Questions And Answers On Two Charges Of Electromagnetic Forces

This problem emphasizes the fact that even if we have two different charges, mutual electrostatic force between them will be same.

1.8 Coulomb’s Law in Vector Form As force is a vector quantity, it has some magnitude as well as direction. We will write coulomb’s law in vector form so that it will represent magnitude as well as direction of electrostatic force. Consider two charges q1 and q2 separated by distance r. First of all we will define . It is the vector joining charge q1 and q2. Unit vector along Where From Coulomb’s law As direction of force is along  Quiz-5 If two charges of magnitude + q1 and – q2 are separated by a distance r, then find out force acting on the charge –q2.

Sol. First define .

We can see that direction of the force is opposite to that of . This is because the force is attractive in nature.

Illustration 5 A charge of 5 C is placed at the origin. Another charge of – 10C is placed at (3, 4). Find the net force acting on – 10C charge.

Sol.

Here = 5 units As, × ×

Illustration 6 A charge of 10C is placed at (1, 5), another charge of 6C is placed at (4, 2). Find the net force acting on 6C charge.

Sol.

× × 1.9 Electric Field Suppose two charges q1 and q2 are placed at points A and B separated by distance r. Force experienced by a charge at B is given as ... (1) Now suppose there is no charge at point B. If we place a unit positive charge (+1C) at B, it would experience some force. So here at point B we can define electric field as force experienced by +1C charge due to charge q1. Electric field is denoted by E. In above case, we can calculate the value of E at point B due to point charge q1 with this

Plot a graph between E and x. Sol. Electric field due to a point charge is given as– Direction of electric field to right and left of origin is as shown. Variation of E at different points on x axis will be as shown in the figure below From the above observations, we can plot the graph between E and x. 1.10 Electric Field Lines The concept of electric field lines was first given by Michael Faraday. With the help of this concept of electric field lines, we can visualize electric field more clearly. Suppose we have a charge + Q at point O. We can draw electric field at any point around it, say at point A. If we move further away from the charge +Q along line OA, magnitude of E decreases, so the size of arrow also reduces. We can do the same for all directions. From above diagram, we can observe two things:

1. Direction of electric field - which is along the arrows.

2. Magnitude of electric field - though we are not getting the exact magnitude of E but we can compare the value of E at any two points by comparing the length of the arrows. Faraday joined all these arrows as shown in the figure below and called them electric field