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13 January 2010

Leading questions about the electric field due to point charges


First of all, this blog is up to six -- count 'em, six -- followers.  Woo-hoo!  Only a few million more and I'm as popular as Bill Simmons.  In all seriousness, I'm glad to have everyone aboard.

The diagram at the right is modified a bit from a Serway electrostatics problem.  The goal in the problems I assign, as you may have guessed, is to find the electric field and the electric potential at point P due to the charges.  Today, I just want to talk about finding the electric field.

I will ask this question differently depending on how long we have been studying electrostatics. 

If it is our first foray into fields due to point charges, I will start with:

i.  Is there a charge at point P?
ii.  Is there a force at point P?
iii.  Is there an electric field at point P?

Don't think that's so silly.  I guarantee you, if you don't start here, a large number of students will assume that point P is "positive," or consists of an electron, or will be confused by the sign of point P, or something like that.  Be sure that the class understands the difference between a point charge and a point in space.

If we've had an introduction to fields produced by charges, I might ask the following series of leading questions:

(a) What is the magnitude and direction of the electric field at point P due to the top-left charge?

(b) What is the magnitude and direction of the electric field at point P due to the bottom-right charge?
(c) Calculate the magnitude and direction of the electric field at point P due to BOTH the top-left and bottom-right charges.
(d) What is the magnitude and direction of the electric field at point P due to the bottom-left charge?
(e) Calculate the net electric field at point P. Include both magnitude and direction.

And only after we have gone through example after example in class and on homework would I ask the (nearly) full monty:

1.  Calculate the magnitude and direction of the electric field at point P.

The final, culminating question, which I dread to ask, but I will ask after we are absolutely clear on how to calculate the electric field at point P:

I.  Consider an electron placed at point P.  Calcualte the magnitude and direction of the force experienced by this electron due to the surrounding charges.

2 comments:

  1. I'm not following but I subscribe by RSS. This is all very helpful, thank you!

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  2. I'm also not a follower, technically, but I check your posts regularly and enjoy your approach.

    PS Micah sent me the link -- he's my nephew.

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