Magnetic Field of Straight Wires
Two identical long current carrying wires are bent into the shapes shown in the following figures. If the magnitude of magnetic fields at the centres P and Q of a semicircular arc are and respectively, then the ratio is ______.

Show answer and solution
Answer: (a)
- (I) Two semi-infinite wires, each , plus semicircle , all into the page: .
- (II) The vertical wire's line passes through Q, so it gives zero. Top wire plus semicircle : .
- So the answer is (a).