# Heya Help with Electro magnitisim Question Please

1. Aug 20, 2008

### vip_uae

Heya everyone could u help me with these questions plz

a charge +2nC is located at (0, 2 m). Another +2 nC charge is
located at (0, -2 m). A third charge, +3 nC, is located at (2 m, 0).
Calculate the total force (in vector form) acting on charge +3nC
from both +2nC charges.

Using the result obtained in part above, calculate the electric field at
the position of the + 3nC charge due to both +2nC charges.

plz guys thanks alot

2. Aug 20, 2008

### tiny-tim

Hi vip_uae!

Show us what you've tried, and where you're stuck, and then we'll know how to help you.

3. Aug 20, 2008

### vip_uae

well i used the normal formula of F13 = (Q1 . Q3)/ 4 pi E0 r13 multiply by Rhat 13 ... and i knw wats rhat13 and r13 thats for the first part of the question i add the F13 AND F23 to give me FTOTAL... is that right?

4. Aug 20, 2008

### tiny-tim

Well, it looks ok …

but it would be more convincing if you showed us some figures.

erm … since you've got that far, what did you want help with?

5. Aug 20, 2008

### vip_uae

its just i am getting a weired answer for the total force i am getting Zero in the end of it...

when i add F13 AND F23 = 0

and i dont understand what to they mean by VECTOR form

6. Aug 20, 2008

### tiny-tim

Vector form means as a multiple of rhat, which you've done.

I don't understand how your F13 and F23 can add to zero, if you're adding them as vectors.

F13 is a multiple of the unit vector rhat13, while F23 is a multiple of rhat23.

Since rhat13 and rhat23 are not parallel, how can any combination of them add to zero?

7. Aug 20, 2008

### vip_uae

awwwwww right sorry i actually missed out one part of it but it gave me a really small number for Xhat 9.537x10^-9

i just found somthing that if i want to have to calculate it in a vector form

F13 = Q1 X Q3 / 4 pi E0 (r13)^2 multiply by Rhat13 + (u0 X Q1 X Q3)/ 4 pi (r13)^2 Multiply [V3 $$\Delta$$(V1 $$\Delta$$ Rhat13 )]

is that formula looks right it said that if a vector form needed Fe which is Electric Force + Magnetic force = the force

Last edited: Aug 20, 2008
8. Aug 20, 2008

### tiny-tim

Is that supposed to be a magnetic force?

Magnetic forces only act on moving charges.

These charges are stationary … so forget magnetic forces!

You're vector-adding a multiple of rhat13 to the same multiple of rhat23.

Use components … what is the resultant?

9. Aug 20, 2008

### vip_uae

see i dont understand what you meant by Multiple of Rhat i knw r hat = r13/moduls r13

10. Aug 20, 2008

### tiny-tim

Yes, you're multiplying the vector rhat13 by (Q1 . Q3)/ 4 pi E0 r13,

and adding to it the vector rhat23 multiplied by (Q2 . Q3)/ 4 pi E0 r23 …

aren't you?

11. Aug 20, 2008

### vip_uae

yes thats wat i am doing exactly

12. Aug 21, 2008

### tiny-tim

ok, so you've got a sloping-down vector plus an equal sloping-up vector …

so their sum (resultant) is … ?

13. Aug 21, 2008

### vip_uae

ya i have got Fnet = 9.536 x 10^ -9 Xhat ... does this answer looks right?

14. Aug 21, 2008

### tiny-tim

I haven't done the figures, but yes it certainly is a multiple of Xhat.

15. Aug 22, 2008

### vip_uae

Thats gr8 thanks alot i have an another question please

when an electron beam shoots through a horizantal line in the Z direction carying 50 A .. how would the beam will deflect ... like the direction of the deflection how would it be :)

16. Aug 22, 2008

### tiny-tim

ok, but same thing … show us how far you've got!

what sort of field is produced? which direction is it in? what is the equation that relates field strength and force?

17. Aug 22, 2008

### vip_uae

Hey tiny tim lol i found the answer some where basicly it will deflect upwards due to the way that the current is rotating ... soo o i got it straight away

18. Aug 22, 2008

### tiny-tim

he-he … now you see one of the reasons why we like to see some work first!

19. Aug 22, 2008

### vip_uae

hahahahahaha ohhhhhhhh i have a problem tooo :P see i have one of the questions asking me about 2 wires carying 10K A in different directions i need to see the force of one wire exerting on the other using the Motor Equation which i knw

F = L ( I upside down triangle B)

I is current L is lenght and B is magnetic Flux density

B = u H

H = I / 2 pi R *I guess*

should i replace each unit and see wat i can get?

20. Aug 22, 2008

### tiny-tim

oooh … I always get confused by questions about induction.

Can you start a new thread on this one, so that some other people will answer?