Question

An$\alpha - $particle moves from $E$ to$W$ in a magnetic field perpendicular to the plane of the paper and into the paper. The particle is deflected towards:
A. East
B. West
C. South
D. North

Answer 1

Hint: Use Fleming’s left hand rule to find the direction of motion of the particle. Draw a proper diagram to understand the direction of motion.

Complete step by step answer:
Observe the diagram

Let us consider that the plane of paper is in the XY-plane. Let us assume that the positive X-axis represents the East direction. Negative X-axis represents the West direction. Positive Y-axis represents North direction and negative Y-axis represents South direction.
Let us assume the third axis i.e. Z-axis perpendicular to the plane of paper, such that, the part above the plane of paper is shown by positive Z-axis and the part below the plane of paper is shown by negative Z-axis.
It is given in the question that, the $\alpha - $particle is moving from East to West. That means, it is moving along the negative X-axis.
The magnetic field is perpendicular to the plane of paper and into the paper. That means, the direction of the magnetic field is along the negative Z-axis.
Now, according to Fleming's left hand rule, if we stretch our thumb, middle finger and forefinger in such a way that they look perpendicular to each other. And if we assume that the middle finger represents the direction of current and the forefinger represents the direction of the magnetic field. Then, the thumb represents the direction of motion.
In simple terms, we can say that, if the direction of current is along X-axis and the direction of magnetic field is along Z-axis, then the direction of motion will be along Y-axis.
From the above explanation, we can conclude that the $\alpha - $particle will deflect along the negative Y-axis. We have already established that it represents the South direction.
Therefore, from the above explanation, the correct answer is, option (C) South

Note:We have considered the deflection towards the negative Y-axis and not along the positive Y-axis because all the remaining quantities have direction along the negative side of their respective axes. This question can be solved by improving visual skills. 3D images are hard to draw on paper. You need to be able to visualize it to understand what will happen in the diagram.