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- Thread starter zezima1
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I've confusion in solving a question. Please help me out. The question is as follows.

There a positive charge q placed 'a' distance far from the center of a disc of radius 'r', on its axial line. The flux of charge on the disc is 1/4th of the total flux of the charge in the space. then deduce a relation between 'r' and 'a'.

the answer is "r = a / √3"

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Philip Wood

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I'm not sure you're going to find any deeper interpretation of the definition of electric flux. The definition proves its worth as soon as you state Gauss's Law.

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Philip Wood

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Th. A couple of hints...

(1) The flux from q that passes through the disc will pass through the spherical cap shown

in my diagram – hope I've got the set-up right.

(2) I've given you a formula (which you'll need) for the area of a spherical cap. You should try and prove it by integration.

With this information and a bit of algebra and elementary trig, you should be able to solve the problem. Incidentally, my answer wasn't quite the same as the one you gave. Quite possibly a slip on my part.

This isn't the only way to do the problem. You can integrate directly the flux passing through the disc, dividing the ring into annuli. You mustn't forget that the field strength changes from annulus to annulus, and so does the angle between the disc surface and the field.

(1) The flux from q that passes through the disc will pass through the spherical cap shown

in my diagram – hope I've got the set-up right.

(2) I've given you a formula (which you'll need) for the area of a spherical cap. You should try and prove it by integration.

With this information and a bit of algebra and elementary trig, you should be able to solve the problem. Incidentally, my answer wasn't quite the same as the one you gave. Quite possibly a slip on my part.

This isn't the only way to do the problem. You can integrate directly the flux passing through the disc, dividing the ring into annuli. You mustn't forget that the field strength changes from annulus to annulus, and so does the angle between the disc surface and the field.

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Edward M. Purcell's book "Berkeley Physics Vol2: Electricity & Magnetism" has me confused on the concept of electric flux.

On page 22 he has a diagram of flowing water implying (like zezima1 says above) that electric flux is analogous to the volume of fluid passing through a surface per unit of time--such as m^3/s

However, on page 24 he talks about bullets. He writes that field strength E is analogous to intensity of particle flow in bullets per unit area per unit time--bullets/m^2/s. He goes on to say that the flux of bullets through any surface surrounding the gun "is just the total number emitted per unit time" (bullets/second).

This is VERY confusing to me as I'm more familiar with neutron physics where the definitions are the OPPOSITE of what Purcell is describing. For instance a neutron source strength (Q) is measured in neutrons/second while neutron flux (phi) is rated in neutrons/cm^2/s.

I would appreciate some help in finding a way forward as this is a huge impasse.

On page 22 he has a diagram of flowing water implying (like zezima1 says above) that electric flux is analogous to the volume of fluid passing through a surface per unit of time--such as m^3/s

However, on page 24 he talks about bullets. He writes that field strength E is analogous to intensity of particle flow in bullets per unit area per unit time--bullets/m^2/s. He goes on to say that the flux of bullets through any surface surrounding the gun "is just the total number emitted per unit time" (bullets/second).

This is VERY confusing to me as I'm more familiar with neutron physics where the definitions are the OPPOSITE of what Purcell is describing. For instance a neutron source strength (Q) is measured in neutrons/second while neutron flux (phi) is rated in neutrons/cm^2/s.

I would appreciate some help in finding a way forward as this is a huge impasse.

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The need to define an electric field has come from the observation that charges experience forces from other charges, they attract or repulse. The electric field is also defined as the force per unit charge. That's also all it is. The electric field is just a description of what forces charges would experience on various locations. The electric field is more of an abstract thing and not really a concrete physical thing. When we define the electric field as the force per unit charge it also means that there's nothing really moving around. Rather it's a description of how something (a charge) might move around when placed in the field. I think this is different from your nuclear flux and nuclear source strength.

In electrostatics we often talk of field lines which we use to represent the electric field. Electric field lines are as abstract as the electric field, but they are of great use in visualizing what happens. Now the field strength can indeed be described as something like bullets. It is important to note that it is a property of an area whereas the flux is a property of volume. Field strength describes how many field lines go through a certain area while electric flux describes how many field lines go in/out a volume.

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