# Find the electric force on a point charge

1. Feb 8, 2016

### henrco

1. The problem statement, all variables and given/known data
A point charge q1 = 4.40 μC is at the origin and a point charge q2 = 6.00 μC is on the x axis at x = 2.49 m.
i) Find the electric force on charge q2
ii) Find the electric force on charge q1

2. Relevant equations
Coulomb's law F = k (q1q2)/d^2

3. The attempt at a solution
Part i)
Using k = 8.99 x 10^9, q1 = 4.40 x 10^-6, q2 = 6.00 x 10^-6 and d = 2.49m
Plug these values into the equation

F = (8.99 x10^9) x (4.40 x 10^-6) x (6.00 x 10^-6)/ (2.49)^2
F = 3.83 x 10^-2 N

Checked this value a couple of times and seems correct.

Part ii)
Since we are looking for the force from q2 to q1, it is the same magnitude but opposite in direction.

F = -3.83 x 10^-2 N

I'm a little unsure of this, but if feels correct. If I'm wrong, any guidance welcome.

2. Feb 8, 2016

### TSny

Your work looks correct. But I wouldn't use a minus sign to indicate the direction of the force in part (ii). If you want to specify the directions of each force, then it would be better to use a descriptive phrase such as "toward the other charge" or "away from the other charge".

[If you are dealing with components of a force, such as the x-component or the y-component, then you could have a negative component. But this would require having a clearly stated coordinate system in which the directions of positive x and positive y are known.]

3. Feb 8, 2016

### henrco

"A point charge q1 = 4.40 μC is at the origin and a point charge q2 = 6.00 μC is on the x axis at x = 2.49 m. "
From the question(extract above) it specifies the forces along the x-axis, so I would take these to be the x-components of the forces.

Therefore should I quote the minus sign for the answer to part ii) ?

4. Feb 8, 2016

### TSny

In this case I would give the direction of the force in (i) as "toward the positive x direction",
or I would state the answer as Fx = 3.83 x 10-2 N, Fy = 0. (Also, Fz = 0 if you usually work in 3 dimensions of space.)

Similarly for (ii) I would give the direction of the force as "toward the negative x direction",
or I would state the answer as Fx = -3.83 x 10-2 N, Fy =0. But I would still not state the answer as F = -3.83 x 10-2 N.