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Moving Charges And Magnetism

Question

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An electric current passes through a long straight copper wire. At a distance 5 cm from the straight wire, the magnetic field is B. The magnetic field at 20 cm from the straight wire would be

$\frac{B}{6}$

$\frac{B}{4}$

$\frac{B}{3}$

$\frac{B}{2}$

Solution

B.

$\frac{B}{4}$

The magnetic field at a distance r from the straight wire

B = $\frac{μ_{o} i}{2 πr}$

or B ∝ $\frac{1}{r}$

or $\frac{B_{2}}{B_{1}} = \frac{r_{1}}{r_{2}}$

∴ $\frac{B_{2}}{B} = \frac{5}{20}$

= $\frac{1}{4}$

⇒ $B_{2} = \frac{B}{4}$

Some More Questions From Moving Charges And Magnetism Chapter

Two moving coil meters, M₁ and M₂ have the following particulars:
R₁ = 10 Ω N₁ = 30,
A₁ = 3.6 x 10^–3 m², B₁ = 0.25 T
R₂ = 14 Ω; N₂ = 42,
A₂ = 1.8 x 10^–3 m², B₂ = 0.50 T
(The spring constants k are identical for the two meters).
Determine the ratio of (a) current sensitivity and (b) voltage sensitivity of M₂ and M_1.

Answer the following questions:
A charged particle enters an environment of a strong and non-uniform magnetic field varying from point to point both in magnitude and direction, and comes out of it following a complicated trajectory. Would its final speed equal the initial speed if it suffered no collisions with the environment?

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