# Homework: Coulomb's Law

1. Jan 14, 2014

### Sneakatone

1. The problem statement, all variables and given/known data
A point charge q1 = -3.9 μC is located at the origin of a co-ordinate system. Another point charge q2 = 7.3 μC is located along the x-axis at a distance x2 = 7.8 cm from q1.
1) What is F12,x, the value of the x-component of the force that q1 exerts on q2?

2. Relevant equations
F=(k*q1*q2)/r^2

3. The attempt at a solution
[(-3.9*10^-6 C)*(7.8*10^-6 C)*(9*10^9 C)]/(0.078^2) m= -45 N

but the answer is wrong I converted everything into metric units.

#### Attached Files:

• ###### h1_oneDa.png
File size:
2.8 KB
Views:
95
2. Jan 14, 2014

### Simon Bridge

Converting to SI units can simplify things, but if you are frequently using different units it can work out better to convert the constants instead.

k=89.876N.cm2/μC2

I think you just had some rounding issues ... I'm getting a slightly different number off the same calculation.

3. Jan 14, 2014

### Sneakatone

when I used your k value with no conversions I got -42.1 N. I guessed the program wanted a more accurate solution thanks.

4. Jan 15, 2014

### Simon Bridge

When I used yours - I got 42.115N and mine got me 42.057N ...

With conversion the calculation works out as:

90*3.9*7.3/7.8^2 = 42.115

5. Jan 15, 2014

### Sneakatone

must have been the order with my calculator . I have a ti-30x IIS which is harder to track parenthesis and slightly longer operations. thanks again

6. Jan 17, 2014

### lightgrav

depends on whether q2 = 7.3 μC , or 7.8 μC .