# Homework Help: Charge Question.

1. Feb 3, 2005

### neoclee

Hi,
Can anyone possibly help me w/ this Question?
Been stuck trying to figure it out four a couple hours (what a waste...)
Thanks in advance!
----------------------------------
A charge of 1 C is fixed in place.
From a horizontal distance 10 km apart a particle of 1 g
and charge 1pc is fired with an initial speed 1000km/h towards the fixed charge.

What is the minimum distance between the two charges ?

I dont even know where to begin!
and whats pc ? Picocoloumbs ?

2. Feb 3, 2005

### MathStudent

use conservation of energy....
do you know the equations for KE and PE?
yes pC is picocoulombs = 10^-12C

Last edited: Feb 3, 2005
3. Feb 3, 2005

### neoclee

i tried, but i cant get anything.
i dont know how PE is related to calculation of distance or radius,
can you atleast point to some formulas that show their relations ?

4. Feb 3, 2005

### MathStudent

$$KE = \frac{1}{2} m v^2$$

$$PE = \frac{kq_1q_2}{r}$$

The second equation is the potential energy for two point charges where r is the distance between q1 and q2...

Remeber that conservation of energy means

$$KE_i \ + \ PE_i = KE_f \ + \ PE_f$$

Decide what should be the initial and final states, and set up the equation... the rest should be apparent from there

Last edited: Feb 3, 2005
5. Feb 3, 2005

### neoclee

Isn't that F=kq1q2/r^2 ?

Im trying it right now.
Thanks alot for your help thus far.

6. Feb 3, 2005

### MathStudent

Thats the equation for the electrical force between two point charges which isn't the same as electrical potential energy.

The potential energy equation is derived from that equation however.

Last edited: Feb 3, 2005
7. Feb 3, 2005

### neoclee

Iv tried to use the help youve provided me to find the answer.
can you please take a look at the attachment and see if my understanding is right and if iv approached the problem properly ?
[The known/unknown values are listed on my first post.]

Thanks again, you help means alot.

#### Attached Files:

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• ###### calc2.gif
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Last edited: Feb 3, 2005
8. Feb 3, 2005

### MathStudent

Where's the attachment?

9. Feb 3, 2005

### neoclee

Sorry, i thought i added them [file limited was exeeded; had to edit them]....

10. Feb 3, 2005

### MathStudent

before even checking if your calculations are correct... you need to go over units

v needs to be in m/s , and g needs to be in kg

11. Feb 3, 2005

### neoclee

they are...I didn't include the unites, but everything is in m/s and KG
edit: actually i missed a deci place for KG, ill have to fix it now..

12. Feb 3, 2005

### MathStudent

oh yeah,, you had the velocity right, I just didn't recognize it

13. Feb 3, 2005

### MathStudent

also in calc1.gif, the equation for KE is 0.5mv^2, you forgot to square the velocity.

14. Feb 3, 2005

### neoclee

Iv made the revision.
Can you please take a look now ?
Thanks again..

#### Attached Files:

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• ###### calc2.gif
File size:
7.7 KB
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15. Feb 4, 2005

### MathStudent

The first part is ok...

in the second, why did you multiply KE_i by PE_i, where in the equation does it say to multiply them?

16. Feb 4, 2005

### neoclee

Changed...
Will you take a look one last time ? {thanks}

#### Attached Files:

• ###### calc2.gif
File size:
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Views:
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17. Feb 4, 2005

### MathStudent

Success!!

18. Feb 4, 2005

### neoclee

lol,
so i take it as thats the right answer...
(its a shame i made so many unnecesary mistakes, Univeristy works really getting up to me...but phys not my area :) ) My friends were saying , i could only solve this problem w/ integration. i guess not ;)

Thanks alot for all your help. I really really appreciate your time towards this.

19. Feb 4, 2005

### MathStudent

no problem... glad I could help

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