Homework Help: Electric field stregth between charges

1. Feb 2, 2012

GRAvit__

1. What is the magnitude and direction of the electric field strength midway between a +60uC charge and a -30uC charge, if the charges are 2.0m apart?

2.E=kQ/r^2

3.
E1=Kx-30u/1
E2=-2.75x10^5

E2=kx60u/1
E2=5.4x10^5

Etotal=E1 + E2 ----> 2.7x10^5 N/C towards the - charge (my book says i have this right)

The problem that i have is that i came across a question that's almost the same but different answer.

1. What is the magnitude and direction of the electric field strength midway between a 75uC charge and a -25uC charge, if the charges are 2.0m apart?

Now i did everything the same as the first one and came to

E1=kx-25u/1
E1=-2.25x10^5

E2=kx75u/1
E2=6.75x10^5

Etotal=E1+E2 ----> 4.5x10^5 N/C towards the - charge (my book says its wrong)

which would work if i took away the (-) from -2.25x10^5 and added them up but thats a different way of doing it from the first so i'm confused as to which way is right.

The only difference in the questions that i see is there's no (+) in front of the 75uC

2. Feb 2, 2012

SammyS

Staff Emeritus
Your answer to the first problem is wrong too. The book is also wrong if it agreed with you.

Does the electric field due to a positive charge point towards it, or point a way from it?

Does the electric field due to a negative charge point towards it, or point a way from it?

3. Feb 2, 2012

GRAvit__

The electric field would point away from a positive charge

The electric field would point towards a negative charge

4. Feb 2, 2012

GRAvit__

(-)-------(m)---------(+)
<----- ......<-------

So you would just take out the negative and add it up? Negative is only referring to direction in this case?

5. Feb 3, 2012

SammyS

Staff Emeritus
Yes.

or they're both negative.