I'm trying to visualize the Lenz's Law on a practical inductor.

The Lenz's Law says that

$$ \mathcal{E} = -\dfrac{\partial \Phi}{\partial t} . $$

Now, assume that there is a magnetic flux inside the coil is in the given direction in the image below, and magnitude of this flux is increasing in time (i.e.; \$\frac{\partial \Phi}{\partial t}>0\$).

My two questions are:

  1. What will be the polarity of induced voltage on the resistor? Please give an answer like "'A' will be more positive than 'B'.".

  2. What will be the direction of current in the system. Please give an answer like "It will be in the 'R' direction.".

enter image description here


1 Answer 1


If the changing magnetic field totally surrounds the coil, inside and out then there will be no net voltage or current but, i think you are probably referring to the flux only being concentrated through the centre of the coil in which case you use the right hand rule.

The thumb points in the direction of the flux that will counter the original flux ie it points to the left of your picture. The fingers of your right hand represent the direction that current flows.

This means that point A is more negative than point B and current flows, using your terminology, will be in the R direction.

  • \$\begingroup\$ If we apply the Right Hand Rule, the current will flow in 'L' direction. I'm confused now. Does the minus sign emerge at this point by making the current direction 'R' instead of what we find by the Right Hand Rule? \$\endgroup\$ May 3, 2014 at 18:24
  • \$\begingroup\$ eh? The thumb points in the direction to counter the flux. The fingers form a grip shape and hence resemble your coil in shape. The current flows along your fingers to the tips. \$\endgroup\$
    – Andy aka
    May 3, 2014 at 18:30
  • \$\begingroup\$ One more question; does the inductor weaken (cancel out) the incoming flux when it turns it to current? \$\endgroup\$ May 3, 2014 at 18:42
  • \$\begingroup\$ it sure does else current would rise and add more flux and soon there would be infinite current and all the rules on conservation of eneegy would be busted wide open plus, every action has an equal and opposite reaction! \$\endgroup\$
    – Andy aka
    May 3, 2014 at 19:04

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