Supplementary Reading - Lecture 22



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Familiar Aspects of Electricity

Today we begin with the discussion of electricity. Consider the following:






Electric Charges and Forces

The effects of electricity have long been known, and it is interesting to study the historical development of the subject. However, it is simplest to start with what has been learned up to the present. Here are the facts:






Everything is Made of Atoms

Famed physicist Richard Feynman once said that the single most important thing learned from scientific studies is that everything is made of atoms. Knowing about atoms helps us understand electricity because we find that atoms are made of tiny particles with electric charge, both positive charge and negative charge. We then learn that charges have the property that same sign repel each other while charges of opposite sign attract. See the website Properties of Matter for an animated introduction to the subject.





Electricity in Our Everyday World

Electric forces hold together atoms and produce chemical reactions. We depend on the flow of electric charges, or electric currents, to make all kinds of appliances work in our modern world. We witness the power of electricity in lightning bolts. See this website on Electricity and Magnetism for an elementary introduction.






Explaining the Electroscope Observations

Negative charges are removed from a piece of fur and deposited on to a rubber rod by vigorously rubbing the rod with the fur. By contrast, a glass rod loses electrons when it is rubbed by a silk cloth. What remains on the rod is a deficit of electrons (negative charge) and, therefore, a net positive charge.

If a rod with an excess of charge, either positive or negative, is put in contact with the metal of the electroscope, some of the excess charge flows on to the electroscope. If the electroscope is of the type pictured in the textbook, both leafs have the same charge and repel each other. The in-class version has a movable needle and a fixed metal plate, but the idea is the same. We see that the needle moves away from the plate because both have charge of the same sign and like charges repel.

If the rod is brought close to the rod, but doesn't touch, the electroscope leafs still separate. This is more subtle and more difficult to explain in words alone. See Figure 19.8a to see what happens. Here we see positive charge on the rod pulling negative charge to the nearby metal surface of the electroscope leaving positive charge on the two leafs. Note that the total charge on the electroscope is zero. What has happened is that the charge gets rearranged as a result of the rod being placed nearby. This suggests that the charge on the rod affects the nearby environment so that charge in the electroscope "feels" its presence. What happens is that an electric field is created by the charge on the rod. This electric field influences the distribution of charge on the electroscope.




Making and Explaining Sparks

We made sparks fly across a gap between two metal spheres. Electrons were transferred by a rotating belt on to one of the spheres. As described above, the charges create an electric field which spans the space between the two spheres. The electrons jump the gap and settle on the other sphere which is grounded. (Grounding has to do with establishing an electrical connection to the earth which has the effect of neutralizing the electrical charge on an object.) The flow of electrons between spheres results in the spark we see. But do we actually see the electrons? No, what we see results from the electrons striking atoms in the air between the spheres. The struck atoms become "excited" and when the atoms jump back to a deexcited state they emit light. That is what we see as the spark.




Attracting the Metal Can or the Wood Plank

We are able to attract a metal can or even a large delicately balanced wood plank with charges on a rubber rod. We explain this by noting, as we explained the electroscope, that the presence of charge creates an electric field. The electric field influences the distribution of charge within the metal can, or even on the atoms in the wood. A negative charge has the effect of drawing positive charge closer to it, and a positive charge draws negative charge closer to it. The net effect is to create an attractive force.




Electric Forces and Electric Fields

Electric fields are illustrated in the animation we access here. We observe electrons in the environment of an electric field caused by two much larger charged objects, one positive and one negative. Note how a field is created by the charges. An electron "feels" the field and experiences a force whose magnitude depends on the strength of the field at the location of the electron. By convention, an electric field flows from positive charge to negative charge.

Another website, Charges and Fields, is similar to the previous one except that here you can experiment by creating more positive, negative and neutral objects with charge and noting how they effect electrons in their environment.

Now look at electrons in orbit around an atomic nucleus as depicted in the Web demo here.
This illustrates the similarity between the electric force and the gravitational force. The force between charges (Coulomb's Law) q1 and q2 separated by distance r is given by
Fcoul = kq1q2 / r2,
where k is a constant.
The gravitational force between masses m1 and m2 separated by distance r is given by
Fgrav = Gm1m2 / r2, where G is the gravitational constant.




Measuring the Electric Field

In the web demo the large + and - charged objects are meant to carry a much larger net charge than that of a single electron (yellow). Individual electrons are considered to be test charges. The electric fields generated by individual electrons are ignored for simplicity compared to the strong fields generated by the so-called "terminals" with + and - charge. The strength of the electric field at any point (E) is determined by the force (F) on a "test charge," here an electron. The strength of the electric field is given by the magnitude of the force divided by the charge of the electron (q). So E = F/q.


Electric Potential Energy

Consider the analogy with gravitational potential energy. You know you can exert a force on an object and move it from one place to another, i.e., do work on it. If you raise an object in a gravitational field you increase its gravitational potential energy. The same concepts apply if you move a charge in an electric field by doing work on it. The work done in moving a charge from one place to another in an electric field, like moving a mass in a gravitational field, is equal to the change in the potential energy, electrical or gravitational, whichever applies. In many cases both apply.




Electric Potential - Volts and Voltage

It is commonly known, if not understood, that "Volts" have something to do with electricity. What has Volts is the electric potential? It is defined this way. If you have a object with charge Q and you move it from one place to another within an electric field by doing work on it so it gains an electric potential energy (U), its electric potential, measured in Volts (V), is given by V = U/Q. U is in Joules and Q is in Coulombs.




R.S. Panvini
11/27/2001