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Exam Questions Papers - Engineering

Q1:

Find the electric charge required on the earth and moon to balance their gravitational attraction if the charge on the earth is 10 times that on the moon.

A 1.73 TC

B 17.3 GC

C 17.3 TC

D 1.73 GC

ANS:C - 17.3 TC

Using law of gravitations, the force between the two point masses is F = M1 = mass of moon = 6.7 x 1022 Kg M2 = mass of earth = 6 x 1024 Kg r = distance between masses = 380 mm G = universal gravitational constant = 6.7 x 10-11 Nm2/Kg2 Let Q1 and Q2 be the charge on moon and earth respectively. Now the gravitational force must be balanced by the force of repulsion Q1Q2 = 4pε0M1M2G but Q2 = 10Q1 = 10 x 4pω0M1M2G ∴ Q2 = 173 x 1012 Q1 = 17.3 x 1012 C Q1 = 17.3 TC.