Force on a metal bar across two wires in a magnetic field

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SUMMARY

The discussion centers on calculating the force on a metal bar across two wires in a magnetic field using the equations F=ma and F=-IlB. Participants emphasize the importance of considering the vector sum of forces acting on the bar, including the magnetic force and applied force. Faraday's law of induction is introduced to relate induced emf (ε) to potential difference (V), while the Blv law is suggested as a preferable method for calculating emf in moving media due to its basis in the Lorentz force. The need to neglect electrical resistance in the wires and bar is also highlighted.

PREREQUISITES
  • Understanding of Newton's second law (∑F = ma)
  • Familiarity with magnetic force equations (F = -IlB)
  • Knowledge of Faraday's law of induction
  • Concept of the Blv law for induced emf in moving media
NEXT STEPS
  • Study the application of Faraday's law of induction in various scenarios
  • Explore the derivation and applications of the Blv law in electromagnetic contexts
  • Investigate the effects of electrical resistance in circuits involving magnetic fields
  • Learn about the Lorentz force and its implications in electromagnetic theory
USEFUL FOR

Physics students, electrical engineers, and anyone studying electromagnetic theory or working with circuits in magnetic fields will benefit from this discussion.

aolivias
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Homework Statement
two parallel wires. The distance between the wires is d from each other and they are connected by a capacitor with the capacitance C. The constant magnetic field B is applied and is perpendicular to the plane of the wires. We put a metallic bar across the parallel wires and start pulling it with force F parallel to the wires. What is the acceleration of the bar?
Relevant Equations
I also attached my work to a file so it can be readable. I also did it here below:

I so far did C=Q/V,
I=dQ/dt,
I=c*dV/dt
F=ma, F=-IlB. I set these equations equal to each other:
ma=-IlB, In the next step I substituted c*dV/dt for I and got:
m*dv/dt=-(cdVlb)/dt
then, I substituted a into dv/dt and solved for a and then for final answer:
a=-(cvlb)/(mt)

I am not sure if this is correct or if I am on the right path.
I need help with the problem above
 

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aolivias said:
F=ma, F=-IlB. I set these equations equal to each other
F = ma should be written as ∑F = ma. The left side is the vector sum of the forces acting on the bar.
The force Fmag = IlB is the magnitude of the magnetic force acting on the bar. But there is also the applied force, F, acting on the bar.

The current is produced by the induced emf, ε, due to the changing magnetic flux through the circuit. So, you will need to bring in Faraday's law of induction. How is ε related to the potential difference, V, across the capacitor?

The problem statement is not clear. I guess you are meant to neglect any electrical resistance in the wires and in the bar.
 
Last edited:
aolivias said:
I need help with the problem above
I agree with post 2 except when it comes to moving media I prefer the Blv law (l=distance, v=velocity) to faraday's law for finding the emf. You can get into trouble with faraday's law in moving media. The Blv law is based on the Lorentz force which has a different basis than faraday's.

Otherwise, just play with the various relations as post 2 suggests.
 

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