What is the magnetic field strength at distance r = 0.05 m from a long straight wire carrying a current I = 10 A in vacuum (mu_0 = 4pi 10^-7 T m/A)?
Answer and explanation
Correct answer: D. 4.0 * 10^-5 T
- Step 1: Ampere's law for a long straight wire: B = mu_0 I / (2 pi * r).
- Step 2: Substitute values: B = (4pi 10^-7 10) / (2 pi * 0.05).
- Step 3: Cancel 2pi: B = 2 10^-6 / 0.05 = 40 10^-6 T = 4.0 10^-5 T.
Answer
4.0 * 10^-5 T.
Why not the others: 2.0 10^-5 T uses r = 0.10 m; 8.0 10^-5 T doubles the field; 1.26 10^-4 T forgets to divide by 2pi.
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