Magnetic Field Of A Straight Wire
A very long straight wire carries a current of 10 A; determine the magnitude of the magnetic field it produces at a perpendicular distance of 5 cm from the wire.
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About This Question
- Subject
- physics
- Chapter
- magnetic effects of current and magnetism
- Topic
- magnetic field of a straight wire
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
4×10−5 T
NCERT Class 12, Chapter 4 (Moving Charges and Magnetism) applies Ampere's circuital law to a long straight conductor to obtain the field at distance r: B=2πrμ0I. The field lines form concentric circles around the wire, their direction given by the right-hand thumb rule. Substituting μ0=4π×10−7 T·m/A, I=10 A, r=0.05 m: B=2π(0.05)(4π×10−7)(10)=0.05(2×10−7)(10)=4×10−5 T. The value 2×10−5 T doubles the distance mistakenly. The value 8×10−5 T halves the distance. The value 4×10−6 T is ten times too small from a unit conversion slip. Plausibility check: the field weakens with distance as 1/r, and a value of order 10−5 T a few centimetres from a wire carrying 10 A is physically reasonable, comparable to Earth's magnetic field.
This easy difficulty physics question is from the chapter magnetic effects of current and magnetism, covering the topic of magnetic field of a straight wire. It appeared in the 2025 exam.
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