The magnetic field around a long straight current carrying wire consists of

  • A. straight lines parallel to the wire
  • B. concentric circles centred on the wire
  • C. lines radiating outwards from the wire
  • D. no field at all

Explanation

The right hand grip rule gives the sense of the circles: point the thumb along the conventional current and the curled fingers show the field direction. The strength falls off as one over the distance from the wire, not as one over the distance squared. Inside a long solenoid, by contrast, the field is nearly uniform and parallel to the axis.

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