Calculating voltage from multiple point charges

[Eric_meyers]

Homework Statement

A point charge 10 km in the air with a charge of 40 coulombs
A point charge 5 km in the air with a charge of -40 coulombs
A point chare 2 km in the air with a charge of 10 coulombs
-Find the electric potential due to these charges at a point which at a height of 8 km and on the vertical line passing through the charges.

Homework Equations

kq1/r1 + kq2/r2

The Attempt at a Solution

So I summed them up

v = k (40C/2000m - 40C/3000m + 10C/6000m) = 7.5E7

My question is, is do I need to convert km to m and was I correct in subtracting the negative 40 coulomb charge from the rest of the positive charges?

 

[Delphi51]

Yes and yes. Looks good to me.

 

[nlightner]

I would like to clarify a few things. First, it is important to note that the equations used in this problem are for calculating the electric potential, not voltage. Voltage is a measure of electric potential difference, while electric potential is a measure of the electric potential energy per unit charge at a given point in space.

Now, to answer your question, yes, it is important to convert all units to the same base unit for consistency. In this case, since the charges are given in coulombs and the distances are given in kilometers, it would be best to convert the distances to meters (1 km = 1000 m). This will ensure that all units are in SI units and will give you the correct answer.

As for your calculation, you were correct to subtract the negative 40 coulomb charge from the rest of the positive charges. This is because the negative charge will have an opposite effect on the electric potential compared to the positive charges. In other words, the electric potential due to the negative charge will be negative, while the electric potential due to the positive charges will be positive. By subtracting the negative charge, you are taking into account its opposite effect on the overall electric potential.

In summary, it is important to convert all units to the same base unit and your calculation was correct in subtracting the negative charge from the positive charges. However, it is important to clarify that the result of your calculation is the electric potential, not voltage.