An electron (mass = 9×10−31kg, charge =1.6×10−19 C ) whose kinetic energy is 7.2×10−20 J is moving in a circular orbit in a magnetic field of 9×10−5weber/m2. The radius of the orbit is
A
1.25 cm
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B
2.5 cm
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C
12.5 cm
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D
25.0 cm
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Solution
The correct option is B 2.5 cm r=√2mKqB=√2×9×10−31×7.2×10−201.6×10−19×9×10−5=2.5cm