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Question:

A particle having the same charge as an 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 move in a straight path, then the mass of the particle is ___?

2.0×10𕒶4kg

1.6×10𕒶7kg

1.6×10𕒵9kg

9.1×10󔼧kg

Solution:

mv²/R = qvB
mv = qBR
(i)
Path is a straight line if qE = qvB
E = vB
(ii)
From equation (i) and (ii)
m = qB²R/E
m = 2.0 × 10⁻²⁴ kg