# Homework Help: What is an additive constant?

1. Jan 31, 2006

### red ink

Im doing some online homework and it says my answer is off by an additive constant???
Some back ground...
How many electrons would have to be removed from one sphere and added to the other to cause an attractive force between the spheres of magnitude 1.00 *10^4 Newtons? Assume that the spheres may be treated as point charges.

I caluclated the charge in one sphere to be 8.43*10^-2. Then calculated how many electrons are needed to get this charge which is 5.268*10^17.

Assume that after some electrons have been removed from it, the first sphere ends up with a net charge of q_1. What would be the charge on the other sphere, q_2, after these extra electrons are added to it?

I entered q_2=(q_1 - 5.268*10^17). It says im off by an additive constant? Did i lose you guys, or do u know whats wrong?

2. Jan 31, 2006

### cepheid

Staff Emeritus

3. Jan 31, 2006

### cepheid

Staff Emeritus
Where'd you get this from? Doesn't make any sense to me.

Hint: If you take 3 electrons off one sphere and slap them onto the other, then obviously suddenly there's a net charge of +3e on the first sphere, and since the second one gets the three extra electrons the first one lost, its net charge is obviously then -3e. Can you see that since you're just transferring charge from one sphere to the other, the net charge on one sphere has to be equal and opposite to that on the other?

4. Feb 1, 2006

### red ink

assuming this is the correct number of electrons needed , 5.268*10^17 ,to create the force, what would q_2 be then?
I totally understand the concept u mentioned, but the online program said i have to have q_1 in my answer to what q_2 equals
E.g.
q_2= q_1 ????
what goes in place of the question marks? rrrr...online homework sucks:yuck:

5. Feb 1, 2006

### cepheid

Staff Emeritus
Close. Equal and OPPOSITE.

6. Feb 1, 2006

### red ink

if q_1= 5.268*10^17
then q_2= -5.268*10^17
are there any nessesary constants i need to express in my answer?