Circuit and electromagnetism basics
When a direct current flows in a long straight wire, which correctly describes the shape and direction of the magnetic field around it?
- AConcentric circles around the wire, in the direction a right-handed screw turns when advancing in the direction of the current
- BParallel to the wire, in the same direction as the current
- CRadially outward from the wire
Show answer and explanation
Answer: A. Concentric circles around the wire, in the direction a right-handed screw turns when advancing in the direction of the current
The magnetic field around a straight current forms concentric circles centered on the wire, circulating in the direction a right-handed screw turns as it advances along the current (the right-hand screw rule). Equivalently, point your right thumb along the current and your fingers curl in the field direction. The flux density is B = μ0I/(2πr), inversely proportional to the distance r from the wire. A radial pattern is the shape of the electric field around a point charge.
- A. Correct. The field forms concentric circles given by the right-hand screw rule.
- B. The field around a straight current is not parallel to the current.
- C. A radial pattern describes the electric field of a point charge, not the magnetic field of a current.
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