# Homework Help: Magnetic force with spring and resistor problem

1. Jun 17, 2007

### floridianfisher

I am having a great deal of trouble solving the following problem. Can anyone help?
A straight wire of mass 10.2 g and length 5.0 cm is suspended from two identical springs that, in turn, form a closed circuit (Fig. P19.66). The springs stretch a distance of 0.51 cm under the weight of the wire. The circuit has a total resistance of 11 . When a magnetic field directed out of the page (indicated by the dots in the figure) is turned on, the springs are observed to stretch an additional 0.30 cm. What is the strength of the magnetic field? (The upper portion of the circuit is fixed.)

2. Jun 17, 2007

### Kurdt

Staff Emeritus
What have you tried so far?

3. Jun 17, 2007

### floridianfisher

I have tried to solve by first find the spring constant by 2kx=mg, then by using that spring constant in a new equation and solving for magnetic force by 2kx-mg=BIL but it is not working out for me.

4. Jun 17, 2007

### Kurdt

Staff Emeritus
Well you seem to have the correct method. Just a point on working out the magnetic force. You can say 2kx = BIL if you use 0.3cm for x.

So is there any more information give about the circuit because you're going to need to know what I is?

5. Jun 17, 2007

### floridianfisher

I am solving for I by I+V/R = 2.18 A

6. Jun 17, 2007

### Kurdt

Staff Emeritus
You have all the information and the correct equation so now just rearrange for B and plug all the numbers in.

7. Jun 17, 2007

### floridianfisher

I get 539.25 T but when I put it in the computer says im wrong?

8. Jun 17, 2007

### floridianfisher

sorry 539.45 T but stilll wrong

9. Jun 17, 2007

### Kurdt

Staff Emeritus
Have you converted the cm to meters before using them?

10. Jun 17, 2007

### floridianfisher

yes I have 2kx=mg, k= 9800 then 2kx-((mg)/(IL))=B
or (10.2*9.8)/(2*0.0051)=9800 and ((2*9800*0.0081)-(10.2*9.8))/(2.18*.05)= 539.45

11. Jun 17, 2007

### Kurdt

Staff Emeritus
Or have you converted grams to kilograms?

12. Jun 17, 2007

### floridianfisher

OMG!!!! I am an idiot... Thanks for your help

13. Jun 17, 2007

### Astronuc

Staff Emeritus
When solving a problem, one should put units with the numbers, so that one will sure to use the proper system, either MKS or cgs.

14. Jun 17, 2007

### Kurdt

Staff Emeritus
Another bit of advice is if you have the answer to hand and your answer looks the same but is a number of factors out then check your units. Thats where the discrepency usually is.