Question

Two point charges A and B whose magnitudes are the same are positioned at a certain distance from each other. A is at origin. Graph is drawn between electric field strength and distance x from A. E is taken positive if it is along the line joining from A to B. from the graph it can be decided that
A. A is positive, B is negative
B. A and B are both positive
C. A and B are both negative
D. A is negative, B is positive

Answer 1

As a very first step, one could read the question well and hence note down the important points from it. You could analyse the graph really well as all the necessary points lie in it. One could recall for which values of the charges, the electric fields would be negative and positive or even zero.

Complete answer:
In the question, we are given two point charges A and B with same magnitudes that are kept at a certain distance from each other. Also, point charge A is kept at the origin and a graph is drawn between electric field strength and distance from A. We are given four options and we are supposed to find the conclusion that can be made from the graph.
As the electric field is found to be along the line joining A to B, we could say that it is positive throughout. Also at none of the points is the field zero, so A and B cannot be both positive. Also at no point is the field found to be negative, so the chance of both being negative is also omitted. If A was negative with B positive, then, the field would have been always negative.
Hence, we found that charge A is positive and charge B is negative. Therefore, option (A) is correct.

Note: Electric field could be defined as the electric force acting per unit charge. The electric field lines would be represented as radially outward from a positive charge and radially toward a negative point charge. Electric fields are actually manifestations of electromagnetic force which is known to be one of the four fundamental forces of nature.