Magnetic field strength

[jpzuniga]

Hello All, I have two solenoids normally used for magnetic lock of doors. The provider says that the have a weigth liftiing force of 5Lb. In fact they are very strong used at their maximum capacity (24V). I want to know some things:
-Is there a way to calculate the strength of the magnetc field (in tesla) in order to predict the agnetic field strength depending on the voltage applied? (Please note that I don't know the number of turns of the coil in the solenoid)
-Somebody nows about a good magnetic field sensor that can detect between a big range?
Thank's!

[danadak]

http://www.nec-tokin.com/english/product/pdf_dl/sensors.pdf

Just google magnetic field sensors, many more vendors.


Regards, Dana.

[jpzuniga]

Thak's Dana! The only problem is that looking at google, amazon, etc, they sell magnetic sensors almost for high school level (these words are from a vendor that contact me), or even for secondary school science projects....so I don't know how accurate they are...

[xygor]

You could use an electromagnet that you do know characteristics of and adjust it to cancel the field of the unknown.

[Kleinstein]

If it is a magnet with a iron core, it is rather difficult to calculate the field strength accurately. So even a poor quality probe is likely better than a back of an envelope calculation.

Even if you don't know the number of turns, one can usually measure resistance and the dimensions. So one can get a reasonable estimate for the number of turns. 

[Ammar]

If you are interested in practicing your vector calculus, look up the Lorentz force. It is not trivial. Otherwise, you will probably need a sensor.

[T3sl4co1l]

The Maxwell Stress is simply:

\$ \sigma = \frac{B^2}{2 \mu_0} \$

Which, you'll notice, is also a unit of energy density (which, multiplied by the volume the field occupies, this gives the total electrical energy stored in the field).

If you have B applied to one end of a magnetic armature and zero to the other end, then this is the pressure applied to the armature.  Multiplied by the area of the pole pieces (that carry the magnetic field), this gives the force.

For example, a 12mm dia. armature at 1.5T (typically the limit for steel pole pieces), with no field leakage, would support about 10kgf.

As Ammar hinted, this generalizes to the Lorentz force and whatnot.  But if you can make extreme simplifications, you don't need to integrate vectors at all.  Use geometry and physics to solve them for you!

Tim

[jpzuniga]

Thank's for all the tips!!