Friday, May 18, 2007

A Question on Lorentz Force

The following MCQ appeared in Kerala Engineering Entrance 2007 question paper. I have seen this question on many occasions with more or less the same options. Here is the question:

A uniform electric field and a uniform magnetic field exist in a region in the same direction. An electron is projected with a velocity pointed in the same direction. Then the electron will

(a) be deflected to the left without increase in speed

(b) be deflected to the right without increase in speed

(c) not be deflected but its speed will decrease

(d) not be deflected but its speed will increase

(e) be deflected to the right with increase in speed

This is a simple question and the most suitable option is (c) since the magnetic field (being parallel to the electron) does not exert any force, but the electric field decelerates the electron (since the charge on the electron is negative).

Now, suppose the question setter had changed the last option as “not deflected but its speed will first decrease and will then increase”. Then, this last option would be the correct option since the electron would be decelerated first and then would be accelerated in the opposite direction. [Remember speed is a scalar quantity and the modified option takes advantage of this].

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