# Charge on a particle to balance its weight

1. Feb 9, 2013

### Fluxthroughme

1. What must the charge of a 1.45g particle be for it to remain stationary when placed in a downward-directed electric field of magnitude 650N/C?

2. Relevant equations
$$E = \frac{F}{q}$$

3. The attempt at a solution

So the field is pointing downwards. E fields point in the direction a positive charge would take, so the charge must be negative to stay balanced. Gravity is also pointing downwards.

So I take the above formula, and I get F = Eq = ma
$$q = \frac{(1.45*10^{-3})*g}{650} = 1.488*10^{-16} C$$
(Using g = $6.67*10^{-11}$) However the answer given is $-21.9\mu C$

Not sure what I'm doing wrong/missing here?

2. Feb 9, 2013

### BruceW

hey man welcome to physicsforums :)
Why are you using g=6.67*10^-11 ?

Edit: or, what units are these?

3. Feb 9, 2013

### Fluxthroughme

I'm trying to balance the weight of the particle (mg) with the force from the electric field (Eq).

Thanks for the welcome ;D

Edit: the original particle is 1.45grams, so I use the $10^{-3}$ to convert that to kg. E is in N/C, and g, well I don't know :P Whatever the units of the gravitational constant are

4. Feb 9, 2013

### Fluxthroughme

Ohhhhh. I see what I've done! Doing dimensional analysis shows I have the wrong units; thanks for that.

I have to use 9.81 instead of the gravitational constant -_-

Thanks :P

Edit: Yeah, thanks ap123 :P I certainly won't make THAT mistake again ;D

5. Feb 9, 2013

### ap123

g is the acceleration due to gravity, not the gravitational constant, ie g should be 9.80m/s2

Edit: OK - you've got it :)