WAEC Physics 2016 Theory — Question 4
Question 4 of 12 from the West African Examinations Council (WAEC) Physics 2016 Theory paper, with the correct answer and a full explanation.
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4. An electron enters perpendicularly into a uniform magnetic field which has a flux density of 0.12T. This results in a magnetic force of 9.6×10⁻¹²N on the electron. Calculate the speed of the electron as it enters the magnetic field. (e=1.6×10⁻¹⁹ C)
Model answer
F = evB sinθ, θ=90° (perpendicular entry) F = evB v = F/(eB) v = 9.6×10⁻¹² / (1.6×10⁻¹⁹ × 0.12) v = 9.6×10⁻¹² / (1.92×10⁻²⁰) v = 5×10⁸ ms⁻¹
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