Lecture Notes (Electric Fields)

Transcription

1 Lecture Notes (Electric Fields) Intro: - the gravitational force and the electrostatic force are both capable of acting through space, producing an effect even when there isn t any physical contact between the objects involved - field forces can be discussed in a variety of ways, but an approach developed by Michael Faraday ( ) is the most practical - in this approach, an electric field is said to exist in the region of space around a charged object; the electric field exerts an electric force on any other charged object within the field. Electric Field: - an electric field cannot exert a force on the charge which generates it, in just the same way as you cannot pick yourself up with your own shoelaces

2 - electric fields are real; there is an energy associated with an electric field which fills space; it is actually possible to covert this energy into heat or work, and vice versa - an electric field is a vector quantity; it has both magnitude and direction; the strength is the force per unit charge - if a body with charge, q, experiences a force, F, at some point in space, then the electric field, E, at that point is: E F q - the units of electric field strength are N/C (newtons per coulomb) - the electric field is depicted with vector arrows; the direction of the field is shown by the vectors and is defined to be the direction in which a small, positive test charge at rest would be moved - the test charge is required to be very small, so it doesn t cause any significant rearrangement of the charge creating the electric field - the direction of the force and that of the electric field at any point are the same

3 - if we start with an example of a negatively charged ball, all the vectors would point to the center of the ball - any charge placed in an electric field experiences a force on it resulting from the electric field at that point Picturing the Electric Field: - a more useful way to describe an electric field is with electrical lines of force (electric field lines) - the electric field lines represent a small number of the infinitely numerous possible lines that indicate direction of field - the electric field is tangent to the field lines at all points

4 - for a single point charge the lines extend to infinity; they point away from a positive charge and toward the negative point charge - for two or more opposite charges, we represent the lines as emanating from the positive charge and terminating on the negative charge

5 - the electric field concept helps us not only understand the forces between isolated, stationary bodies, but also what happens when charges move - the electric field is a storehouse of energy which may be transported over long distances; this energy may be directed through metal wires or teamed up with a magnetic field to move through empty space Van de Graaf Machine: - in 1929, Robert J. Van de Graaf designed and built an electrostatic generator that bears his name - everyday objects are made up of equal amounts of positive electric charge and negative electric charge in almost perfect balance; we might say that ordinary matter is made up of cancelled-out electricity - a VDG machine takes the mixed-up positive and negative charges of matter, sorts them out, then pulls them far apart from each other - a VDG is a charge uncanceller or charge separator; the machine moves charge continuously, so it also acts as a mechanical charge pump - the machine operates by use of a motor driven pulley

6 - the needles attract negative charges from the belt, causing one side of the belt to be positively charged - the charged belt attracts electrons from a second comb of needles found at the top of the device; this enhances the positive charge of the metal dome - a discharge occurs when the excess positive charge builds up on the dome; this is due to a strong electric field - when this field builds up enough strength, it will ionize the air around the dome; charges on the dome now have a way to leak off into the air which will produce lightning discharges - VDG machines were originally used as power supplies for the early particle accelerators used in research into radioactivity - big VDGs are also used to power high energy X-ray machines; if you want to treat cancer with radiation, make X-ray photos of locomotive engines, or sterilize food with gamma rays, you'll want to buy a Van de Graaff powered X-ray generator