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shea
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< Mentor Note -- thread moved to the EE forum from the Homework forums since it is a more general set of questions >
So, we are going into Kirchhoff's Laws in class, and my entire understanding of circuits, which took me a while to form, is again falling apart.
A physical understanding is absolutely essential to everything I do in physics, and I want to be as strict with it as I can while studying classical mechanics.
My new issue is with Kirchhoff's rules. I definitely understand the reasoning behind them, matter is conserved, as well as energy, so of course the junction rule describes matter, and the loop rule describes the conservation of energy through voltage. However, the instant I see this intuitive law in action, I fail to picture what is actually occurring on the atomic level.
To understand circuits, I had to learn electro-chemistry to try and understand the inner workings of a battery. I understand at this point that all batteries are effectively two separated electrodes, an anode and a cathode. By connecting the two together with a wire, their chemical composition (e.g, zinc vs copper) results in a redox reaction in which the atoms of the anode release their electrons into the wire, which travel around the new circuit, and on the way interact with various resistors.
One important point I inferred from this understanding is that electrons can NOT flow from the cathode to the anode (at least for commercial, non-rechargeable batteries).
Returning to the above circuit, let me quickly solve it so I have the reality of what is happening, and use it to describe my confusion.
Find the current through the circuit.
https://physicsforums-bernhardtmediall.netdna-ssl.com/data/attachments/97/97581-1aad8b8e3ec1d4ec37153371c25a5d91.jpg [If not visible, its attached below. Sorry, new to the forum :) ]
ε1 = 150 V ε2=50 V
R1 = 3 Ω R2 = 2 Ω
Using the loop rule, taking positive to be a counterclockwise current:
ε1 - ε2 - iR1 - iR2 = 0
Plugging in some numbers:
i = 20 A
Now, the reason why I am confused.
Initially, it makes sense that it should be a current in the clockwise direction, the first battery pushes harder than the second, which results in a net push. However, when looking at what is happening atomically, I become confused. Firstly, let's assume that these are fresh batteries, in which the cathode has had no time to accumulate significant negative charge from the reaction.
When a circuit is connected, there should be a potential difference supplied through a wire and resistor, etc. However, a battery consists of two completely separate electrodes, in which there is no means for an electron to flow in between the two ends apart from through the wire itself. So, when this particular circuit is connected, both batteries should have no "wire" to impart this potential, as they are cut off from a complete loop by another gap between electrodes.
I began to think that perhaps its the fact that there is a potential difference between the two negative ends of the batteries, resulting in a current anyway. But let me provide a counterexample which messed that up for me, using the above problem. Using the ground as a reference point, let's say the first battery's negative terminal is practically grounded (0 V potential difference to the ground), and the positive terminal has a very large positive charge relative to the ground. This results in a potential difference of 150 V between the negative and positive terminals of the first battery. The second battery once again has a grounded negative terminal, and a large positive charge on its positive terminal, resulting in a 50 V potential difference between the electrodes. Of course, this is not necessarily the way the battery actually is made, but it is a valid possibility. Once the above circuit is connected, there is no potential difference between the negative terminals, as they are both grounded, but there is a 100 V difference between the positive terminals. Electrons would be biased towards flowing towards the first battery's positive terminal as it has a higher voltage, and the wire connecting the positives would develop a current. As electrons flowed into the first battery's positive terminal, the voltage of that battery would decrease, as the difference in charge between the battery's electrodes decreased. The other battery would experience the opposite, gaining a little more voltage, as difference in charge increased. However, the relative potentials of the negative terminals to the ground would still remain zero, despite changing with respect to the positive terminals. As there are no new electrons being supplied by the imbalance on the positive terminals, it would quickly normalize to no current and no potential difference. By my logic, which must be flawed somewhere of course, this is not the correct theory for how the battery works.
Could someone please help me out here? I love thinking about this stuff, but circuits have thrown me into a world of solid state and quantum stuff which is a lot harder for me to know for sure I am on the right track.
Thanks in advance,
Shea
So, we are going into Kirchhoff's Laws in class, and my entire understanding of circuits, which took me a while to form, is again falling apart.
A physical understanding is absolutely essential to everything I do in physics, and I want to be as strict with it as I can while studying classical mechanics.
My new issue is with Kirchhoff's rules. I definitely understand the reasoning behind them, matter is conserved, as well as energy, so of course the junction rule describes matter, and the loop rule describes the conservation of energy through voltage. However, the instant I see this intuitive law in action, I fail to picture what is actually occurring on the atomic level.
To understand circuits, I had to learn electro-chemistry to try and understand the inner workings of a battery. I understand at this point that all batteries are effectively two separated electrodes, an anode and a cathode. By connecting the two together with a wire, their chemical composition (e.g, zinc vs copper) results in a redox reaction in which the atoms of the anode release their electrons into the wire, which travel around the new circuit, and on the way interact with various resistors.
One important point I inferred from this understanding is that electrons can NOT flow from the cathode to the anode (at least for commercial, non-rechargeable batteries).
Returning to the above circuit, let me quickly solve it so I have the reality of what is happening, and use it to describe my confusion.
Find the current through the circuit.
https://physicsforums-bernhardtmediall.netdna-ssl.com/data/attachments/97/97581-1aad8b8e3ec1d4ec37153371c25a5d91.jpg [If not visible, its attached below. Sorry, new to the forum :) ]
ε1 = 150 V ε2=50 V
R1 = 3 Ω R2 = 2 Ω
Using the loop rule, taking positive to be a counterclockwise current:
ε1 - ε2 - iR1 - iR2 = 0
Plugging in some numbers:
i = 20 A
Now, the reason why I am confused.
Initially, it makes sense that it should be a current in the clockwise direction, the first battery pushes harder than the second, which results in a net push. However, when looking at what is happening atomically, I become confused. Firstly, let's assume that these are fresh batteries, in which the cathode has had no time to accumulate significant negative charge from the reaction.
When a circuit is connected, there should be a potential difference supplied through a wire and resistor, etc. However, a battery consists of two completely separate electrodes, in which there is no means for an electron to flow in between the two ends apart from through the wire itself. So, when this particular circuit is connected, both batteries should have no "wire" to impart this potential, as they are cut off from a complete loop by another gap between electrodes.
I began to think that perhaps its the fact that there is a potential difference between the two negative ends of the batteries, resulting in a current anyway. But let me provide a counterexample which messed that up for me, using the above problem. Using the ground as a reference point, let's say the first battery's negative terminal is practically grounded (0 V potential difference to the ground), and the positive terminal has a very large positive charge relative to the ground. This results in a potential difference of 150 V between the negative and positive terminals of the first battery. The second battery once again has a grounded negative terminal, and a large positive charge on its positive terminal, resulting in a 50 V potential difference between the electrodes. Of course, this is not necessarily the way the battery actually is made, but it is a valid possibility. Once the above circuit is connected, there is no potential difference between the negative terminals, as they are both grounded, but there is a 100 V difference between the positive terminals. Electrons would be biased towards flowing towards the first battery's positive terminal as it has a higher voltage, and the wire connecting the positives would develop a current. As electrons flowed into the first battery's positive terminal, the voltage of that battery would decrease, as the difference in charge between the battery's electrodes decreased. The other battery would experience the opposite, gaining a little more voltage, as difference in charge increased. However, the relative potentials of the negative terminals to the ground would still remain zero, despite changing with respect to the positive terminals. As there are no new electrons being supplied by the imbalance on the positive terminals, it would quickly normalize to no current and no potential difference. By my logic, which must be flawed somewhere of course, this is not the correct theory for how the battery works.
Could someone please help me out here? I love thinking about this stuff, but circuits have thrown me into a world of solid state and quantum stuff which is a lot harder for me to know for sure I am on the right track.
Thanks in advance,
Shea
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