Transcript Chapter 33
Electric Fields and Potential • Electric field – a force field that fills the space around every electric charge or charges Electron • Example: Proton • An electric field has both magnitude and direction • Therefore, an electric field can be represented using… VECTORS!!! Force Field Lines • The vectors are drawn by how they will affect a small positive test charge The field of a positive charge near a negative charge Two positive charges Two negative charges So… the strength of the electric field, E, is given by Electric Field = Force ÷ charge E=F÷q For example: A 0.5 C charge experiences a force of 20 N when placed in an electric field. What is the strength of the electric field, E? E=F÷q= 20 N ÷ 0.5 C = 40 N/C The electric field near a charged piece of plastic or styrofoam is around 1000 N/C. The electric field in a television picture tube is around 10,000 N/C. The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C! The further you go from an electric charge, the weaker the field becomes. Electric Shielding There is no way to shield from gravity, but there is a way to shield from an electric field…. Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface) That’s why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering. The covering shields them from all outside electrical activity. “Faraday Cage” Electric Shielding Shielding is important in electronic devices such as televisions and computers Electrons repel toward the outside of any conducting surface Net charge inside is zero Electrons flow outward evenly, but pile up on sharp corners Voltage • Electric potential is commonly called VOLTAGE. • Voltage = Energy / charge or you can rearrange to say Energy = Voltage x charge Remember, one Coulomb is a HUGE amount of energy! • Energy = Voltage x charge • Let’s say you rub a balloon on your head and transfer 3000 V. How much energy is that? Energy = 3000 V x 0.000001 C Energy = 0.003 J That’s not much energy! High Voltage does not necessarily mean that there’s a lot of useful energy or that something is dangerous. When you change the position of a charge in an electric field, the Electric Potential (Voltage), V, changes. The change in Potential (“pressure”), called the “Potential Difference” is given by Electric Field 3 meters DV = Ed For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given by DV = Ed = 4000 N/C x 3 m = 12,000 V • Static devices Electroscope: the separation of metal leaves indicates the presence of static charge Van de Graaff generator: http://www.youtube.com/watch?v=I2G0IdTWGQU&list=PL17 E6343C2B87F2A7 charge is delivered by a rubber belt to a metal dome http://education.jlab.org/frost/live_vdg.html Electrophorus a device used to transfer electric charge Capacitor – a device used to store electrical energy Capacitors are Used in – camera flashes – defibrillators – Computers: tiny capacitors store the 1’s and 0’s for the binary code – Many keyboards have a capacitor beneath each key that records every key stroke. – Virtually every electronic device Vocabulary • • • • • • Capacitor Electric field Electric potential energy Electric potential Volt voltage