish tank) is polarized, as is laser light and "glare."
So, light is obviously a wave.  However, we can shine light of certain wavelengths (ultraviolet) and they visibly knock electrons around.  This suggests that light doesn't just act as a wave!  We'll hit on the details of this more firmly in quantum mechanics.