# Electric force and its proportionality on charge(q)

Discussion in 'Physics' started by logearav, Jan 5, 2012.

1. ### logearav Thread Starter Member

Aug 19, 2011
248
0
The Electric field due to a source charge Q is defined in terms of some test charge q. Electric force F is proportional to q. Upto this i understood. Now the next line of my book says, Since F is proportional to q, the ratio F/q does not depend on q..
I dont understand the text highlighted. Why F/q does not depend on q?
Revered members, please throw light on my query.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Well if F is directly proportional to 'q' then F=K*q, where K is some constant of proportionality.

Hence the ratio F/q=(K*q)/q=K, which being a constant is independent of q.

logearav likes this.
3. ### timekeeper New Member

Oct 24, 2007
8
1
Let Q be the charge which causes an electric field, q1 the test charge and r the distance between them. The Force that q1 experiences at distance r because of Q is:
This is your first equation.

Now take a different test charge q2 and put it at the same distance r again. For q2 the Force because of Q is:
This is your second equation.

Now, notice that is equal to !!! And since we define
As you can see, we have reached to an equation which considers only source charge Q and distance r from Q.

The Electric field of a given charge Q at a distance r from it is always the same, no matter what test charge q you put there.

Last edited: Jan 6, 2012
logearav likes this.
4. ### logearav Thread Starter Member

Aug 19, 2011
248
0
Thanks for your valuable replies t_n_k and timekeeper. It is so helpful to me