B Not sure about the magnetic flux (phi) depending on a cosine function

[Antoha1]

TL;DR Summary

Hi, school student here. I have a question about magnetic flux. I know it is BAcosa. But can it actually be negative?

Hi, school student here.
I have a question about magnetic flux.
I know it is BAcosa. But can it actually be negative? Based on cosine function it can be, because it is related to it. But based on my logic, magnetic flux is just how much magnetic field (for example lines) goes through the Area (A). (or I may be wrong). EXAMPLE: The frame which Area A=5cm^2 is placed in a magnetic field (perpendicular to B)(B=0,1T). It is then rotated through a=120 degrees. What is the change in magnetic flux? Im thinking about dPhi=B(A1-A2)=BA(1-((abs)(cosa)))
But if it can be negative then it should be just dPhi=B(A1-A2)=BA(1-cosa)

I am adding specific problem to solve. Having this problem right here:
A frame with an area of 5 cm^2 and a resistance of 2 Ω is placed in a uniform magnetic field with an induction of 0.1 T. A galvanometer is connected to the frame. What charge will flow through the galvanometer when the frame is rotated through an angle of 120 degrees? At the beginning of the observation, the plane of the frame is perpendicular to the lines of magnetic induction.

Thank you for helping.

 

[Dale]

Yes, it can be negative. That just means that the flux is entering the other side.

 

[Ibix]

The flux is signed, yes. Remember that magnetic fields have a direction: you want the same strength of field in the opposite direction through the same surface to have the opposite sign.

 

[Antoha1]

Yes, it can be negative. That just means that the flux is entering the other side.

So, talking about the change in the flux. If the frame in the field is turned 180 degrees, upside down, does the change in the flux become 2*phi(initial)?

 

[Ibix]

So, talking about the change in the flux. If the frame in the field is turned 180 degrees, upside down, does the change in the flux become 2*phi(initial)?

Check your signs.

Also note that there's LaTeX here - you can write $\Phi=BA\cos\theta$. (If that doesn't show as maths, refresh the page.)

 

[Dale]

frame in the field

What do you mean by this? By “frame” do you mean a loop of wire?

 

[Antoha1]

Yes, it can be negative. That just means that the flux is entering the other side.

So, talking about the change in the flux. If the frame in the field is turned 180 degrees, upside down, does the change in the flux become 2*phi(initial

What do you mean by this? By “frame” do you mean a loop of wire?

 

[Antoha1]

What do you mean by this? By “frame” do you mean a loop of wire?

Yes. Like frame in DC motor

 

[Dale]

Yes. Like frame in DC motor

OK. Frame in physics usually refers to a coordinate system. A loop of wire in a motor is typically called a loop.

If the field makes an angle $\theta$ with a loop and then the loop is rotated by $\pi$ then the new angle will be $\theta+\pi$.

 

[nasu]

So, talking about the change in the flux. If the frame in the field is turned 180 degrees, upside down, does the change in the flux become 2*phi(initial)?

If the initial flux is $\phi_0$, the final flux will be $- \phi_0$ and the change will be $-2 \phi_0$.