# Motional emf -easy question

Motional emf -easy question(re-edited)

Hi guys ,:

i'm learning about the motional emf , because i will have to take an exam soon, but I've found something really confusin for me well :
they say that when a conductor is moving in a direction perpendicular to the field with constant velocity under the influence of some external agent. The electrons in the conductor experience a force FB = qv x B that is directed along the length ℓ, perpendicular to both v and B . Under the influence of this force, the electrons move to the lower end of the conductor and accumulate there, leaving a net positive charge at the upper end. As a result of this charge separation, an electric field is produced inside the conductor.ok I understand all this (I'm talkin about the upper figure on the attachment)

Question(lower figure on the attachment):
BUT when the conductor slides along two conducting rails they say that there is a magnetic field that goes to the left , Why???? I mean they just said that it goes downwards and they now say that it goes to the left? it's really confusing , it would be great if anyone can help me please ..By the way i'm sorry for my bad english

Last edited:

I have fixed the attachment problem

rude man
Homework Helper
Gold Member

Hi guys ,:

Question(lower figure on the attachment):
BUT when the conductor slides along two conducting rails they say that there is a magnetic field that goes to the left , Why???? I mean they just said that it goes downwards and they now say that it goes to the left? it's really confusing , it would be great if anyone can help me please ..By the way i'm sorry for my bad english

I don't think there is a magnetic field pointing to the left.

By Lenz' law, the generated field must oppose the external field. So the generated field must point out of the page (since the external field points into the page).

The induced current travels around the closed circuit with R in the path. Everywhere along the circuit you can imagine circular magnetic loops per Ampere's law.

In the diagram, FB is the restoring force
In example 1'the induced emf causes Charges to Move up (FE =qvxB)
To counter this, FB restores equilibrium by moving opposite charges to the bottom of the wire CreatIng a Potential difference

For diagram two, the current travels CCW in the system when Fapp is applied. However'this offsets the System
equilibrium as the FLUX through a closed loop has changed'
Therefore, there is an induced force on the rod FB to return the state to equallibrinm.The key differences in these examples is that the second is a closed loop that you are now having to deal with flux,

Thank you !!!