Verified Solution • 2025–26 Session moderate

QPages 230-234: Q1: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Pages 230-234: Q1 Solution: An alpha particle is projected vertically upward with a speed…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often flip the cross‑product direction by misreading the field’s orientation or by using the left‑hand rule for a positive charge. Treat the magnetic field as a vector pointing from south to north, place the thumb along the upward velocity, and extend the fingers toward the field; the palm then points west for a +2e alpha particle. Double‑check the sign of the charge (alpha = +2e) and the orientation of the vectors before assigning the force direction.

An alpha particle is projected vertically upward with a speed of

3.0 \times 10^4 \text{ km s}^{-1}

in a region where a magnetic field of magnitude 1.0 T exists in the direction south to north. Find the magnetic force that acts on the

\alpha

-particle.

Step-by-Step Verified Solution

**Answer:**

F = 9.6 \times 10^{-12} \text{ N}

, directed towards west. **Step-by-Step Explanation:** Given:

q = 2 \times 1.6 \times 10^{-19} \text{ C}

,

v = 3 \times 10^4 \text{ km/s} = 3 \times 10^7 \text{ m/s}

,

B = 1 \text{ T}

. The velocity is vertically upward and the magnetic field is from south to north. The force

\vec{F} = q\vec{v} \times \vec{B}

. Using the right-hand rule, the force is directed towards **west**.

F = qvB = 2 \times 1.6 \times 10^{-19} \times 3 \times 10^7 \times 1 = 9.6 \times 10^{-12} \text{ N}

.

All Questions in this Chapter

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