# Help with physics problem

1. Jan 13, 2008

### rcmango

1. The problem statement, all variables and given/known data

An electric field of 281,000 N/C points due west at a certain point. What are the magnitude and direction of the force that acts on a charge of -7.4 µC at this spot?

2. Relevant equations

3. The attempt at a solution

I just took (-7.4*10^-6)*281000

...to get -2.0794 N, and that seems incorrect.

2. Jan 14, 2008

### rohanprabhu

The answer you have is correct because the Force is given by:

$$F = qE$$

which is exactly what you did. Does your textbook [or whatever] say something else? Because there is no calculation error either [http://tinyurl.com/35fzsu].

Also, as you've got a (-) sign, the direction of the force will be opposite to that of the field i.e. due east.

3. Jan 14, 2008

### rcmango

oh, thats what it was. The negative sign. That is not supposed to be there, apparently.

thankyou.

4. Jan 14, 2008

### rohanprabhu

Actually, the sign is supposed to be there [or not be there]. Basically, it depends on what we are talking about. When we are concerned with a vector, we talk about it's magnitude and direction. If you were asked purely the magnitude, it is always greater than 0 i.e. +ive. But, the (-) sign indicates the direction. If I say that the force is -10N due west, it means that the force was 10N due east.