1. Find the electric field at a point midway between two charges of + 3.0 x 10^-9 C and + 60 x 10^-9 C seperated by a distance of 30 cm.  2.

3. Originally Posted by european SENS
Find the electric field at a point midway between two charges of + 3.0 x 10^-9 C and + 60 x 10^-9 C seperated by a distance of 30 cm.

Find the electric field for each one at the desired distance and use superposition.

Cheers  4. what is superposition.  5. so the equation for electric field is E = [k*q] / r^2

so we have

[9 x 10^9 * 3.0 x 10^-9] / 30^2

and

[9 x 10^9 * 60 x 10^-9] / 30^2

Now what do i do with them  6. Originally Posted by european SENS
so the equation for electric field is E = [k*q] / r^2

so we have

[9 x 10^9 * 3.0 x 10^-9] / 30^2

and

[9 x 10^9 * 60 x 10^-9] / 30^2

Now what do i do with them

Firstly, I think the distance you want is half-way, right?

Then imagine the directions of the fields and they will either add or subtract. That is superposition.

Cheers  7. so yes they should be halfed...

[9 x 10^9 * 3.0 x 10^-9] / 15^2

and

[9 x 10^9 * 60 x 10^-9] / 15^2

and i think that they would face opposite directions since the same charges repel one another. does that mean i subtract the two?  8. Originally Posted by european SENS
and i think that they would face opposite directions since the same charges repel one another. does that mean i subtract the two?
Sounds reasonable....  9. do i subtract the top from bottom or bottom from top to get my final answer?  10. Originally Posted by european SENS
do i subtract the top from bottom or bottom from top to get my final answer?

Either way, but try to imagine which direction the result will point.  11. 0.12 - 2.4 = -2.28

and

2.4 - 0.12 = 2.28

im thinking that the negative answer is correct since they repel and dont face one another.  12. Originally Posted by european SENS
0.12 - 2.4 = -2.28

and

2.4 - 0.12 = 2.28

im thinking that the negative answer is correct since they repel and dont face one another.

Well, 2.28 is the magnitude (assuming the math is correct - I'm too lazy to check).

But what you want to imagine is, if you placed a positive test charge at that location, which way would it move?

Generally, there are two ways to do these type of problems;
1. Keep track of all the signs and make sure you do everything correctly (e.g., adding instead of subtracting). This, I would call the meticulous approach. For me, personally, this gets tedious. But that's just me.

or
2. Play fast and loose, use intuition, and imagine the final result. This, as you might have guessed, is what I usually do.

Cheers  Bookmarks
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