Forces of attraction and repulsion among non-metallic
substances were reported in early Greek times. Artisans found
that when they cut and polished amber (hardened tree sap) to make
jewelry, the amber attracted or repelled bits of paper, hair,
and other light-weight, non-metallic materials. One can imagine
that one of the eye-catching features of this phenomenon is the
idea of action-at-a-distance. Whereas in most cases if a person
wishes to cause an object to move, he or she must physically touch
the object to pull or push it. However, with the phenomenon we
now call electrostatic attraction/repulsion, objects interact
with each other with no physical contact.
1. There are two kinds of charge. 2. Like charges repel. 3. Unlike charges attract. 4. Charge is quantized.
|The idea that charges repel and attract one another was quantified by the French scientist Charles Coulomb who used a sensitive torsion balance to measure the forces between charged objects. He concluded that these forces were directly proportional to the product of the charges and inversely proportional to the square of the distance between them. The form of this relationship, later named Coulomb's law, looks remarkably like Newton's law of universal gravitation, a fact that has prompted scientists to look for connection between the two rules ever since.|| Coulomb's Law
Unit of charge
The SI unit of charge is the Coulomb, named after you-know-who, and represents a number of charges, specifically
1C = 6.25 x 10^18 charges
Ten to the eighteenth power seems like a big number and it is compared to most things that we count. But let's put it in context with something like Avogadro's number, about 10^23. Let's say we have a mole of copper (about 63 g) and we wish to give this object a residual charge of one Coulomb of positive charge. To do this, we must remove 10^18 electrons. A mole of atoms could be divided into 10^18 piles of 10^5 atoms. We get a Coulomb by removing 1 electron from each pile of 100,000.
|Let us consider two charges Q1 = +6 x 10^-6 C and Q2 = 5 x 10^-6 C separated by a distance of .4 m Determine the magnitude and direction of the force acting on each charge|
|Because both charges have the same sign, the forces are repelling and F21 points left. The is no easy way to generate a negative sign show this. The safest strategy is to report that F21 = 1.69 N points left|
1. Solve force problems for the absolute value of F. Do not install + and -.
2. If more than one charge acts on a given charge, apply Coulomb's law, pairing the charge in question with each of the others, and add the forces vectorially
|To explain the action-at-a-distance, we develop the idea of the electric field, a region of interaction around any charged object that starts at the charge and radiates (outward for positive charges, inward for negatives) in all directions to infinity. Thus any charge falling in this space (which is to say every charge in the universe) is either attracted to or repelled by the charge causing the field. If we bring a test charge into a space where a field exists, the charge will feel a force which could be large or small. Let's say that the force is large; is the large magnitude due to the field being large or that the test charge is large. To determine field strength, we define a quantity called Electric Field intensity E|
| Let us draw an
analogy to gravitational fields. Pretend that we are visiting the
planet Glork and we attempt to move a small box. It turns
out the box is very heavy. Because w = mg still works
on Glork, we can conclude that either a) the box is very massive; or b)
the gravitational field on Glork is very strong
If we define gravitational field initensity (GFI) to be force/ testmass we get the familiar g. Thus E is to electric fields as g is to gravitional fields. It is important to note that the gravitational field is always ppresent everywhere around the mass that produced it. So, too, is the electric field around a point charge..
point charges located at the corners of a square (side = a meters); the
fourth corner is vacant. Each charge creates a electric field
around itsef extending from the charge to infiinity in all directions
and necessarily has a piece of the action when it comes
See other supporting text at
Try these applets
You will explore these ideas more thoroughly by visiting the web sites listed below.
sites are links to an entire course in E & M
An exciting yet safe encounter with lightning
can be found at Boston's Museum of Science.
Visit the Theatre of Electricity at http://www.mos.org/sln/toe/toe.html
TO ELECTROSTATIC PROBLEMS 1
Here are some useful sites for investigating practical applications related to electric fields.
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