Physics 2 Question

Question

The players of electrodisc are playing the popular three disc variant of electrodisc where each disc has the following charges: q_a=-5q, q_b=4q, q_c=0. In the first quarter of the game the opposing team is able to touch disc A and B together then separate. Then the home team scores by toucing disc C to disc A and then separates them. The opposing team scores again by toucing disc C to B and then separates them. give your anwsers in terms of q. “a”. 

a) what is the final charge of C? 

b)before the objects touch, what is their total change? 

c)After the objects touch, what is their total energy? 

 

 

 

Answer 1

Initial conditions:

 

q_a = -5q

q_b = 4q

q_c = 0

 

A and B touch and then separate.  Assuming even distribution of charge after touching, 

 

q_a' + q_b' = -5q + 4q = -q

q_a' = -q/2

q_b' = -q/2

 

C and A' touch and then separate.

 

q_c' + q_a'' = q_c + q_a' = 0 - q/2 = -q/2

q_c' = -q/4

q_a'' = -q/4

 

C' and B' touch and then separate.

 

q_c'' + q_b'' = q_c' + q_b' = -q/4 - q/2 = -(3/4)q

q_c'' = -(3/8)q

q_b'' = -(3/8)q

 

(a)

 

q_c'' = -(3/8)q

 

(b)

 

I assume you meant to ask "before the objects touch, what is their total CHARGE?"  The problem statement says "CHANGE".

 

q_a + q_b + q_c = (-5 + 4 + 0)q = -q

 

This total amount of charge is constant throughout the problem.

 

(c)

 

Is this part worded correctly?  You asked for total ENERGY after they touch.  It would have to be total electrostatic energy, which depends on the distances between the charges, which are not given in the problem statement.  Perhaps you meant to ask "After the objects touch, what is their total CHARGE?"  The total charge after touching is the total charge before touching, since total charge must be conserved.