Calculus Practice Test 2026 | Exam Prep Questions And Answers

If the loop radius R is doubled while keeping current I fixed, how does the magnetic field at the center change?

Doubles Halves Becomes quarter Remains the same

Correct Answer:

Halves

Explanation:

At the center of a circular loop, the magnetic field from a current I is B = μ0 I / (2R). This shows the field is inversely proportional to the radius for a fixed current. If the radius is doubled, the denominator doubles, so the field becomes half of its original value. The intuition is that spreading the same current over a larger loop weakens the magnetic influence at the center. The magnitude is halved (the direction would still be along the axis given by the right-hand rule, if you care about orientation).

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