Question

Two point charges $a$ and $b$ whose magnitudes are same are positioned at a certain distance from each other along the positive $x-$ axis. $a$ is placed at the origin. A graph is plotted between the electric field strength and distance $x$ (measured from $a$). $E$ is taken to be positive if it is along the line joining the two charges from $a$ to $b$. From the graph it can be decided that

• A. $a$ is positive, $b$ is negative
• B. $a$ is negative, $b$ is positive
• C. $a$ and $b$ both are negative
• D. $a$ and $b$ both are positive

Answer 1

The correct option is A $a$ is positive, $b$ is negative

From the graph it is evident that the field is always in the positive direction and it is not zero at any point.

If both $a$ and $b$ are positive or negative then there must exist a point between them where the field must be zero. Since, that is not reflected in the graph these two options can be neglected.

If $a$ is negative and $b$ is positive, then the direction of the electric field will be from $b$ to $a$. According to the convention we have chosen, the field will always be negative. Since, in our graph the field is always positive. Hence, this option can be neglected too.

This means that $a$ must be positive and $b$ must be negative.

Hence, option (a) is the correct answer.