A 0.00260-kilogram air inflated balloon is given an excess negative charge, q1 =

Question

A 0.00260-kilogram air inflated balloon is given an excess negative charge, q1 = -4.00 × 10-8 C, by rubbing it with a blanket. It is found that a charged rod can be held above the balloon at a distance of d = 0.0800 m to make the balloon float. In order for this to occur, what polarity of charge must the rod possess? How much charge, q2, does the rod have? Assume the balloon and rod to be point charges. The Coulomb force constant is 1/(4 0) = 8.99 × 109 N·m2/C2 and the acceleration due to gravity g = 9.81 m/s2.

Explanation / Answer

weight acts downward and electrostatic force must act upward because net force must be zero

because rod is above ballon and force should be upward the rod should attract the ballon and so it should have positive polarity because ballon has negative polarity(Unlike charges attract)   

Weight must be balanced by electrostatic force

F = mg

8.99*109 *(-4*10-8)*q /(0.08)2 = 0.0026 *9.81

q= 7.25 *10-6

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