# Potential difference and current flow

I've been taught that electrons move from cathode to the anode (negative and positive). But what happens when I apply positive voltages at +10V at one end of the circuit and +6V at the other? How would electrons flow if both terminals are positive? Could some please clarify how we determine the sign that we put before the value of a voltage?

Also, why does current not flow in an open circuit? If I connect one end of a wire to a battery and let the other hand hang without any connection, what is the potential at the loose end, the one that is in contact with the air? Is it 0V?

The second question may seem like a dupe but I looked it up and it was slightly different from what I've asked, the answer for the other question slightly differs too and it didn't clear my doubt either.

• Some very basic searches on google or stack exchange should reveal numerous answers to these questions at varying skill levels. Have you attempted to research this yourself? You likely need to read about Ohm's law, series and parallel resistance and kirchoff's current/voltage laws. Also include a diagram when asking about specific circuits. One hint is in "Potential difference". If you put two sources at +10 volts in parallel, there will be no potential difference between the points you've connected and no current will flow. What is the difference between +10V and +6V? 4V. – K H Mar 14 at 7:09
• I did research and I couldn't find any satisfactory answer. – noorav Mar 14 at 7:13
• Then search again - this has been asked on here several times... – Solar Mike Mar 14 at 7:14
• Now you know some better search terms. If you read up one what I've suggested you'll know the answer. If you run into terms you don't understand, you may have to look them up too. Don't be afraid to spend substantial amounts of time reviewing fundamentals as it will only make it easier for you to progress at more advanced levels. "Basic electrical theory" "electrical fundamentals" or "how does electricity work?" are examples of other search terms you should be able to find an adequate answer with. – K H Mar 14 at 7:20

I've been taught that electrons move from cathode to the anode (negative and positive)

That's generally true but remember that ions can move in electrolytes or solutions. Instead, for circuit analysis I recommend that you think in terms of conventional current rather than electron flow.

But what happens when I apply positive voltages at +10 V at one end of the circuit and +6 V at the other? How would electrons flow if both terminals are positive?

simulate this circuit – Schematic created using CircuitLab

Figure 1. Current flow between to points at positive potential.

The resistors in Figure 1 don't know anything about where your ground reference is. From the resistor's point of view each of the circuits in Figure 1 are the same. There is a 4 V potential difference across it with the left side 4 V higher than the right. Therefore 4 mA will flow from left to right in each case.

Another way of thinking about this is think what would happen if you connect a 10 V battery to a 6 V battery as shown in Figure 1a? Answer: current will flow from the higher potential to the lower.

Also, why does current not flow in an open circuit?

How could it? There has to be a circuit for charge to flow.

If I connect one end of a wire to a battery and let the other hand hang without any connection, what is the potential at the loose end, the one that is in contact with the air?

It is the same as the potential between the battery terminals before you connected the wire. You've only extended the terminals.

Is it 0 V?

No. This should be clear by now.

• thank you. I have always struggled visualizing current flow. Could you please provide me with some sources or videos that would better my understanding? I would really appreciate it. – noorav Mar 14 at 7:39
• Sorry, but no. You'll have to do your own research. – Transistor Mar 14 at 8:09

I've been taught that electrons move from cathode to the anode (negative and positive). But what happens when I apply positive voltages at +10V at one end of the circuit and +6V at the other? How would electrons flow if both terminals are positive? Could some please clarify how we determine the sign that we put before the value of a voltage?

What happens when one reservoir of water is at 1000 m above sea level and the other one at 600 m? What changes if we don't use sea level as a reference but the base of the Eiffel Tower or some other place?

Also, why does current not flow in an open circuit? If I connect one end of a wire to a battery and let the other hand hang without any connection, what is the potential at the loose end, the one that is in contact with the air? Is it 0V?

What happens if water flow is blocked by a dam? At which level is the water at the upper side of the dam, at which at the lower end?

(There are some limits to this analogy between water flow and electric currents, but as far as the current question goes, it should help understanding.)

• As I go deeper with the water flow and current analogy, it really ends up confusing me. Visualization of current flow is something I've always struggled with – noorav Mar 14 at 7:37
• @noorav You shouldn't really go deeper with the water analogy. I personally think it could do more harm than good. I'm an engineer, not a plumber. I have absolutely no idea what happens if you connect a motorized valve to a pipe with one diameter on one side and another diameter on the other side, while connecting it to a dam etc... – pipe Mar 14 at 13:06
• You must understand, what the limits of an analogy are. This may be more difficult than simply accepting something as one-of-its-kind. Still, we wouldn't have Maxwell's equations without physical analogies. Maxwell himself thought extensively in terms of an "incompressible viscoelastic fluid" (in "On Fourier's Lines of Force"). – Frank from Frankfurt Mar 14 at 13:50

I've been taught that electrons move from cathode to the anode (negative and positive).

The direction of conventional current was defined to be from the positive to the negative before we know about the electron. So from a physics perspective on the quantum scale the electron flows the other way but it is really not significant information while designing electrical circuits, though it's nice to know about.

http://web.engr.oregonstate.edu/~traylor/ece112/beamer_lectures/elect_flow_vs_conv_I.pdf

Since the convential current flow is defined from the positive to the negative node you should think about it as flowing this way even though it's the oposite in reality.

simulate this circuit – Schematic created using CircuitLab

The flow will therefore be from the 10V battery to the 6V battery and effectively you are charging the 6V battery with 40mA current.

Also, why does current not flow in an open circuit? If I connect one end of a wire to a battery and let the other hand hang without any connection, what is the potential at the loose end, the one that is in contact with the air?

Potential to what? You see potential only makes sense compared to something else. The potential to it's ground would still be the same but current would simply not flow. Electricity always want to come back to its ground, an analogy to potential energy in physics can be used. A ball on top of a hill always "wants" to fall to because that's the natural way of things, but it can be blocked from falling.

Air is a sufficient insulator to stop current from flowing from your wire the ground -> given that the voltage is low. With high enough voltage current can actually flow through air, just like thunderbolts will hit the ground.

This book is great for learning about basic circuit analysis.

https://www.amazon.com/Engineering-Circuit-Analysis-David-Irwin/dp/0470873779