# Homework Help: Electric Field of Earth problem

1. Jan 26, 2007

### stylez03

1. The problem statement, all variables and given/known data
Electric Field of the Earth. The earth has a net electric charge that causes a field at points near its surface equal to 153 N/C and directed in toward the center of the earth.

1. What charge would a human with a mass of 58.0 kg have to acquire to overcome his or her weight by the force exerted by the earth's electric field?

2. What would be the magnitude of the repulsive force between two people each with the charge calculated in part (a) and separated by a distance of 120 m?

2. Relevant equations

$$E = \frac{1}{4*pi*e_o}*\frac{q}{r^2*\vec{r}}$$

$$G = 9.81 & m/s$$

3. The attempt at a solution

I couldn't find any relevant information in the electric field section that I could map to this problem to help me solve it. Does anyone have a suggestion on where to start?

Last edited: Jan 26, 2007
2. Jan 26, 2007

### Chi Meson

Your looking at the wrong equation here. This is a simple problem. Just a matter of balancing two forces.

How are charge, electric field, and electric force related?

3. Jan 26, 2007

### stylez03

Electric force on a charged body is exherted by the electric field created by other charged bodies?

4. Jan 26, 2007

### Chi Meson

I meant what is the (very simple) equation relating the three quantities?

5. Jan 26, 2007

### stylez03

I can't seem to find the equation that relates to all three, I'm looking at the index of current chapter of all the equations in this chapter and I can't find one that does?

6. Jan 26, 2007

### Chi Meson

I'm surprised. Well it's easy: F=qE . Notice how it is similar to the equation for weight: F=mg . In both cases the force is found as the product of a quantity of matter times the strength of a field.

Balance those forces.

7. Jan 26, 2007

### stylez03

So it should be q = F/E because for part 1 it's looking for the charge.

q = 58.0 kg / 153 ??

8. Jan 26, 2007

### marcusl

Hmm, I gave stylez the same equation in another thread about 12 hours earlier :uhh:

9. Jan 27, 2007

### Chi Meson

What's the difference betwen mass and weight?

10. Jan 27, 2007

### stylez03

Mass is a measurement of the amount of matter something where as weight is the measurement of the pull of gravity on an object.

11. Jan 28, 2007

### stylez03

EDIT:

Thanks I found the right equation. For part B it asks:

What would be the magnitude of the repulsive force between two people each with the charge calculated in part (a) and separated by a distance of 120 m?

Is there a separate to find the repulsive force?

Last edited: Jan 28, 2007
12. Jan 28, 2007

### stylez03

$$\frac{1} {4*pi*8.85*10^-12} * \frac{-3.72*10^-9^2} {120^2}$$

Though the online program says I'm off by an additive constant.

Last edited: Jan 28, 2007
13. Jan 29, 2007

### mace2

where did the 10^-9 come from?

14. Jan 29, 2007

### stylez03

For some of the problems when they mention charge, the example I had to build off of was to take it and multiply it by 10^-9. The equation I'm using is the following:

$$\frac{1} {4*pi*8.85*10^-12} * \frac{q1*q2}{r^2}$$

Should q1 and q2 just be -3.72 and since q1 = q2 it should just be q^2 ? Is this the equation you would use to find the repulsive force?

15. Jan 29, 2007

### mace2

Yeah, that will give you the electrostatic force. Since they're both negatively charged they will repel.

Don't guess about 10^-9, just work with what you have. The math doesn't lie.

q1 & q2 & so on are all measured in Coloumbs.

16. Jan 29, 2007

### stylez03

I tried:

$$\frac{1} {4*pi*8.85*10^-12} * \frac{-3.72^2}{120^2}$$

The units of the solution is in Newtons

17. Jan 30, 2007

### mace2

um, maybe try (-3.72)^2

18. Jan 30, 2007

### stylez03

nope, didn't work

19. Jan 30, 2007

### mace2

i have no idea. maybe they don't like your rounding? what is an "additive constant"?

20. Jan 30, 2007

### stylez03

I don't know, but I ran out of trys for the online program.

21. Jan 30, 2007

### Kurdt

Staff Emeritus
What answer did you get through your calculations? You probably made a small error while calculating the result.