Jump to content
Sign in to follow this  

Help with Sign of L*dI/dt in an LC circuit

Recommended Posts

One concept I always struggle with imparting to my kids in AP Physics C: E&M each year is how to write the sign for the potential drop across an inductor in an RL or LC circuit.  We, of course, always start out with Faraday's Law (and avoid KVL since it is invalid with a changing magnetic flux):


png.latex? $$\oint {\vec E \bullet dl =


But then, when replacing the time rate of change of the magnetic flux through the inductor with L*dI/dt, there's always the question of whether the current is increasing or decreasing, and how best to describe this to students.  If we follow the circuit in a clockwise fashion from the positive side of the battery, the first electric field I see for the voltage source is the negative sign, hence -V on the left-hand-side.  Then, as we follow the circuit the rest of the way around, I end up with the equation:


png.latex?$$ - {V_C} =  - {Q \over C} =


which is opposite in sign to most of the standard answers I can find.  I have regularly "wiggled around" this problem by telling students to think about which way the current through the inductor is changing to choose your sign, but there has to be a more elegant solution!  This, below, makes sense to me...




Why, then, in problems like AP-C FR EM2 from 2005 as you do the same basic analysis in part B is the sign of LdI/dt reversed?  I struggle with this discrepancy every year.  Any help you can provide would sure be appreciated!






Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

Terms of Use

The pages of APlusPhysics.com, Physics in Action podcasts, and other online media at this site are made available as a service to physics students, instructors, and others. Their use is encouraged and is free of charge. Teachers who wish to use materials either in a classroom demonstration format or as part of an interactive activity/lesson are granted permission (and encouraged) to do so. Linking to information on this site is allowed and encouraged, but content from APlusPhysics may not be made available elsewhere on the Internet without the author's written permission.

Copyright Notice

APlusPhysics.com, Silly Beagle Productions and Physics In Action materials are copyright protected and the author restricts their use to online usage through a live internet connection. Any downloading of files to other storage devices (hard drives, web servers, school servers, CDs, etc.) with the exception of Physics In Action podcast episodes is prohibited. The use of images, text and animations in other projects (including non-profit endeavors) is also prohibited. Requests for permission to use such material on other projects may be submitted in writing to info@aplusphysics.com. Licensing of the content of APlusPhysics.com for other uses may be considered in the future.

  • Create New...