enter image description here

Description of figure -

A homopolar generator in which disc is rotating with constant angular frequency with respect to ground frame and a uniform steady magnetic field is present perpendicular to disc plane .

I wanted to understand what will be effect on current through load due to relative motion of load in two cases (below) for homopolar generator and in general any generator.


Is current through loadZ (figure) same in following cases?

1.a brush is attached(fig) to circumstances of disc whose velocity (of brush not disc)with respect to ground frame is zero and then load is connected to a that brush and axel(center).

2.if load is directly connected between axel(center) and circumference of disc without brush i.e load is also rotating with same velocity as disc.

I thought current should be same in both cases but I'm not sure because we mostly read in books only 1st case is mentioned almost in every book but not 2nd case.

hope someone can help me !

  • \$\begingroup\$ There is no voltage if the brushes contact always the same sector of the disc. \$\endgroup\$
    – Janka
    Dec 4, 2020 at 14:23
  • \$\begingroup\$ Hi @Janka ,but what is reason for no voltage ? \$\endgroup\$
    – user215805
    Dec 4, 2020 at 14:30

1 Answer 1


If the disk velocity in case 1 is zero, the current will be zero.

The total resistance in the loop that includes the load is the resistance of the load plus the resistances of the conductors connecting the load, the resistance of the connections of the conductors at the center and circumference and the resistance of the disk. Sliding connections at the center and circumference will presumably be higher than fixed connection methods. The effect on current will be determined by the difference in the connection resistance compared to the total resistance in the loop.

For current to flow in the second case, the magnetic field must be prevented from passing through the moving conductors that connect the load between the circumference and the center of the disk.

  • \$\begingroup\$ Thanks for answer but I think you misunderstood 1st case disc is always rotating with constant angular velocity relative to ground frame of reference ,it is brush which has zero velocity with respect to ground \$\endgroup\$
    – user215805
    Dec 4, 2020 at 14:20
  • \$\begingroup\$ I suspected that the velocity of the disk was not zero, but the statement of the first case actually states that the velocity of the disk is zero. \$\endgroup\$
    – user80875
    Dec 4, 2020 at 14:34
  • \$\begingroup\$ Ok ,I edited it to avoid any confusion! \$\endgroup\$
    – user215805
    Dec 4, 2020 at 14:37
  • \$\begingroup\$ but in 2nd case if load form a closed loop with both ends of disc , isn't it impossible to prevent magnetic field to pass through moving conductor ? \$\endgroup\$
    – user215805
    Dec 4, 2020 at 15:56
  • \$\begingroup\$ Yes, it would very likely be impossible. \$\endgroup\$
    – user80875
    Dec 4, 2020 at 16:44

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