Magnetic field on a solenoid: Y-intercept

[SetepenSeth]

EDIT: While going over the data again I noticed the sensor was giving us the data in µT, I'm editing the data I wrote to reflect this change of x10^-3 in magnitude.

1. Homework Statement
On lab class we are exploring the relation of a magnetic field on a solenoid (B) with its number of turns (loops) and the current that goes through. Using 3 different coils (with different number of loops / length / diameter) and using a field sensor we obtained data, the graphs of this data ( B in relation with I ) shows the expected lineal behavior, in all cases however, there is a significant Y-Intersect rounded to 0.0002 T which we can't explain as we expected it to be much closer to zero, the standard error ranges 7 x10^-7 .

Also, if we calculate the number of loops using the equation bellow, the number of loops is close to what we counted on the first couple of measures, but the value decreases as I increase, we can't explain that either.

Homework Equations

B=µo I n

n= N/L

Where

B= Magnetic field
µ0=Vacuum Permeability
I= Current
N= Number of loops
L= Solenoid length

The Attempt at a Solution

[/B]
Some possibilities were ruled out- Sensor calibration (Ran the experiment again with very similar results)
- This should also rule out Earth's magnetic field as it was zeroed before any itineration
- Wires providing the current from the power supply (This one hasn't been completely ruled out but can't explain how would they increase the field in 0.0002 T)

Attached is an example of the graphs we are seeing, the other two are very similar to the first.

 

[John Morrell]

I'm not sure what you mean by your second question, but is it possible that you're measuring the Earth's magnetic field? That would give you a non-zero value for a zero current. The Earth's magnetic field ranges from about 25-65 micro teslas, which seems large enough to account for the error you're getting.

Another possibility is that you are picking up some paramagnetic effects from the core of the solenoid. The individual atoms of certain materials have magnetic moments which line up when put in a magnetic field and strengthen that field. I don't know the physics behind that quite as well, but it's another possibility.