In a velocity selector, an electric field E = 1.2 * 10^5 V/m and a magnetic field B = 0.4 T are perpendicular to each other and to the path of charged particles. What speed must a particle have to pass undeflected?
Answer and explanation
Correct answer: D. 3.0 * 10^5 m/s
- Step 1: For zero deflection, the electric force balances the magnetic force: q E = q v * B.
- Step 2: Cancel charge q: v = E / B.
- Step 3: Calculate: v = (1.2 10^5) / 0.4 = 3.0 10^5 m/s.
Answer
3.0 * 10^5 m/s.
Why not the others: 4.8 10^4 m/s computes E B; 0.33 10^-5 m/s computes B / E; 1.5 10^5 m/s divides by 0.8.
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