A beam of electrons is moving with constant velocity in a region having electric and magnetic fields of strength 20 V m⁻¹ and 0.5 T at right angles to the direction of motion of the electrons. What is the velocity of the electrons? (a) 8 m s⁻¹ (b) 5.5 m s⁻¹ (c) 20 m s⁻¹ (d) 40 m s⁻¹

Answer: (d). Constant velocity ⇒ electric and magnetic forces cancel: eE = evB v = E/B = 20/0.5 = 40 m s⁻¹ So the answer is (d) 40 m s⁻¹.

A beam of electrons is moving with constant velocity in a region… | NEET 1996 (AIPMT)

Q131NEET 1996 · AIPMTEasy

Moving Charge in a Magnetic Field

A beam of electrons is moving with constant velocity in a region having electric and magnetic fields of strength $20\text{ V m}^{-1}$ and $0.5\text{ T}$ at right angles to the direction of motion of the electrons. What is the velocity of the electrons?

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Answer: (d)

Constant velocity $\Rightarrow$ electric and magnetic forces cancel: $eE = evB$
$v = \dfrac{E}{B} = \dfrac{20}{0.5} = 40\text{ m s}^{-1}$
So the answer is (d) $40\text{ m s}^{-1}$ .

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