Question

Two point charges, Q1 = -30 ?C and Q2 = 56 ?C , are separated by a distance of 12 cm. The electric field at the point P (see the figure(Figure 1)) is zero. How far from Q1 is P?

Answer 1

Suppose at distance 'x' from charge Q1 net electric field is zero, then

Electric field is given by:

E = kQ/R^2

Since we know that direction of electric field due to +ve charge will be away from charge and due to -ve charge will be towards the charge. In given case since both charge are opposite, So Electric field will be zero when

Enet = E1 - E2 = 0

E1 = E2

kq1/r1^2 = kq2/r2^2

q1 = -30 uC

q2 = 56 uC

r1 = x

r2 = 12 + x

Using above values:

30/x^2 = 56/(12 + x)^2

(12 + x)/x = sqrt (56/30) = 1.366

12 + x = 1.366*x

0.366x = 12

x = 12/0.366 = 32.79 cm

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