# The Naked Scientists Forum

### Author Topic: Magnet Strengths  (Read 2608 times)

#### parsley

• Jr. Member
• Posts: 12
##### Magnet Strengths
« on: 21/04/2006 16:10:04 »
I am doing an investigation into magnetic fields and the electrolysis of copper sulphate, using a strong magnet to accelerate varying concentrations of copper sulphate along a channel.
I am having a bit of trouble calculating the strength of the magnet (in Tesla) though. I have been using a clamp stand pole, and running a current through it, on a top pan balance, with the magnet fixed over the clamp stand (with the clamp stand between the poles of the magnet), so that when I put a current through the pole, either it or the magnet has to move, and as the magnet can't, it does, causing a change in mass on the top pan balance, which I used to calculate the force. Using F=BIL I could then work out B (in theory). However, my values for B are varying between 0.01T, and 0.15T, whereas it was thought I should have about 0.2T. There is a bit of a difference there, and no consistency either. It is a big squarish magnet, which cost about £180, and weighs approximately 2.8kg.
Does anyone know what strength a magnet like this usually is, or how I could calulate B more reliably? I have already been through several science resources catalogues and can find nothing of any use (unless anyone can tell me the siginificance of 68,000 mA?!).

Cheers!

"I just set fire to the table!"
Bring on the chemicals!

#### DocN

• Full Member
• Posts: 94
##### Re: Magnet Strengths
« Reply #1 on: 21/04/2006 21:40:32 »
"The magnetic fields generated by currents and calculated from Ampere's Law or the Biot-Savart Law are characterized by the magnetic field B measured in Tesla. But when the generated fields pass through magnetic materials which themselves contribute internal magnetic fields, ambiguities can arise about what part of the field comes from the external currents and what comes from the material itself. It has been common practice to define another magnetic field quantity, usually called the "magnetic field strength" designated by H. It can be defined by the relationship
H = B0/#956;0 = B/#956;0 - M

and has the value of unambiguously designating the driving magnetic influence from external currents in a material, independent of the material's magnetic response. The relationship for B can be written in the equivalent form
B = #956;0(H + M)

H and M will have the same units, amperes/meter. To further distinguish B from H, B is sometimes called the magnetic flux density or the magnetic induction. The quantity M in these relationships is called the magnetization of the material.

Another commonly used form for the relationship between B and H is
B = #956;mH

where
#956; = #956;m = Km#956;0

#956;0 being the magnetic permeability of space and Km the relative permeability of the material. If the material does not respond to the external magnetic field by producing any magnetization, then Km = 1. Another commonly used magnetic quantity is the magnetic susceptibility which specifies how much the relative permeability differs from one." see--

http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/magfield.html

#### The Naked Scientists Forum

##### Re: Magnet Strengths
« Reply #1 on: 21/04/2006 21:40:32 »