Physics
Physics
Problem
A vertical straight wire carries a current of 3 A downward. Three points P, Q and R lie on a horizontal line through the wire at perpendicular distances 2 cm, 4 cm and 8 cm respectively from the wire. a. (i) Taking the magnetic field strength at P as , express the field strengths and in terms of . b. (ii) Arrange the three points in order of decreasing magnetic field strength and justify your answer. c. (iii) If the current in the wire is doubled while the positions of P, Q and R remain unchanged, by what factor does the magnetic field strength at Q change?Intuitive explanation
The magnetic field around a current-carrying wire forms concentric circles, and its strength depends only on your perpendicular distance from the wire—closer points experience stronger fields. Here, you're comparing the field at three different distances, so use the inverse relationship to see how the field weakens as you move away. The direction at each point follows the right-hand rule: curl your fingers around the wire in the direction of the field circulation, with your thumb pointing downward along the current.
Intuitive explanation
The magnetic field around a current-carrying wire forms concentric circles, and its strength depends only on your perpendicular distance from the wire—closer points experience stronger fields. Here, you're comparing the field at three different distances, so use the inverse relationship to see how the field weakens as you move away. The direction at each point follows the right-hand rule: curl your fingers around the wire in the direction of the field circulation, with your thumb pointing downward along the current.