Register to reply 
Two point charges electric field 
Share this thread: 
#1
Mar107, 06:53 PM

P: 67

What are the magnitude and direction of the electric field midway between two point charges, −14.0 µC and +13.0 µC, which are 7.40 cm apart?
I used the F = ((k)(q1)(q2))/r^2 F= ((9x10^9)(14)(13))/(7.40^2)) F= 2.99 x 10^10 however that answer is wrong. (The correct answer is 177000000 N/C). Which equation am I suppose to use? 


#2
Mar107, 07:16 PM

HW Helper
P: 1,540

The equation you have there is for the force between two point charges, not the electric field due to a point charge. Take another look at your notes and see if you can figure out which equation it should be. You need to consider the force due to each charge at the centre, i.e. the electric field at the centre due to Q1 and the electric field at the centre due to Q2. It is a vector quantity. Make sure you are consistent with your units.



#3
Mar107, 07:21 PM

P: 67

F= ((k)(q))/r^2 so would i perform this equation twice? once for q1 and once for q2? sorry, i am still not understanding 


#4
Mar107, 07:27 PM

HW Helper
P: 1,540

Two point charges electric field
It isn't "F", it's "E".
Yes, that is the equation to use. You need to use it for both charges. The resultant electric field at the midpoint will be the vector sum of the fields from the two charges, E=E1+E2. 


#5
Mar107, 07:37 PM

P: 67

E1 = ((9x10^9)(14) / (7.4^2)) =  2.3 x 10^9 E2 = ((9x10^9)(13) / (7.4^2)) = 2.14 x 10^9 E = ( 2.3 x 10^9) + (2.14 x 10^9) = 1.6 (correct answer should be 1.77 x 10^8) 


#6
Mar107, 07:45 PM

HW Helper
P: 1,540

You don't have the correct value for r. If the distance between the two charges is 7.4 cm, what is the distance from each charge to the midpoint of their separation?
Also, you are not being consistent with the magnitudes of the quantities. 14uC needs to be 14x10^6 C, so your units work out. Do you see what I mean? Include your units in the calculation so you can make sure they work out properly. Check your conversion from cm to meters as well (you must do this). 


#7
Mar107, 07:52 PM

P: 67

thank you! you explained it very well and i won't forget converting my numbers! :)



#8
Mar107, 08:16 PM

HW Helper
P: 1,540

You should have all the information now to get the answer. Just put it all together in the right way!



Register to reply 
Related Discussions  
Electric Field due to Two Point Charges  Calculus & Beyond Homework  4  
Electric Field and point charges problem  Introductory Physics Homework  1  
Electric Field Due to Point Charges?  Advanced Physics Homework  3  
Electric field strength due to two point charges  Introductory Physics Homework  1  
Electric field of discrete point charges  Introductory Physics Homework  3 