A uniform magnetic field acts right angles to the direction of motion of electrons. As a result, the electron moves in a circular path of radius 2 cm. If the speed of electrons is doubled, then the radius of the circular path will be (a) 2.0 cm (b) 0.5 cm (c) 4.0 cm (d) 1.0 cm

Answer: (c). r = mv/qB ⇒ r ∝ v Speed doubled ⇒ r' = 2× 2 = 4 cm So the answer is (c) 4.0 cm.

A uniform magnetic field acts right angles to the direction of motion… | NEET 1991 (AIPMT)

Q133NEET 1991 · AIPMTEasy

Moving Charge in a Magnetic Field

A uniform magnetic field acts right angles to the direction of motion of electrons. As a result, the electron moves in a circular path of radius $2\text{ cm}$ . If the speed of electrons is doubled, then the radius of the circular path will be

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Answer: (c)

$r = \dfrac{mv}{qB} \Rightarrow r \propto v$
Speed doubled $\Rightarrow r' = 2\times 2 = 4\text{ cm}$
So the answer is (c) $4.0\text{ cm}$ .

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Moving Charge in a Magnetic Field question from NEET 1991 (AIPMT)

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