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.

 The study of electrostatics became quite popular among natural philosophers in the seventeenth and eighteenth centuries. Prominent among these was Benjamin Franklin who, when not working as a founding father or as an ambassador to France, found time to do elementary research into the nature of electricity. He had some successes, showing that lightning was in fact an electrostatic phenomenon and inventing the lightning rod to protect buildings. And like any other thinker who dares to speculate on what might be true, he held some beliefs that were later shown to be less than correct. For instance, he believed that electric charge was a single "fluid". An object was positively charged if it had an excess of fluid, negatively charged if it had a deficiency. (This scheme is not unlike our modern view of heat where an object is hot if it contains lots of heat energy, cold if it does not.)

For a look at Benjamin Franklin the scientist, go to

The modern view of charge can be summarized by four simple statements.

1. There are two kinds of charge.

2. Like charges repel.

3. Unlike charges attract.

4. Charge is quantized.

The fourth statement, that charge exists in discreet lumps, is a twentieth century notion that we will revisit when we get to modern physics.

   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.

Sample Problem - Coulomb's Law
 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  
 Problem-solving strategies
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

why grounding

 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..


Consider three 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

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You will explore these ideas more thoroughly by visiting the web sites listed below.


 These sites are links to an entire course in E & M
http://library.advanced.org/16600/intermediate/static electricity.shtml




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


Here are some useful sites for investigating practical applications related to electric fields.



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