I was told this idea works for any sign of the source charge: in any potential, a positive test charge will always naturally move towards a lower potential and a negative test charge towards a higher potential.
I can kind of understand why a positive charge would move towards the region of lower potential (because don’t things in general want to do that? but if the source charge doesn’t matter, I’m just confused).
Can someone explain like I’m stupid why a negative charge would move towards higher potential?
In: 1
This question is philosophy rather than physics.
There are no objective ways to determine whether the potential is high or low, or whether the charge is positive or negative. These are what philosophers call “useful fiction”. What you can see, at the end of the day, is that certain charge move in a certain way under the influences of other charges. If you assign certain charge a value of positive, some other charge negative values, and write out an equation for the electric potential based on them, you can find the correct way these things moves. That’s why it’s useful.
But the choice of assigning positive or negative charge is arbitrary. It’s a human invention. Assigning values at all to charge allows us to write simpler theory than not doing it, but there are multiple valid way of doing the assignments. Hence it’s “fiction”. For example, you can rewrite the entire Newtonian physics without even talking about force at all; but it’s quite inconvenient to study it without using the concept of force. If someone asked on ELI5 “why greater force causes greater acceleration” the answer would be similar “force is an useful fiction; that’s how we define the concept of force”.
>the source charge doesn’t matter
Here is something I can say that’s part of physics. The source charge do not have instant effect on the test charge: the limit is the speed of light. You can confirm this without even looking at Einstein’s special relativity: even from Maxwell’s equation, the effect of the source charge only move at the speed of light (in fact, this issue is what lead people to realize that either Maxwell is wrong or Newton is wrong, and special relativity confirmed that it’s Newton). Therefore, from modern perspective, we do not consider the source charge to act directly on the test charge. What happened is that there is an electromagnetic (EM) field that permeate the entire universe, and other charge particles interact with this EM field: the EM field cause acceleration on the charged particle, but the charged particle also cause ripple in the EM field. In this formulation, we were able to explain the delay in the effect of the source charge on test charge.
(note that the test charge also distort the EM field; that’s why people define test charge to be an ideal small charge: the charge is too small to cause any changes to the EM field, but big enough for it to be affected by the EM field; this is just a non-mathematical way of talking about order of approximation)
Latest Answers