1) Two parallel long wires carry the same current and repel each other with a force F.per unit length. If both these currents are doubled and wire separation tripled, the force per unit length becomes:

A. 2F/3
B. 2F/4
C. 2F/9
D. 4F/9
E. 6F

explain your answer.

2) Solenoids are used to generate uniform magnetic fields. There is a relation B=m0nI. Explain why the magnetic field B inside the solenoid is proportional to n?

3) In Ampere's law, the integration must be over:

A. Any surface
B. Any closed surface
C. Any path
D. Any closed path
E. any closed path that surrounds all the current producing the magnetic field.