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### Course: Physics library>Unit 13

Lesson 4: Magnetic flux and Faraday's law

# Flux and magnetic flux

Introduction and intuition for flux and magnetic flux.

## Want to join the conversation?

• What would be a good definition of flux? And what are its units?
• Flux a measure of how much of a vector field (ex. magnetic or electric) is going through a particular surface. Specifically it is the integration of a field through a surface. There are some useful properties related to electric and magnetic fields, such that the electric field flux through a closed surface is equal to the changed enclosed in the surface, or the rate of change of magnetic flux is equal to the induced voltage around the surface.
https://en.wikipedia.org/wiki/Magnetic_flux
• How can the number of field lines passing through a closed surface (for instance a conducting coil) be determined? Or how do I know how many field lines are passing through any closed surface?
• yeah, its a good question. Field lines are a dodgy concept so dont worry if you are not too clear on it.

The way I see it is like this...
a) field lines do not exist : they are just an idea or model to help us visualize the magnetic field and its relative strength. (the density of the lines)
b) whats more useful is the idea of flux or flow and, although it is 'visualised' by the drawing of lines, it is difficult to imagine what it really is BUT this IS related to field strength. I still find it easy to imagine something 'flowing' through the magnet like a fluid... :)
We do know that field strength B=flux / area

or flux = BA

And in any case, we are usually only interested in flux when it is changing. (ie electromagnetic induction)

OK??
• how is magnetic flux produced?
• Magnetic flux is not really "produced". It is simply a measure of the relative component and density of a given magnetic field that is normal to a predefined area. It is sort of like saying that length is produced. Length is simply a measure of how long something is.
(1 vote)
• If the magnetic flux depends on density of the magnetic field lines, will the magnetic flux be greater near the poles of the magnet (because magnetic field lines are more concentrated near the poles)?
• Yes, it will. Notice that the total flux in a closed region, however, must always be 0. If you draw an open surface near a positive pole, there'll be many lines crossing it, but if you close the surface, the number of entering lines must equal the number of exiting lines.
• So the magnetic flux measures the strength of a magnetic field? Upon that, is it proper to say 'this magnetic field has more flux than the other?'
• Consider Khan's description of a rectangular surface patch that intersects the magnetic field lines, since each point of that surface patch has a magnetic field line passing through it, and since there are an infinite number of points in that patch, then wouldn't that imply an "infinite" magnetic flux per patch?
(1 vote)
• No, that's silly, right?
Let's apply your reasoning to a more familiar situation. You put a box on the floor. Each point on the floor has some weight pushing on it, and there are an infinite number of points, so the force of the box on the floor must be infinite, right?
What's wrong with that reasoning?
• The identification of electric flux is same as megnatic flux ?
• I think yes, because we are dealing '~ flux'. The flux means the amount of something passing through a surface. That's means that the magnetic flux and electric flux concept is same but the 'something' is different.
• So by this convention, couldn't electric current (Q/t) be also called the electric flux?
It is essentially the same thing..
• flux means flow. so what you say makes sense.

How do the equations work out in your model?
(1 vote)
• Just to clarify. I understand that both the magnet and the current produce magnetic field lines. What should I be calling them individually?
(1 vote)
• I am not aware of there being a difference in terminology.

They are both magnetic fields: one produced by an electric current and the other by a permanent magnet...