Solved Problems- Electrostatics

‫ية الهندسة‬‫ك‬ Faculty of  Engineering  Physics II (MP108) 

Solved Examples

Question

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A charged comb often attracts small bits of dry paper that then fly away when they touch the comb. comb . Explain.

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When the comb is nearby, molecules in the paper are polarized and the paper is attracted.

During contact, charge from the comb is transferred to the paper by conduction.  Then the paper has the same charge as the comb, and is repelled. 

Question

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Where is the location  of +Q to be in equilibrium? 

Where is the location  of +Q to be in equilibrium? 

Where is the location  of +Q to be in equilibrium? 

Where is the location  of +Q to be in equilibrium? 

Where is the location  of +Q to be in equilibrium? 

Coulomb’s Law

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Problem No. 1

Two identical  conducting small  spheres are placed  with their centers  0.3 m apart. One is  given a charge of   12 nC and the other  a charge of  18 nC. 

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a) Find the F exerted by

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one sphere on the other. b) The spheres are connected by a conducting wire. Find the F each exerts on the other after they have come to equilibrium

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F is Attraction Force

b) The spheres are  connected by a  conducting wire. 

Redistribution of   charges will be  occurred . 

After spheres have  come to equilibrium. 

Each sphere has a  charge of  

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Repulsion Force

Question ?

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A light, uncharged metallic sphere suspended from a thread is attracted to a charged rubber rod . After it touches the rod, the sphere is repelled by the rod. Explain.

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First case

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Final Case

By induction

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There are +ve charges on near and – ve charges far sides of the sphere

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Attraction force between sphere & rod is greater than Repulsion force between sphere & rod

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There is touch between sphere and rod

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Finally Repulsion force between sphere & rod

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Problem No. 2

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Four identical point charges particles (q = + 10 μC ) are located on the corners of a rectangle. The dimensions of the rectangle are L= 60 cm and W = 15 cm. Calculate the magnitude and direction of the total F exerted on the charge at the lower left corner by the other three charges.

Problem No. 3

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Three point charges

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are located at the corners of an equilateral triangle as shown in the figure.

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Calculate the total F on the 2μ C charge.

Problem No. 4

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Two known charges, 12 μ C and 45 μ C, and an unknown charge are located on the x axis. The -ve charge is at the origin, and the +ve charge is at x = 15 cm.

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The unknown charge q is to be placed so that each charge is in equilibrium under the action of the electric forces exerted by the other two charges. Find the required location, magnitude, and sign of the q.