Answer: mu 0 I/(2 R).
- A mu<sub>0</sub> I/(4 pi R)
- B mu<sub>0</sub> I/(2 R)
- C mu<sub>0</sub> I/(R)
- D 2 mu<sub>0</sub> I/R
Correct answer: B. mu<sub>0</sub> I/(2 R)
Explanation: B at center of circular loop = mu<sub>0</sub> I/(2R). This comes from Biot-Savart law integration around the full circle.
The magnetic field around a long straight current-carrying wire forms concentric circles, with direction given by the right-hand rule (point thumb along I, fingers curl in the direction of B).
Concept context
Magnetic force on moving charges and currents, the Biot-Savart law, Ampere's law, and the cyclotron - how electric currents create and respond to magnetic fields.