(a) If a +2.5 uC charge is placed at (-3.0 cm, 0) and another + 2.5 uC charge is

Question

(a) If a +2.5 uC charge is placed at (-3.0 cm, 0) and another + 2.5 uC charge is placed at (+3.0 cm, 0) then what is the stored electric potential energy of these two charges?

(b) if a third + 2.5 uC charge is placed at (0, +3.0 cm), then what is the stored electric potential energy of these three charges?

(c) if the third charge is released from the rest from the position (0, +3.0 cm), then what will its change in kinetic energy be?

(d) By using conservation of energy what will its change in kinetic energy be?

(e) given that its mass is 1.5x10^-15 kg, what wil its speed be when it is significantly far away from the other two charges?

Explanation / Answer

a) r12 = 6cm

U12 = kQ1Q2 / r12 = 9 *10^9 * 2.5*10^(-6) * 2.5 * 10^(-6) / 0.06   

= 937.5*10^(-3) = 0. 9375 J

b)

U = U12 + U13+U23 = 3 * U12 = 3 * 0.9375 = 2.8125 J

c)

from the energy conservation when the third charge at far away from Q1 and Q2

U = U12+U13+U23 = KE3 + U12

2 * U12 = KE3

Ke3 = 2* 0.9375 = 1.875 J

e)

Ke3 = 1.875 = m3 v^2/2 = 1.5*10^(-15) * v^2 /2

v = 5*10^(-8) m/s

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