EMF induced in a wire loop rotating in a magnetic field

AI Thread Summary
The discussion centers on the derivation of the electric field induced in a wire loop rotating in a magnetic field, specifically through the integral ε = ∫(v × B) dl. Participants express confusion about the emergence of the electric field despite the magnetic field being time-independent. The role of the Lorentz force in causing electron movement and charge separation, leading to an electric field according to Gauss's law, is highlighted. An equilibrium condition is introduced, stating that the electric force equals the magnetic force, which is crucial for understanding the relationship between the electric field and the motion of charges. The conversation suggests that this equilibrium condition is valid under quasi-static conditions where changes in B and v are slow.
LCSphysicist
Messages
644
Reaction score
162
Homework Statement
.
Relevant Equations
.
1655168640334.png


To solve this problem, we need to evaluate the following integral: $$\epsilon = \int_{P}^{C} (\vec v \times \vec B) \vec dl$$

The main problem is, in fact, how do we arrive at it! I can't see why a Electric field arises at the configuration here. The magnetic field of the rotating sphere is time independent ##(\frac{ d \omega }{dt}) = 0##. The magnetic dipole at the center is also time independent.

So why do a electric field arise? Worst: Why do it arise and is equal to ##\vec v \times \vec B##?
 
Physics news on Phys.org
Have you studied motional emf?
 
kuruman said:
Have you studied motional emf?
It is more about the difficult to see that the area to calculate the flux is constant in avarage, but in infinitesimally time it is varying?
 
Herculi said:
So why do a electric field arise? Worst: Why do it arise and is equal to ##\vec v \times \vec B##?
This link might be helpful.
 
The Lorenz force ##F_L=(v\times B)q## makes the electrons move, and this creates charge separation or simply charge density ##\rho\neq 0##. This charge density creates electric field according to Gauss's law $$\nabla\cdot \mathbf{E}=\rho$$.
How do we know that the line integral of this electric field equals to $$\int_C^P \mathbf{E}\cdot d\mathbf{l}=\int_C^P(\mathbf{v}\times\mathbf{B})\cdot d\mathbf{l}$$.

Well we also impose the additional equilibrium condition $$F_E=F_B\Rightarrow \mathbf{E}q=-(\mathbf{v}\times\mathbf{B})q$$

I am feeling we 'll have to open some can of worms if we going to discuss why this equilibrium condition holds but anyway that is my take on this problem.

P.S In my opinion the equilibrium condition holds approximately in the quasi static approximation, that is when B and v are independent of time or vary slowly in time so that the charges move to the equilibrium position almost instantaneously.
 
Last edited:
Thread 'Collision of a bullet on a rod-string system: query'
In this question, I have a question. I am NOT trying to solve it, but it is just a conceptual question. Consider the point on the rod, which connects the string and the rod. My question: just before and after the collision, is ANGULAR momentum CONSERVED about this point? Lets call the point which connects the string and rod as P. Why am I asking this? : it is clear from the scenario that the point of concern, which connects the string and the rod, moves in a circular path due to the string...
Back
Top