كلية الهندسة Faculty of

Engineering

Physics II

(MP108)

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Solved Examples

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Question

?

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A charged comb often attracts small

bits of dry paper that then fly away

when they touch the comb. Explain.

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When the comb is

nearby, molecules in

the paper are

polarized and the

paper is attracted.

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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.

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Question

?

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Where is the location

of +Q to be in equilibrium?

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Where is the location

of +Q to be in equilibrium?

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Where is the location

of +Q to be in equilibrium?

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Where is the location

of +Q to be in equilibrium?

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Where is the location

of +Q to be in equilibrium?

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Coulomb’s Law

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Problem No. 1

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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.

a) Find the F exerted by

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

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b) The spheres are

connected by a

conducting wire.

Redistribution of

charges will be

occurred.

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After spheres have

come to equilibrium.

Each sphere has a

charge of

Repulsion Force

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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

Final Case

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By induction

There are +ve charges on

near and –ve charges far sides of the sphere

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

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There is touch between

sphere and rod

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.

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Problem No. 3

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Three point charges

are located at the

corners of an equilateral

triangle as shown in the

figure.

Calculate the total F

on the 2μ C charge.

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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.

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.

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With all my best wishes for

You

Dr.Mayssa Amin

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