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Class 12 › Physics › Moving Charges and Magnetism
CBSE2020Class Class 12 · Physics2 Marks · Short✅ Verified
A long straight wire carries a current of 2 A. An electron travels with velocity $4 \times 10^4$ m/s parallel to the wire at a distance of 0.1 m, in a direction opposite to the current. What force does the magnetic field of the current exert on the moving electron?
✅ Answer & Solution
Magnetic field due to wire at distance $d = 0.1$ m:
$$B = \frac{\mu_0 I}{2\pi d} = \frac{4\pi \times 10^{-7} \times 2}{2\pi \times 0.1} = \frac{8\pi \times 10^{-7}}{0.2\pi} = 4 \times 10^{-6} \text{ T}$$
Force on electron moving opposite to current (i.e., effectively like opposite current — wires attract if same direction, so electron moving opposite to current is repelled):
$$F = evB = 1.6 \times 10^{-19} \times 4 \times 10^4 \times 4 \times 10^{-6}$$
$$F = 1.6 \times 10^{-19} \times 1.6 \times 10^{-1} = 2.56 \times 10^{-20} \text{ N}$$
Direction: away from the wire (repulsive).
