what is the concept of flux and flux density in electromagnetics?

Question

How is the concept of electric flux and magnetic flux useful in calculating the fields?

Answer 1

Maxwell's equations require that in empty space the magnetic and electric fields are divergence free: Div B=0 , Div E=0. An equivalent way of saying that, is that for any closed surface, the integral of the normal (to the surface) component of the magnetic field over the surface is zero. For an electric field the integral of the normal component of the electric field over the surface gives the total net charge enclosed (zero in empty space).

There are other fields with this property as well. The word flux ("flow") probably comes by analogy with hydrodynamics. The velocity v(x) of an incompressible irrotational fluid (water for example) is an example of a divergence free field. Div v = 0 just means that the total amount of water flowing across any closed surface is always zero (unless the surface encloses a source) because for each element of water flowing in, an equal amount must flow out.

Any field with this property can be represented visually by drawing continuous lines in space such that the direction of the lines gives the direction of the field, and the magnitude is proportional to the density of lines per unit area in a plane perpendicular to their direction. That is, electric and magnetic fields are identified as directional flux densities. The actual lines have no physical existence; they're a conceptual device.

Each line starts at a source (positive charge) and terminate at a sink (negative charge), or else (especially in the case of a magnetic field) must form a closed loop. This makes it possible to visualize easily the structure of a field particularly when the geometry is simple.

For example it's obvious that a point charge produces a field in the radial direction which decreases as the square of the distance from the charge since the lines of flux must radiate outward from the charge.

Answer 2

actually flux is defined as the total number of lines of field passing through a surface ( be it magnetic field or electric field)the formula is b.a cos(thetha) . if cos value is 0 the flux is maximum and if cos value is 90 flux value is minimum. regarding flux density i dont know about that .