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For help with this problem, see Example 20-4 in our text. Two charges, +q1 = q = 4.9 %u03BCC and q2 = +3q are held in place at locations d and -d respectivly as shown above. d=7 cm. A third charge q3 =+4q is held at rest at a point A on the y axis at y=d.

1) What is the electrical potential at point A due to q1?

V1 =

2)What is the electrical potential at point A due to q2?

V2 =

3)What is the total electrical potential at point A due to both charges?

VTotal =

4) What is the total potential energy of q3 when it is located at point A?

U3 =

5)Now lets assume q3 is released from point A, although q1 and q2 remain fixed in place. q3 is accelerated away from the other two charges due to the Coulomb force. What is the kinetic energy of q3 when it is a large distance (i.e. infinitely far) away from the other two charges?

KE3 =

Explanation / Answer

1) V = Kq1/sqrt2*d = 445477.27 V/

2) V' =3V = 1336431.81 V/

3) V'' = 4V = 1781909 V

4) PE = q3*V'' = 34.92 J

5) KE at inf. = PE at this point using energy con. = 34.92 J

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