The current in a DC circuit is associated with the alignment and movement of electrons. At a resistor the free movement of electrons is inhibited, and there is a build up of electrons that creates a voltage across the resistor. The electrons move through the resistor at a rate corresponding to the circuit current, as shown for the simple 2 resistor diagram below. enter image description here

If there is a build-up of electrons in the areas as shown by the blue ellipses then I would expect that they would effect the strength of the induced magnetic field in their vicinity.

I have been looking for but cannot find what the strength profile of the induced magnetic field profile at a fixed distance $d$ from the center of the wire conductor and across a resistor (assuming both wire and resistor to have the same cross-sectional area, and thus thin or thick film resistors and such like could not be used) around a DC circuit. To eliminate conjectural theoretical arguments, I would really prefer direct measurements from an appropriate lab experiment.

In the sketch below,
I have shown 3 possibilities:

  1. constant (blue);

  2. increasing at resistor boundary and reduced across resistor (maroon); and

  3. reduced across the resistor (green).

Possibly it would look like option 4 (i.e. something different to any of the ones shown).

A description of what the profile looks like and an explanation of why would be appreciated. A reference link to an experiment verifying the profile would be fantastic.

Thank you in advance.

  • $\begingroup$ Assuming $d$ is larger than cross section. What physical law do you use to calculate magnetic field? $\endgroup$
    – npojo
    Sep 8, 2018 at 8:16
  • $\begingroup$ homework question? $\endgroup$
    – jim
    Sep 8, 2018 at 8:30
  • $\begingroup$ You need to take npojo's hint. It might help to idealise the set-up by making the top portion of the circuit (the resistor and horizontal portion of connecting wires) very long compared with $d$, and to also to keep the 'bottom' of the circuit (not shown) a long distance away. $\endgroup$ Sep 8, 2018 at 8:31
  • $\begingroup$ Ampere's law shows B=µI/2πd. Assuming d to be a reasonable distance (within millimeters) of the wire/resistor does a change in µ between the wire and resistor alter the profile, and/or can a build-up of electrons at the wire/resistor interface change B, or a combination of such factors? (P.S. not a homework problem) $\endgroup$
    – Excentrix
    Sep 8, 2018 at 8:56
  • $\begingroup$ What kind of resistor? A film resistor will have a different field from a wirewound resistor. For a film resistor, remember the current is the same all along the circuit. $\endgroup$
    – hdhondt
    Sep 8, 2018 at 9:53

1 Answer 1


Since the current is constant through the resistor, I can't imagine any reason why the magnetic field strength wouldn't be constant at fixed distance to the resistor either. That's Ampere's Law.

  • $\begingroup$ The current is constant, but the voltage across the resistor is caused by a build up of electrons that cannot all get through the resistor at once. Does such a build up cause a variation in the induced magnetic field in the vicinity of the buildup. I will try to edit the question to clarify why it was asked. $\endgroup$
    – Excentrix
    Sep 11, 2018 at 3:08
  • 1
    $\begingroup$ Build up of elections? I've never heard of such a thing. $\endgroup$ Sep 11, 2018 at 12:20
  • $\begingroup$ Why wouldn't electrons build up at a barrier restricting their forward movement? Haven't you ever been caught in merging traffic because of a road narrowing or a partial road blockage? Seems illogical that they would not. $\endgroup$
    – Excentrix
    Sep 11, 2018 at 14:15
  • $\begingroup$ That's not what a resistor does. A resistor prevents a larger amount of current flowing in the first place. You have fewer cars in the first place with a resistor according to Ohm's law, not a backup. $\endgroup$ Sep 12, 2018 at 15:03
  • $\begingroup$ That is an assumption that I am not saying is wrong: I would just like some proof of that this is the case (as opposed to the build-up of electron concentrations). Hence the original request for a reference to experiments showing a flat-line (option 1) magnetic profile to confirm it to be the case. $\endgroup$
    – Excentrix
    Sep 14, 2018 at 4:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.