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A particle having the same charge as of electron moves in a circular path of radius 0.5 cm under the influence of a magnetic field of 0.5 T. If an electric field of 100 V/m makes it to move in a straight path, then the mass of the particle is (Given charge of electron = -1.6 × 10⁻¹⁹ C)

1. 1.6 × 10⁻¹⁹ kg
2. 1.6 × 10⁻²⁷ kg
3. 9.1 × 10⁻³¹ kg
4. 2.0 × 10⁻²⁴ kg

Correct answer: 2.0 × 10⁻²⁴ kg

Solution

Straight-line motion needs qE=qvB so v=E/B=100/0.5=200 m/s. From r=mv/(qB), m=qBr/v=(1.6e-19*0.5*0.005)/200 = 2.0e-24 kg.

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