# Homework Help: Magnitude of e field

1. Feb 22, 2012

### sasuke07

1. The problem statement, all variables and given/known data
What is the magnitude of the E field at the origin due to a single charge (either the + or the -) located 5 cm above the horizontal axis?

2. Relevant equations
KQ/D^2

3. The attempt at a solution
in an earlier question it asked What is the direction and magnitude of the E field at the origin due to the single negative charge (ignore all others) on the horizontal axis?" SO i used columbs law "KQ/d^2, (8.99x10^9x30x10^-9)/.05^2 and i got 1.1x10^5 which was the correct answer, but this questions answer is supposed to be 5.4X10^4. I was wondering why this is the case as all the numbers are the same from an earlier question, exepct now its vertical.

#### Attached Files:

• ###### Chargelines.gif
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2. Feb 22, 2012

### kushan

3. Feb 22, 2012

### sasuke07

the earlier question was "What is the direction and magnitude of the E field at the origin due to the single negative charge (ignore all others) on the horizontal axis?"

4. Feb 22, 2012

### kushan

and the present question?

5. Feb 22, 2012

### kushan

I didn't get how all the numbers are same in both question , aren't they different

6. Feb 22, 2012

### sasuke07

Because in both questions you are still 5 cm away from the e field, and the charges are still 30 nc. The first question the negative charge is to the left of the center and in the second question the charge (the problem said could be + or -) is 5 cm above the center. So aren't the numbers the same if i were to use the same equation?

7. Feb 22, 2012

### sasuke07

Sorry in question 1 the negative charge is 5 cm to the left of the origin and question 2 the charge is 5 cm above the origin.

8. Feb 22, 2012

### kushan

but the number of charges are different , they are infinite
and also they are distributed along lines not at a point

9. Feb 22, 2012

### kushan

just for the sake of imagination
think of a person pushing a box with the help of a rope alone
and think many people pushing a box with many ropes

10. Feb 22, 2012

### sasuke07

aren't there only 1 charge in both questions as question 1 says to ignore the other charges and question 2 says there is a single charge? would i have to use a different formula?

11. Feb 22, 2012

### kushan

i really wish if you could post both the question exactly separated by space , i am confused

12. Feb 22, 2012

### sasuke07

Sorry
1."What is the direction and magnitude of the E field at the origin due to the single negative charge (ignore all others) on the horizontal axis?"

2.What is the magnitude of the E field at the origin due to a single charge (either the + or the -) located 5 cm above the horizontal axis?

I know how to do 1, its just question 2 thats tripping me up.

13. Feb 22, 2012

### kushan

where is the particle placed in Q1

14. Feb 22, 2012

### sasuke07

particle is at the origin, and the negative charge is on the left

15. Feb 22, 2012

### kushan

left ? what is the coordinate?

16. Feb 22, 2012

### sasuke07

If you look at the picture the particle would be at the center and the negative charge is on the x axis to the left of the origin. So it would be 5 cm away, as the entire horizantel axis is 10cm.

17. Feb 22, 2012

### kushan

see magnitude of electric field will be same for all points whose distance from the particle is same

18. Feb 22, 2012

### kushan

locus will be a sphere , where the whole surface will have same magnitude at all points on the surface

19. Feb 22, 2012

### kushan

See magnitude of electric field is same for all points which are equidistant to the particle ( which is a locus of a hollow sphere )

20. Feb 22, 2012

### sasuke07

Thats what i was thinking, but on the answer sheet the it said the magnitude of
1. was 1.1x10^5 N/C to left and for 2 it said the magnitude was 5.4X10^4 N/C. Im stumped as to how the magnitude could be different when all the numbers used is essentially the same.

21. Feb 22, 2012

### kushan

There might be something else you are missing
start completely new :0

22. Feb 22, 2012

### sasuke07

would you have an idea of what i could be missing. I posted everything related to the 2 questions and i can't think of anything. The only formula you could use is Columbs law, so ya

23. Feb 22, 2012

### kushan

See i can only think if you write both the questions exactly as they are without omitting anything , with the figure and everything given

24. Feb 22, 2012

### sasuke07

Im uploading a copy of where i am getting the questions. on the quiz they are questions 4 and 7. And you have the picture from my first post. Thats all the information that i recieved for this assignment.

#### Attached Files:

• ###### Review Test Submission: Electric Field Skills Validation.pdf
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25. Feb 22, 2012

### sasuke07

questions 7 and 8 are the only 2 that i can't figure out for myself. So if you could help me out with either of the 2 that would be awesome.