Coulomb’s Law
Methods to electrically charge an object Conduction: Direct contact: will transfer electrons, such as touching your car door in the winter Friction: rubbing your feet against carpet, hair against a balloon
Induction: no direct contact Start with a neutral object. Then, bring an electrically charged object near, but not in contact with, a neutral object The charges in the neutral object will be “induced” to separate to get closer or farther from the charged object. If provided a pathway, the separated electrons will leave. The object is now positively charged.
Electrostatics devices Electroscope: the separation of metal leaves indicates the presence of static charge Van de Graaff generator: charge is delivered by a rubber belt to a metal dome Electrophorus a device used to transfer electric charge
Coulomb’s Law Calculates the magnitude of the electric force between two charges Each charge experiences equal but opposite forces k is a constant, k = 9 x 109 N·m2/C2 (Since we are interested in the MAGNITUDE of the force, do not include the signs of negative charges)
Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation Differences: Gravitational Force is based on MASS. Coulomb’s law is based on CHARGE. Gravity is ALWAYS an attractive force. The Electric Force can attract and repel. “G” is a tiny number, therefore gravity force is a relatively small force. “k” is a huge number, therefore electric force is a relatively large force.
Both laws are INVERSE SQUARE LAWS “The Force varies with the inverse of the distance squared.” At twice the distance, d2 = 22 in denominator = ¼ the Force, At three times the distance, 32 in denominator, = 1/9 the Force At half the distance, (1/2)2 in denominator = 4 times the Force Now if one CHARGE, q, doubles…. The Force doubles since they are directly related.
Get a calculator and let’s practice one… What is the magnitude of the electrostatic force between two charges, q1 = 3.2 mC and q2 = -24.8 mC separated by a distance of 2.4 mm? (milli = 10-3, micro = 10-6) USE THE EXPONENT BUTTON!!! F = 9E9 * 3.2E-3 * 24.8E-6 ÷ 2.4E-3 2 F = 124000000 N or, for Quest: 1.24E8 or 1.24e8
Remember…. Force is a VECTOR- it always points in a specific direction Remember….Force is a VECTOR- it always points in a specific direction! If more than two charges are present, we must find the VECTOR sum of the forces acting on an individual charge. + -
Electric Charge is measured in Amps Volts Coulombs Farads Get a clicker and calculator. Log in and get ready to answer some questions. Electric Charge is measured in Amps Volts Coulombs Farads
Opposite charges Attract Repel May attract or repel, depending on their relative size. Neither attract nor repel.
Objects with a neutral charge are Attracted only by objects with a net positive charge. Attracted only by objects with a net negative charge. Not attracted by any kind of charge. Attracted by objects with either positive or negative charge.
The charge of one electron = - 1.6 x 10-19 C 4. How much excess charge is present in an object that has 3 x 1018 more electrons than protons? (Use correct sign!)
If the distance between two charges tripled, the magnitude of the electrostatic force between them would change by a factor of 3 6 9 1/3 1/6 1/9
6. Robert Millikan measured the elementary charge of an electron by charging and then suspending oil drops using a charged plate as shown: At the most basic level, which law of physics did Millikan apply to measure the charge of an electron? A. Faraday’s Law of mutual inductance B. Coulomb’s Law C. The law of conservation of Energy D. Newtons 1st Law E. Zeroth Law - - - - - - - - - - - - -
7. The development of _______ at room temerature could revolutionize the electric industry? A. Conductors B. Insulators C. Semiconductors D. Superconductors
8. The force between a 150 kg dog and the Earth is 1470 N 8. The force between a 150 kg dog and the Earth is 1470 N. How close must an electron and a proton be to generate the same force? A. 9.90 X 10-7 m B. 9.80 X 10-13 m C. 3.96 X 10-16 m D. 1.57 X 10-31 m