What is the reason for still measuring a current in the circuit even when the potential difference is zero? Please explain using concepts from Physics II.

asked by @taham1 • almost 2 years ago • Physics • 5 pts

Q: What is the reason for still measuring a current in the circuit even when the potential difference is zero? Please explain using concepts from Physics II.

MY ANSWER:

Current will flow as long as there is a difference in potential.

Ohm's Law applies. I = V/R As long as V is greater than zero there will be finite I.

The reason is voltmeter has very high resistance and an ammeter has a very small resistance. when we connect voltmeter parallel to the circuit element the current through voltemeter is approximately zero. so, it reads zero voltage.

But the current flowing through the circuit element passes through the ammeter. so, ammeter reads that current.

TEACHER RESPONSE: If there's still current being measured when no signal is being applied, where do you think that residual current is coming from?

can you please fix my answer no there is no circuit for this problem

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1 answer

I honestly have no idea what your teacher is getting on about. There has to be something up with the question. Current can only flow in a circuit when it is propelled by a voltage, other wise its potential energy will never change. If you have a source, like a battery, charges move through a circuit because that source gives them a potential energy (eV, where V is the voltage of the battery), and the charges dispense all that energy moving through the circuit. No voltage, no movement, because the charges have no potential energy to convert into kinetic energy.

Now, the particular phrasing of your professors response makes me feel like there is something missing in the question, or the question is poorly written by your professor: "where do you think that residual current is coming from". A residual current implies that there's some sort of "hidden" current-reservoir, that as soon as the voltage drops, that releases more current into the circuit. There are things that can store charge/current (specifically capacitors and inductors), BUT they cannot store that charge without any voltage.

With respect to what I just said in the previous paragraph, the main issue with this problem is "...even when the potential difference is zero." The potential difference across what? A resistor? A source? A capacitor? You can't just say "the voltage in the circuit" -- that statement makes no sense. The two points you choose to measure voltage across are ALWAYS going to have different voltages if they are different points. Plus, if you choose the SAME two points on a circuit, the voltage is ALWAYS zero (that's Kirchhoff's loop rule).

My recommendation is to talk to your professor and get clarity on what the problem is actually asking, because it doesn't make any sense in its current form. Let me know his response and I'll see if I can figure it out based on that.

answered by @doug • almost 2 years ago