write short notes on BH curve.
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BH curve is the graph plotted between B (i.e. flux density) and H (i.e. magnetic field strength). H is taken on x-axis and B is taken on y-axis. Different magnetic materials have different BH curve. And non magnetic material have a straight line instead of curve.
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 Suppose there is a coil wound on a core, core may be iron core or steel core or any other. And we apply emf to coil. By changing variable resistance we can vary current. If we decrease the resistance current will increase and if we increass the resistance current through the coil will decrease.

 Also remember if the core shape is straight it is called solenoid and if the core shape is circular it is called toroid.

 When we give the emf to coil and variable resistance is at maximum position means current flowing the coil is minimum and as soon as the current is increase coil get magnetize.
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 Initially the B and H increases slowly but after instep B increases rapidly. And after knee point the B start getting saturated. And at Bmax there is a straight line called saturation. After Bmax the B will not increase further. 

 BH curve is also called as magnetization curve. 
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This is  a very nice and comprehensive answer.
@ibekwe Thanks....
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B-H curve of magnetic material: 

 The B-H curve is the graphical representation of relation between flux density (B) and applied field strength (H), with H plotted on the x-axis and B plotted on the y-axis. Typical B-H curve is as shown in figure below: 

image

The B-H curve can be described by dividing it into 3 regions. Region OA: For zero current, H = 0 and B is also zero. The flux density B then increases gradually as the value of H is increased. However B changes slowly in this region. Region AB: In this region, for small change in H, there is large change in B. The B-H curve is almost linear in this region. Region beyond B: After point B, the change in B is small even for a large change in H. Finally, the B-H curve will tend to be parallel to X axis. This region is called as saturation region.

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image

The B-H curve is the relation between Flux density B in Wb/m2 (on Y Axis) and H in AT/m (on X axis) It is also known as magnetization characteristics. This curve particularly useful to know the characteristics of magnetic material which is generally used to construct pole of an electric machine. It also gives the behavior of the material to get magnetized with rise of current (AT) Initially the material doesn’t have any flux, hence the curve starts at point o. As the process of magnetization starts & current increases, the flux density in the material also increases in proportion with the rise in current. The rise in flux density with rise in current will continue up to point ‘A’ till maximum flux density occurs. At point ‘A’ material gets magnetically saturated and the curve becomes flat (parallel to X axis) 

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