phys chapter 19 notes

# phys chapter 19 notes - Chapter-19 You are expected to...

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Chapter-19 Coulomb’s Law You are expected to: 1. Use Coulomb’s law to calculate the electric forces between charges. 2. Calculate the E -field due to a discrete charge distribution and simple continuous charge distributions. 3. Know the distinction between the electric force F (see eqs.[1], [2] below) and the E -field 4. Use Gauss’ law to calculate the E -field for symmetric continuous charge distributions. 5. Solve problems that contain concepts from this chapter + some simple concepts from PHYS-101. 1. Coulomb’s Law According to Coulomb’s Law, the magnitude of the electrostatic force between two charged particles with charges Q 1 and Q 2 and separated by a distance r is given by: 12 2 e QQ Fk r = …[1] Where k = 9x10 9 N.m 2 /C 2 is the Coulomb constant. The unit of charge is taken as a Coulomb (C). The constant k is also written as k = 1/4 πε o where the constant ε o is called the permittivity of free space o = 8.854x10 -12 C 2 / N.m 2 . Since force is a vector quantity, in the vector form Coulomb’s law is expressed as: 12 12 2 ˆ F k r = G r …[2] r + + ^ r 12 Q 1 Q 2 F 21 F 12 Where is a unit vector directed from Q 12 ˆ r 1 to Q 2 . The electric charge is quantized in units of 1.6*10 -19 C ( magnitude of the charge of an electron).

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Problem Solving: 1.1 Finding force on a given charge due to a discrete charge distribution. Break up the problem in three parts: [1] Direction :draw the force vectors at a given charge location due to the other given charges. (Remember: like charges repel, unlike charges attract.) A well-labeled diagram will be very helpful. [2] Magnitude : find the magnitude of various force vectors using eq.[1] [3] after step[2] treat the problem as a vector manipulation problem. The net force F on a given charge due to a discrete charge distribution is simply the vector sum of the forces produced by individual charges in the charge distribution at the location of the charge of your interest. Study problem 2 below to clarify the points mentioned above. 2. The E-field The magnitude of the E-field generated by a charge Q is given by E = kQ/r 2 . The direction of E at a given location is the direction in which a force would be exerted on a unit positive test charge placed at that location. The E -field due to a discrete charge distribution is simply the vector sum of the E -fields produced by individual charges in the charge distribution.
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phys chapter 19 notes - Chapter-19 You are expected to...

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