Objectives Charging by Conduction Charging by Induction Electroscopes Coulomb’s Law
Charging by Conduction When a charged metallic object is brought in contact with a neutral metallic object, it acquires some of the charge.
Charging by Induction If a negatively charged object is brought close to a neutral metal rod that is grounded on one end, the positive charges in the neutral rod will be attracted to the negative charges on the object The negative charges on the neutral object will be attracted to ground
Nonconductors won’t become charged by conduction or induction, but will experience charge separation Polarization of Charge in an Insulator
The electroscope is a device used for detecting electric charge. The Electroscope
The electroscope can be charged either by Induction or conduction, respectively. Charging of the Electroscope In either case, the leaves will repel each other
The charged electroscope can then be used to determine the sign of an unknown charge. The greater amount of charge deposited on the leaves, the greater the separation
Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them. Coulomb’s Law
Coulomb’s Law: This equation gives the magnitude of the electrostatic force between two point charges. Coulomb’s Law
The force acts along the radial line connecting the charges Unlike charges attract each other Like charges repel each other
Unit of charge: coulomb, C. The proportionality constant in Coulomb’s law is then: k = 8.998 x 10 9 N·m 2 /C 2. Charges produced by rubbing are typically around a micro-Coulomb: 1 μC = 10 -6 C. Coulomb’s Law
Charge on the electron: e = 1.602 x 10 -19 C. Electric charge is quantized in units of the electron charge. Coulomb’s Law Q = n · e
The Coulombic constant k, can also be written in terms of ε 0, the permittivity of free space: Coulomb’s Law
Conceptual Example 21-1: Which charge exerts the greater force? Two positive point charges, Q 1 = 50 μC and Q 2 = 1 μC, are separated by a distance. Which is larger in magnitude, the force that Q 1 exerts on Q 2 or the force that Q 2 exerts on Q 1 ?
Coulomb’s Law Example 21-2: Three charges in a line. Three charged particles are arranged in a line, as shown. Calculate the net electrostatic force on particle 3 (the -4.0 μC on the right) due to the other two charges.
Coulomb’s Law Example 21-3: Electric force using vector components. Calculate the net electrostatic force on charge Q 3 shown in the figure due to the charges Q 1 and Q 2.