1. A point charge q 1 = 4.05 nC is placed at the origin, and a second point char

Question

1. A point charge q1 = 4.05 nC is placed at the origin, and a second point charge q2 = -3.05 nC is placed on the x-axis at x=+ 20.0 cm . A third point charge q3 = 2.10 nC is to be placed on the x-axis between q1 and q2. (Take as zero the potential energy of the three charges when they are infinitely far apart.)

A) What is the potential energy of the system of the three charges if q3 is placed at x=+ 10.0 cm ?

B) Where should q3 be placed between q1 and q2 to make the potential energy of the system equal to zero?

2. A point charge has a charge of 2.80×1011 C . Take the potential to be zero at an infinite distance from the charge.

A) At what distance from the point charge is the electric potential 85.0 V ?

B)At what distance from the point charge is the electric potential 35.0 V ?

3. A very long insulating cylinder of charge of radius 2.50 cm carries a uniform linear density of 16.0 nC/m .

A) If you put one probe of a voltmeter at the surface, how far from the surface must the other probe be placed so that the voltmeter reads 200 V ?

Explanation / Answer

2 A)

Q = charge = 2.80 x 10-11 C

V = electric potential = 85 Volts

r = distance from the charge

Electric potential is given as

V = k Q/r

85 = (9 x 109) (2.80 x 10-11)/r

r = 0.00296 m

b)

Q = charge = 2.80 x 10-11 C

V = electric potential = 85 Volts

r = distance from the charge

Electric potential is given as

V = k Q/r

35 = (9 x 109) (2.80 x 10-11)/r

r = 0.0072 m

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