# Magnetic flux density vs magnetic flux

1. ### chanderjeet

25
i'm having trouble distinguishing between magnetic flux and magnetic flux density...can someone please explain to me?

also why is it sometimes used interchangeably?

2. ### tiny-tim

26,043
Hi chanderjeet!

Magnetic flux, Φ, is a scalar, measured in webers (or volt-seconds), and is a total amount measured across a surface (ie, you don't have flux at a point).

Magnetic flux density, B, is a vector, measured in webers per square metre (or teslas), and exists at each point.

The flux across a surface S is the integral of the magnetic flux density over that surface:
Φ = ∫∫S B.dS
(and is zero for a closed surface)

Magnetic flux density is what physicists more commonly call the magnetic field.

It is a density per area, rather than the usual density per volume.

(and they can't be used interchangeably)

Similarly, electric flux is a scalar, measured in volt-metres, and electric flux density (also a density per area), E, is a vector, measured in volts per metre (and is more commonly called the electric field).​

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3. ### chanderjeet

25
that really cleared things up...thank you.

4. ### Bob S

There is also H, magnetization, measured in amp-turns per meter (in MKS units).

Bob S

5. ### MikeHansen

3
Some sources say that magnetic flux density is not related to current but to VOLTAGE. I dont understand why. I always thought that the magnetic field in a wire is caused by the movement of charged elements (e.g electrons) in a wire. Can you explain this?

6. ### MikeHansen

3
In some litterature this formula is used:

Bmax = Vmax/ (2*pi*f * A * n)

Where Bmax is the max magnetic flux density, Vmax is the max voltage over a coil with n turns wound on a toroid core with area A. 2*pi*f is the rate of voltage change per time unit.

It looks like the magnetic flux density is reduced :

If we increase the number of turns. Why?
If we increase the area. Why?
If we increase the rate of change of voltage applied. Why?

It looks like the magnetic flux density is increased:
If the max VOLTAGE is increased. Why? Isnt it the current that causes the flux density to increase?

I would appreciate a thorough and intuitive explaination!

Mike Hansen