1.) A pith ball with charge +1.5nC is fixed at the origin. A second +1.5nC pith

Question

1.) A pith ball with charge +1.5nC is fixed at the origin. A second +1.5nC pith ball floats vertically above the first. Modelling both pith balls as point charges, compute the centerto-center separation of the two pith balls. The mass of a pith ball is 6.0 × 105kg. Note, the distance calculated may or may not be larger than the radius of the pith ball.

(a) 5.9 × 103m (b) 3.4 × 105m (c) 6.3 × 101m (d) 1.3m (e) 150m

2.)Lightning strikes the power lines, briefly transferring a linear charge density of 1.0 × 103 C m . Model the power lines as a single infinite straight uniform line charge. Calculate the magnitude of the electric field at the ground a distance of 5.0m from the wire.

(a) 3.6 × 105 N/C (b) 1.8 × 106 N/C

(c) 3.6 × 106 N/C (d) 7.2 × 105 N/C

(e) 3.5 × 108 N/C

Explanation / Answer

1) pith ball will float means net force in vertical direction is zero.

Gravity force - electric force = 0

Mg - kqq/d^2 = 0

6 x 10^-5 x 9.81 - [ 9 x 10^9 x 1.5 x 10^-9 x 1.5 x 10^-9 / d^2 ] = 0

20.25 x 10^-9 / d^2 = 58.86 x 10^-5

d = 5.87 x 10^-3 m or 5.9 x 10^-3

Ans ( a)

2) Using Guass law ,

E.A = qin / e0

E ( 2pi * r *L) = (1 x 10^-3 x L) / e0

E =10^-3 / (2pi*r*e0)

Where r = 5m

e0 = 8.854 x 10^-12

E = 3.6 x 10^6 N/C

Ans ( c)

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