Potential difference and current flow

Question

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.

Answer 1

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.

Answer 2

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.)

Answer 3

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