# Don't know where to begin mag field question

#### dewdrop714

Don't know where to begin....mag field question

1. Homework Statement

http://s685.photobucket.com/albums/vv212/dewdr0p714/?action=view&current=physicsexam2q2.jpg

Each support string connects to a corner of the rectangular ceiling. The current carrying wires are .5m apart. The current carrying wires have a mass per unit length of .150 kg/m. The currents are identical and in opposite directions.
Point A is 1m away from the wire on the right
Point B is exactly between the 2 wires
Point C is 1.5 m away from the wire on the left

(all the points are in the same plane as the 2 wires)

b) Determine the magnetic field B at Point A, Point B, and Point C

2. Homework Equations

i think this formula is supposed to be used but I'm not sure...B = F / |q0|(vsinθ)

3. The Attempt at a Solution

I just dont know where to begin with this problem...can anyone give me any pointers?

Related Introductory Physics Homework Help News on Phys.org

#### lanedance

Homework Helper
Re: Don't know where to begin....mag field question

hi dewdrop, do you know amperes law?

the equation you gave is for force on a moving charge in a magnetic field

#### dewdrop714

Re: Don't know where to begin....mag field question

i tried to solve the problem....can someone please check my work? i think i may have made a mistake somewhere along the way
I = 300 is given for the question

B for Point A = [μ0I / 2pi]*[(1/x) - (1/r+x)] = (4pi *10^-7)(300) / (2)(pi) * (1/1m) - (1/1.5m) = 1.97*10^-4 T

B for Point B = 2μ0I / (pi)(r) = 2*(4pi *10^-7)(300) / (.25)(pi) = 9.6 *10^-4 T

***i guess my biggest question is what am i supposed to use for the radius value?

#### lanedance

Homework Helper
Re: Don't know where to begin....mag field question

r will be the distance from the point to the current carrying wire

points
- be careful with direction, at each point is the magnetic field from each wire up, or down?
- define a positive direction before attempting the problem
- eg. B is +ve in the up direction
- also writing B = 2μ0I / (pi.r) makes more sense as we know what is divided by

that said, your first calc looks ok (field opposite directions)
2nd 2 contributions at radius of 0.25 - also looks good

Last edited:

#### dewdrop714

Re: Don't know where to begin....mag field question

thanks for your help! =]

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving