# Finding electric field intensity

1. Nov 2, 2015

### Prathamesh

3 point charges q1 q2 & q3 are placed in vaccum as shown.
Find electric field intensity at B,if q1=2C,q2=-4C & q3=-2C

2. Relevant equations

3. The attempt at a solution

2. Nov 2, 2015

### phoenix95

First find the electric field intensity at the point due individual charges, then find the resultant.

Note:
Electric field intensity due to a charge q at a distance r in the direction of r is given by
E=(1/4πε0)(q/r3)r

3. Nov 2, 2015

### Prathamesh

I have tried it up to this....
Is it right ?
If yes , what to do next and how???
Plz help

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4. Nov 2, 2015

### Prathamesh

Thank you!!!
But while finding resultant vector , can I first find resultant of intensity (E') due to q2 & q3 and then simply subtract intensity due to q1 (E1) from E'(though E' is not exactly opposite to E1)?

5. Nov 2, 2015

### phoenix95

Hey, are you sure the units of charge are in C, not nC or anything?
If it is C, then the Es you calculated are wrong... You have write it in terms powers of ten, otherwise use the correct units. (Keep in mind the electrostatic force due to one coulomb of charge is extremely large)

Keeping in mind the directions of Es, that is a correct way to do it...

6. Nov 2, 2015

### Prathamesh

Yeah... sorry , Charges are in nC.

so , finding resultant is like this? In solutions it is given that even there is angle θ(θ≠180) between 2 vectors , simply subtract them....
I didnt understand that.....

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7. Nov 2, 2015

### phoenix95

Yes.

First see if you get the correct resultant . I didn't catch the 'angle' thing... If you get correct answer then will you please post what exactly the solution says? I'll try my best to explain...

8. Nov 2, 2015

### Prathamesh

Solution says after finding E' (which comes 18*√5=40.248N/C) to get final result subtract E1 (9N/C) from it. Thus final result becomes 31.248 N/C.
But I think,to get final result vector sum of E' and E1 should be done which gives final result =√1620+81+2*18√5*9*cosθ=√ 1701+362.232cosθ

9. Nov 2, 2015

### phoenix95

Correct. In order do what the solution says, the vectors E' and E1 have to be exactly opposite. You are right.

10. Nov 2, 2015

### Prathamesh

since cosθ comes -0.94≈-1 , it is approximately same as subtracting E1 from E' .....
Thank you.....