TransWikia.com

Complete expression for current induced in a conductive loop

Physics Asked by George Robinson on December 13, 2020

What is the complete general expression for the electric current $i$ induced in a coreless conductive loop of inductance $L$ and resistance $R$ when it is subjected to a given external magnetic flux, which is varying linearly from zero to some $Phi_{EXT}$ in time interval $t_1-t_0$. Assume $i(t_0)=0$

According to the Lenz Law, the current induced in the loop generates a counter-flux $Phi_L$ that opposes the external flux $Phi_{EXT}$ , which attempts to thread the loop.

The Net flux through the loop is equal to the sum of the external flux attempting to thread the loop and the counter-flux due to the induced current $Phi_{NET} = Phi_{EXT}+Phi_L$.

When the resistance of the loop is zero then the opposition of the counter-flux to the external flux is complete ( $Phi_L=-Phi_{EXT}$ ) and $Phi_{NET}$ remains constant perpetually.

The expression for the induced current must reduce to $i(t)=0$ when R is infinite.
Also, this expression must reduce to $i(t)=frac{Phi_L(t)}{L}$ or to $i(t)=frac{-Phi_{EXT}(t)}{L}$ when R is zero (see this answer).

Obviously, the expression:

$$i(t)=-frac{d Phi_{EXT}}{d t}/R$$

…does not fulfill the latter condition, because it ignores the opposing magnetic flux $Phi_L$ generated by the current induced in the loop.

One Answer

Recall Lenz's Law- The induced electromotive force with different polarities induces a current whose magnetic field opposes the change in magnetic flux through the loop in order to ensure that the original flux is maintained through the loop when current flows in it. That's how the negative sign in the second equation came into being, and I think you have understood that. But look at your first equation! There need not be a negative sign there. Perhaps you thought it would be negative because of Lenz's Law..and that's a mistake. Look at the part of the definition in bold. By assigning a negative sign to the right-hand side of the equation, you are implying that the emf induced(and hence the current) is opposite to the direction of flux itself- and that's a wrong notion. As for the link you provided, go back and check the first answer. There is no negative sign in the equation. Hope I've helped you.

Answered by ZMeson on December 13, 2020

Add your own answers!

Ask a Question

Get help from others!

© 2024 TransWikia.com. All rights reserved. Sites we Love: PCI Database, UKBizDB, Menu Kuliner, Sharing RPP