Moving bar enclosing a changing magnetic field generates a current

AI Thread Summary
The discussion focuses on the calculation of current generated by a moving bar in a changing magnetic field. The magnetic flux is expressed in terms of the magnetic field and the position of the bar, leading to a derived current equation that depends on the bar's velocity. Participants emphasize the need to solve a differential equation due to the non-constant acceleration of the bar, suggesting that Newton's second law must be applied. A method for simplifying the integration process is introduced, allowing for the separation of variables without involving time directly. The conversation concludes with appreciation for the assistance provided in tackling the problem.
besebenomo
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Homework Statement
A circuit with resistance R, a moving bar of lenght l and mass m has initial velocity along the x-axis = -v and at t=0 the bar is in position x=2l. The circuit lays in a region with depending on the x-coordinate. Find the current flowing in the circuit.
Relevant Equations
Faraday's law
The amplitude of ##\vec{B}## is given by:
$$B(x) = B_{0} - B_{0} \frac{x}{2l}$$ for ##0 \leq 0 \leq 2l##
Immagine 2022-09-03 113102.png


This was my attempts at finding the flux of B:

$$\Phi(B) = (B_{0} - B_{0} \frac{x}{2l})(2l-x(t))l = B_{0}2l^2-2B_{0}x(t)l+ B_{0}\frac{x(t)^2}{2}$$

and the current: $$ i = -\frac{d \Phi(B)}{dt} \frac{1}{R} = -\frac{B_{0} v(t) (2l -x(t))}{R}$$

Is this approach correct?
 
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besebenomo said:
Is this approach correct?
The current depends on the velocity, so you will have to find that first. I would start with $$I=-\frac{1}{R}\frac{d\Phi}{dt}=-\frac{1}{R}\frac{d(BA)}{dt}$$then use the magnetic force $$\mathbf{F}_M=I\mathbf{L}\times\mathbf{B}$$in Newton's second law. The acceleration is not constant. You will have to solve a differential equation.
besebenomo said:
The circuit lays in a region with depending on the x-coordinate.
Is something missing from this sentence?
 
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kuruman said:
Is something missing from this sentence?
Yes sorry, I meant B is depending on the x-coordinate.
kuruman said:
he acceleration is not constant. You will have to solve a differential equation.
Yes, I know. The problem is that I am not used to solving differential equation when also ##x(t)## is the equation... Is there any way I can rewrite ##i## as a function of only ##v(t)##? That way would be so much easier to solve. Maybe I did something wrong when I computed the flux?
 
You need to write Newton's second law and bring the mass in the equation. What is the net force acting on the bar?
 
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kuruman said:
You need to write Newton's second law and bring the mass in the equation. What is the net force acting on the bar?
The force acting on the bar is the magnetic force, in this case: $$\vec{F}_{mag} = i \vec{l}\times \vec{B} = - \frac{-B_{0} v(t) (2l-x(t)) l}{R} (B_{0} - B_{0} \frac{x(t)}{2l})$$
Then I solve the differential equation, if I didn't make mistakes along the way:
$$m \frac{dv(t)}{dt} = - \frac{-B_{0} v(t) (2l-x(t)) l}{R} (B_{0} - B_{0} \frac{x(t)}{2l}) = - \frac{B_{0}^2 v(t) (2l^2+ \frac{1}{2}x(t)^2)}{R}$$
$$\Longrightarrow \frac{dv(t)}{v(t)} = -\frac{B_{0}^2}{mR}2l^2 dt -\frac{B_{0}^2}{2mR} x(t)^2 dt$$

This is where I get stuck, if there wasn't the second term to the right I would just integrate but I don't really know what to do in this case. That's why I was thinking I did make a mistake in the conceptualization of the problem
 
A useful method for handling this (some people call it a trick, I call it the chain rule of differentiation) is this
$$a=\frac{dv}{dt}=\frac{dv}{dx}\frac{dx}{dt}=v\frac{dv}{dx}$$ It allows you to separate variables with ##v(x)## on one side and ##x## on the other. Then you integrate. No variable ##t## anywhere.
 
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kuruman said:
A useful method for handling this (some people call it a trick, I call it the chain rule of differentiation) is this
$$a=\frac{dv}{dt}=\frac{dv}{dx}\frac{dx}{dt}=v\frac{dv}{dx}$$ It allows you to separate variables with ##v(x)## on one side and ##x## on the other. Then you integrate. No variable ##t## anywhere.
Thanks a lot you've been really helpful!
 
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