Quick question about magnetic flux

In summary: Flux is the dot-product of B dot dA integrated over your surface, your surface A being the area of the loop. But how can you expect flux where there's no B? So the area of integration is that of the solenoid cross-section, not that of the loop.And the fact that flux is a dot-product means if the angle of the solenoid axis is 60 degrees to the loop axis then your flux thru the loop will be ∫B⋅dA = BπR2cos(60 deg.) with R = solenoid cross-section radius.Think: why would there be no reduction in flux until you get to 90 degrees
  • #1
pbj_sweg
12
0

Homework Statement


Question asks for the flux through the loop when the loop is both perpendicular and at an angle to the solenoid.
solenoid diameter = 2.2 cm
loop diameter = 6.8 cm
B inside solenoid = 0.22 T

34.EX5.jpg


Homework Equations


∫B⋅dA = Φ_B

The Attempt at a Solution



B is constant so can pull out of the equation. ∫dA is the area of the solenoid, and therefore π(r_sol)^2. I got my flux to be 8.4*10^-5 Wb in both cases, which is correct. My only question is, why do we use the area of the solenoid and not the area of the loop if we're asked to find the flux through the loop? By using the radius of the solenoid, aren't we neglected all the parts of the loop that are outside of the solenoid and have no B field going through it?
 
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  • #2
pbj_sweg said:

Homework Statement


Question asks for the flux through the loop when the loop is both perpendicular and at an angle to the solenoid.
B is constant so can pull out of the equation. ∫dA is the area of the solenoid, and therefore π(r_sol)^2. I got my flux to be 8.4*10^-5 Wb in both cases, which is correct.
Flux = ∫B⋅dA. Say the angle were 90 deg, then the B field would be in the plane of the loop and the dot product would be zero.
So I don't agree with the given answer.
My only question is, why do we use the area of the solenoid and not the area of the loop if we're asked to find the flux through the loop? By using the radius of the solenoid, aren't we neglected all the parts of the loop that are outside of the solenoid and have no B field going through it?
Faraday says emf = dΦ/dt and you can't have any dΦ/dt where there is never any Φ.
 
  • #3
rude man said:
Flux = ∫B⋅dA. Say the angle were 90 deg, then the B field would be in the plane of the loop and the dot product would be zero.
So I don't agree with the given answer.

I'm sorry. I didn't specify. The angle in question is 60°. I agree that if the B field was 90° to the loop's area vector, then the flux would be zero.

rude man said:
Faraday says emf = dΦ/dt and you can't have any dΦ/dt where there is never any Φ.

I understand your reasoning here, but isn't the definition of flux based on the magnitude of the area which the field is going through? If we wanted to calculate the flux of the solenoid, we'd take the cross-sectional area of the solenoid, but because we're being asked to calculate through the loop, why don't we use the area of the loop? I guess I'm just confused about what flux fundamentally is.
 
  • #4
pbj_sweg said:
I'm sorry. I didn't specify. The angle in question is 60°. I agree that if the B field was 90° to the loop's area vector, then the flux would be zero.
I understand your reasoning here, but isn't the definition of flux based on the magnitude of the area which the field is going through? If we wanted to calculate the flux of the solenoid, we'd take the cross-sectional area of the solenoid, but because we're being asked to calculate through the loop, why don't we use the area of the loop? I guess I'm just confused about what flux fundamentally is.
Flux is the dot-product of B dot dA integrated over your surface, your surface A being the area of the loop. But how can you expect flux where there's no B? So the area of integration is that of the solenoid cross-section, not that of the loop.

And the fact that flux is a dot-product means if the angle of the solenoid axis is 60 degrees to the loop axis then your flux thru the loop will be ∫B⋅dA = BπR2cos(60 deg.) with R = solenoid cross-section radius.

Think: why would there be no reduction in flux until you get to 90 degrees, then all of a sudden the flux would drop to zero? Doesn't make sense, does it?
 

1. What is magnetic flux?

Magnetic flux is a measure of the amount of magnetic field passing through a given area. It is represented by the symbol Φ, and its unit is Weber (Wb).

2. How is magnetic flux calculated?

Magnetic flux is calculated by multiplying the magnetic field strength (B) by the area (A) perpendicular to the field: Φ = B * A. It can also be calculated by integrating the magnetic field over the area.

3. What is the difference between magnetic flux and magnetic flux density?

Magnetic flux is a measure of the total amount of magnetic field passing through a given area, while magnetic flux density (B) is a measure of the strength of the magnetic field at a specific point in space. B is related to Φ through the equation B = Φ/A, where A is the area perpendicular to the field.

4. How does magnetic flux affect an object?

Magnetic flux can induce an electric current in a conductor, which can then produce a magnetic field of its own. This phenomenon is known as electromagnetic induction and is the basis for many devices such as generators and motors.

5. What factors affect the magnitude of magnetic flux?

The magnitude of magnetic flux is affected by the strength of the magnetic field, the area perpendicular to the field, and the angle at which the area is oriented with respect to the field. It is also affected by the magnetic permeability of the material through which the flux passes.

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