Magnetism

[Corky]

I am researching the relationship between flux density(B) and field strength(H) using electrical theory but i have looked at several websites and i get the impression these are the smae thing or is it, B=the amount of magnetic force in agiven area whereas H= the magnetism present in the whole magnetic circuit? any help or resources will be much appreciated.

 

[ramuna]

Have a look at this: http://info.ee.surrey.ac.uk/Workshop/advice/coils/terms.html#strength

 

[Corky]

hi ramuna, this is the best site so far but i still jsut cant visulise what they both are, i understand the flux density is how much flux in a permendicular area to the conductive material, but i cant see what field strength is in comparrison it reads very similar up to the point of being a measurement per metre my heads hurting with this whole magnetism part of the course and my instructor is not the best by a long way i had to explain to him the duty cycle or a square wave was a ratio of working time and 'off time' and had to give in to let him tell me that the duty cycle was the full wave on time+off time. so ive sort of give up asking him any help would be apprieciated

 

[ramuna]

Hi Dale,
First of all, magnetism IS a hard subject to understand, so don't feel that you're an exception in finding it tricky. It took me a long while to grasp the basics and even now I only know as much as I need to.
Next, to your question. The easy part to understand, as you point out, is magnetic flux density. It is analogous to the pressure in a tube with water flowing inside it. This pressure is force per unit area where the area is that of a vertical slice across the tube. Replace the tube with a solid magnet and replace force by magnetic flux. Again imagine cutting a vertical slice across the magnet. With respect to the area of this slice, flux density is the amount of magnetic flux per unit area, passing through the magnet.

Now we leave magnets altogether and instead consider wires through which electric currents are passing. As you know, such wires have a circular magnetic field around them (whose field directions are given by the right hand grip rule https://www.gcse.com/energy/em4.htm ). Magnetic field strength concerns the relationship between the the amount of current passing through the wire and the strength (ie amount) of the resultant circular magnetic field. Note the clear difference: Mag. flux density concerns magnets whereas mag field strength concerns current bearing wires.

If however, such a wire is bent into a loop, a linear (as opposed to a circular) magnetic field is generated through the loop, ie field direction is perpendicular to the plane of the loop. And if instead of a single loop, the wire is bent into a coil of several loops (such a coil's formal name is a solenoid), the linear magnetic field is proportionately strengthened. This linear magnetic field is the exact replica of the magnetic field through a magnet. It is as if an imaginary magnet exists inside the solenoid. Incidentally, the relationship between the linear magnetic field and the current in the loop is given by Ampere's law (discussed further here: **broken link removed** ). And since we are back with magnets, albeit imaginary ones, we can apply our concept of mag flux density to such a magnet. This is the inter-relationship between mag flux density and mag field strength.

To summarise: mag flux density relates to magnets; mag field strength relates to current carrying wires; bending such wires into loops produces 'magnets', and by this method we can relate mag field strength with mag flux density.

 

[Corky]

Hi Ramuna, that is litteraly the best explination on the internet and you have made it extremely clear, and easy to understand thanks for putting a bit of thaught into the answer

 

[ramuna]

Thank you Dale for your fulsome compliments ! It makes what we do worthwhile. And good luck with your studies.