Website Notes: Fields (Part 2)  Electric Fields. INTRODUCTION: In ancient Greece, amber became widely valued around 1600 BC. Greeks were fascinated by it. The ancient Greek word for amber is "elektron", meaning  originating from the Sun. The Greeks were also the first to describe the electrostatic properties of amber. Ancient Romans loved amber as well. From the writings of Thales of Miletus it appears that Westerners knew as long ago as 600 BC that amber becomes charged by rubbing. There was little real progress until the English scientist William Gilbert in 1600 described the electrification of many substances and coined the term electricity from the Greek word for amber. As a result, Gilbert is called the father of modern electricity. One of nature's most spectacular display of electricity is the lightning observed during a thunder storm. Benjamin Franklin (17061790) found that electricity originates from charges, positive or negative. We know now that all material bodies possess electric charges. Electrons carry negative charges while protons carry positive charges in the nucleus of an atom. 1. The electric force that stationary objects exert on each other is called the electrostatic force. This force depends upon the distance between the two point charges and the amount of charge on each. Experiments have demonstrated that the greater the charge and the closer they are to each other, the greater the force. 2. If charges have unlike signs, each charge is attracted to one other, whereas like charges repel each other. These attractive forces and repulsive forces act along the line between the charges, and are equal in magnitude but opposite in direction (in accordance with Newton's 3rd law). 3. The French physicist CharlesAugustin Coulomb (17361806) experimented with electric force between two point charges (the unit of charge is the Coulomb, C). His work resulted in a law. Coulombs Law is defined: The magnitude of the electrostatic force (F), exerted by one point charge on another point charge is directly proportional to the magnitudes of the two point charges, and inversely proportional to the square of the distance (r) between the charges. 4. For a pair of charges q_{1} and q_{2}, separated by a distance r, Coulomb's Law may be stated as follows: F = k(q_{1}q_{2}/r^{2}). 5. The constant of proportionality, k = 8.99x10^{9} Nm^{2}/C^{2}. Such a force is transmitted by the presence of an electric field. The electric field E due to a point charge q is, E = F/q , or E = k(Q/r^{2}). 6. Electric force and electric field are vectors. Hence, they have magnitudes and directions. The electric force F and electric field E are related as follows: F=qE, where the force is on charge q due to the presence of an electric field at the position of q. 7. When an electric field is confined between two parallel metal plates, the field is given by E = σ/ε_{o}, with σ, being the surface charge density, and ε_{o} is the Permittivity of Free Space, or ε_{o} = 8.85x10^{12} C^{2}/Nm^{2}. 8. The Principle of superposition also applies to the electric fields produced by multiple charges. That is, the net electric field at a point due to several charges is the vector sum of the electric fields due to individual charges. 9. For example, when more than two charges are present, the net force on any one charge is equal to the vector sum of each of the forces produced by other charges. 10. In other words, the force on charge q_{1} due to the presence of charges q_{2} and q_{3}, is the superposition of the forces exerted by q_{2} and q_{3}. That is, the net force F on charge q_{1} is, F_{net} = F_{12} + F_{13}. where, F_{12} is the force on q_{1} due to the presence of charge q_{2} and F_{13} is the force on q_{1} due to charge q_{3}. 11. While solving a problem, it is useful to rewrite equation given above in its component form as follows: F_{x} = (F_{12})_{x} + (F_{13})_{x} F_{y} = (F_{12})_{y} + (F_{13})_{y} F_{net} = [(F_{x})^{2} + (F_{y})^{2}]^{1/2} θ = tan^{1} [F_{y}/F_{x}] 12. And still, we need these steps to solve any problem in Physics: (i) read the problem and identify the given variables (ii) determine what you are asked to solve for (iii) find the correct equation to use (iv) use Algebra, Trigonometry, and/or Calculus to isolate the unknown (v) substitutein the given information and simplify. View the PowerPoint™: Click HERE.

