Rotating Disk in Magnetic Field

[NewEinstein]

Hi I am having a difficulty understanding how to write the circuit equation inside a disk which rotates in magnetic field.
the disk is made of conductor material.
the resistance of the disk is: r
the answer is: ir=emf+v(a)-v(b)
if someone could explain it would be great!
and how do i know where is the positive charge and negative charge
thanks!

 

[tommyxu3]

You can tell where the positive charges are by the direction of the Lorentz force that any charge has, with the direction of every charge's instantaneous velocity is clear.
The equation just unveils the voltage between A and B?

 

[NewEinstein]

You can tell where the positive charges are by the direction of the Lorentz force that any charge has, with the direction of every charge's instantaneous velocity is clear.
The equation just unveils the voltage between A and B?

oh thank you now i got it the lorentz field is always in the radial direction out of the disk so the positive charge is on b..
and yes that is the voltage between A and B but i also got it now from Ohm`s law