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`“Life is like riding a bicycle.  To keep your balance you must keep moving.”–Albert Einstein`

## Saturday, July 5, 2008

### AP Physics B and C- Electrostatics: Free Response Practice Questions on Electric Field and Potential

The following free response practice question which carries 15 points is to be answered in about 17 minutes and is meant for AP Physics B. But it will be useful to AP Physics C aspirants as well:

Two identical thin rings of radius R are arranged coaxially with a separation R. A charge +Q is sprayed uniformly on one ring while a charge –Q is sprayed uniformly on the other.

(a) Calculate the electric potentials at the centres of the rings

(b) What is the electric potential on the common axis, midway between the rings? Justify your answer.

(c) A point charge +q is moved very slowly from the centre of the positively charged ring to the cenre of the negatively charged ring. Calculate the work done by the external agency for moving this charge.

(d) The charge +q and the negatively charged ring are now moved to a very large distance from the positively charged ring. Now, determine the electric field at the cntre of the positively charged ring.

The following free response practice question which carries 15 points is to be answered in about 15 minutes and is meant for AP Physics C aspirants:

Two small identical conducting spheres of radius R carrying charges Q1 and Q2 (Q1 > Q2) attract each other with a force of magnitude F when they are separated by a distance r (between their centres) in air. When the spheres are brought into contact and then separated to the initial distance r, they repel each other with a force of the same magnitude F. Now, answer the following questions:

(a) It is given that the charge Q1 is positive. Calculate the electric potential difference between the centres of the spheres before bringing them into contact.

(b) What are the charges on the two spheres after bringing them into contact? Justify your answer.

(c) Calculate the electric field midway between the spheres after bringing the spheres into contact and separating them to the distance r.

(d) Calculate the ratio of the initial charges (Q1/Q2) on the spheres.

(e) Keeping the separation at r itself, the charges on the spheres are now changed so that one sphere carries positive charge +4q and the other sphere carries negative charge –q. Calculate the distance of the null point (where the electric field is zero) from the centre of the negatively charged sphere.

Try to answer the above questions. I will be back soon with model answers for your benefit.