Negative voltage and negative current - Page 1
Forum section: electronics / beginners

author: decuser
author_rank: Newbie
timestamp: August 29, 2022, 11:56:23 am
content: I've been thinking about how voltage (potential difference) works in a circuit. Given a circuit consisting of a 6 volt battery and two 1 ohm resistors with ground marked between the resistors. I have finally come to grips with the fact that we call the voltage drop between the positive terminal and ground +3 volts and the voltage drop between the negative terminal and ground -3 volts.What's been bothering me lately is the fact that the current only flows in one direction through the circuit - from the positive terminal through both resistors into the negative terminal. This makes some sense in that the ground is not the lowest potential in the circuit, the negative terminal is. So, conventional current flows from the highest potential, the positive terminal of the battery, through the first resistor, maybe a tiny bit of current goes to ground, the rest goes on through the second resistor, to the negative terminal.But, when I asked some folks if current is negative in a negative voltage situation, they say yes. What is going on here?Does negative voltage mean there will be a negative current or not? It seems to me that if I insert diodes into the circuit on either side of the ground, both will point the same direction, with the anode on the positive terminal side in both cases, which seems to indicate that current doesn't reverse.Thanks for comments, clarifications, and corrections!

author: agehall
author_rank: Frequent Contributor
timestamp: August 29, 2022, 12:44:30 pm
content: Voltage is measured between two points. Often times between ground and a point where the ground is a somewhat arbitrary point in your circuit. Often times ground is picked so that all other points in the circuit would be at a positive voltage potential but if you pick it as in your example, part of the circuit would be at a negative potential and then current becomes negative to make the math work.But do remember that it is all just with respect to how you measure.

author: golden_labels
author_rank: Super Contributor
timestamp: August 29, 2022, 01:05:45 pm
content: Most importantly: this is a mathematical model. Those are numbers and, while they describe a physical process, they are not the process itself. Initially it may be helpful to imagine them as actual charges flowing in a wire. But going too far this path and you miss the goal: letting your brain grasp a new concept.The current direction is indeed a convention. In fact, if electrons are charge carriers, they move in the opposite direction (from negative to positive terminal). So it’s a matter of uschoosing— the critical word here, so I will repeat:choosing— which direction we call “positive current”. Sometimes we can’t predict beforehead, which direction the charge is flowing. Of course you can guess that, if you only have a resistor between two terminals. But one rarely sees such trivial cases.Just see a simplebridge circuitin the attachment. There is no way you can tell which direction charge flows in R3 without calculating it. But to make the calculations, you must assume some current direction. But if you do that, it may happen that you have chosen wrong. The effect will be a negative current.Which only means: the actual current direction is opposite of what you have chosen it to be. If you used a diode in series with R3, it would conduct in the calculated direction. The meaning of a negative voltage is similar: choosing wrong direction for calculations. Note that “wrong” in here doesn’t mean “invalid”. Those are numbers and the model works perfectly fine with both positive and negative values as long as you are consistent with your choices.A non-electrical analogy could be sitting in a stopped train and not knowing, which direction it will go. You may assume it will go the direction you are facing and will say “this will be positive velocity”. Then the train starts moving and you realize it’s going the other way. The velocity is negative from your chosen perspective.Also note that negative currents are not necessarily associated with negative voltages. With resistive elements they always are. But soon you will find out about inductive and capacitive elements, where that is no longer true.

author: decuser
author_rank: Newbie
timestamp: August 29, 2022, 01:09:06 pm
content: @agehall So, the current between the positive terminal of the battery to ground is in the same direction as the current between the ground and the negative terminal of the battery, but because of the choice of ground, we reverse the leads on the voltmeter and get a negative reading of voltage and similarly, we should reverse the leads on the ammeter and get a negative reading of current - for math, but physically, there's no current running in reverse in this example?

author: golden_labels
author_rank: Super Contributor
timestamp: August 29, 2022, 01:11:31 pm
content: Yes, that is right. It’s a matter of how you choose to measure/calculate the current/voltage.

author: fordem
author_rank: Regular Contributor
timestamp: August 29, 2022, 01:18:43 pm
content: Technically, your circuit didn't have a "ground" - it had an arbitrarily chosen reference point.To use golden_labels' train analogy, depending on where you're standing (your point of reference), the same train, moving in the same direction, at the exact same point in time could be either coming towards you or going away from you.

author: adeuring
author_rank: Contributor
timestamp: August 29, 2022, 01:32:49 pm
content: Have a look at Kirchhoff's current law:https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws#Kirchhoff's_current_law:QuoteSo, the sign of the current makes mostly sense when taking some node, or "observation point", to tell if current flows via a certain connection into the "observation point" (positive sign) or out of it (negative sign).Specifically in your example: If your take the "ground node" as the observation point, current flows from the positive terminal of the battery via a resistor into GND - that can be called the "positive current"; and current is flowing from GND via another resistor to the negative terminal - that can be called the "negative current".Another example where current has a sign is an amp meter, or multimeter in current mode: Most modern digital multimeters are able to show a sign before the measured value. The sign indicates if the current is flowing from the red terminal to the black termial (positive) or from the black termial to the red terminal (negative).

author: madires
author_rank: Super Contributor
timestamp: August 29, 2022, 01:37:24 pm
content: Quote from: decuser on August 29, 2022, 11:56:23 amMaybe it would be easier to understand the concept when you take two 3V cells in series and call their connection point ground. The virtual ground in your example opens a can of worms for beginners. But let's stick to your example.Quote from: decuser on August 29, 2022, 11:56:23 amThe whole current has to flow through the two resistors in series, since the virtual ground isn't connected to anything else. It's just a reference point in your example circuit.Quote from: decuser on August 29, 2022, 11:56:23 amAs above, the current flows through both resistors, from the battery's positive terminal to the negative. The direction doesn't suddenly change between the resistors. Now comes the confusing part. Everything depends on the point of view. If you picture youself standing at the connection point of the two resistors you could 'see' current entering the connection point via the top resistor and leaving it again towards the bottom resistor.

author: TimFox
author_rank: Super Contributor
timestamp: August 29, 2022, 01:57:56 pm
content: To think about which direction or polarity the current is flowing through a given point (or plane) in the circuit:Many ammeters are internally a small resistor ("shunt") with a voltmeter measuring the voltage across that resistor.The shunt is between the red and black terminals of the ammeter.If a positive current flows into the red and out of the black terminal, the voltmeter will indicate a positive voltage, hence a positive current.If that current be negative, instead, the voltmeter will indicate a negative voltage, hence a negative current.