Answer: B = curl A (del x A).
- A B = curl A (del x A)
- B B = div A in the majority of cases studied
- C B = grad A as widely reported
- D B = A x r in standard practice
Correct answer: A. B = curl A (del x A)
Explanation: Magnetic field B = del x A (curl of the vector potential A). This is a fundamental relation in electromagnetism.
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.