Details on Voltage


by mmango
Tags: voltage
mmango
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Feb5-13, 08:21 PM
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Hi all, I just completed my first circuits course (which I did decent in). One thing that has always been bothering me is my understanding of voltage. If you have a general understanding of even the basics of circuit analysis you could probably clarify to me my understanding of voltage.

So consider the following circuit and the flow of electrons in it (not the positive charge): http://postimage.org/image/4uqaqz96p/

I have always been told that voltage is considered the "flow" or the "pushing" factor in a circuit. Kind of like gravity the charge "drops" from a point of high potential to low potential doing work, which is dictated by the voltage and the amount of charge. My understanding is that this occurs in the resistor as the electrons move from a positive to a negative terminal and that generally resistors release some sort of energy.

My first set of questions come from the source of the potential energy. My understanding is that the source is the battery (battery = voltage source)? If the electrons were to flow to the positive terminal of the battery does the battery increase the potential energy analogous to someone raising a ball? If the battery is providing an increase of potential energy, where is this energy coming from in the first place (could it be chemical energy from the battery)?

I have some more questions, but I want to tackle this bit by bit so I have a better understanding of voltage and its relation to a circuit.

Thanks in advanced and sorry if this is in the wrong section.
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cepheid
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Feb5-13, 08:27 PM
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Welcome to PF,

Quote Quote by mmango View Post

My first set of questions come from the source of the potential energy. My understanding is that the source is the battery (battery = voltage source)? If the electrons were to flow to the positive terminal of the battery does the battery increase the potential energy analogous to someone raising a ball?
Yeah.

Quote Quote by mmango View Post
If the battery is providing an increase of potential energy, where is this energy coming from in the first place (could it be chemical energy from the battery)?
Yes. (Meaning "yes it is coming from chemical energy," not just "yes it could be)."
mmango
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Feb5-13, 08:37 PM
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Thanks for the fast response.

Another question I have refers to the idea of current forming from this voltage. How do the electrons begin flowing from the voltage? Do electrons start in the battery and flowing out from it or is it electrons from the material itself? And what causes the electrons to flow if the potential energy is used up at the resistor?

Drakkith
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Feb5-13, 09:32 PM
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Details on Voltage


Could someone elaborate on how the battery increases the potential energy? I'm not clear on that. And potential energy of what? The charges, the battery, or something else?
cepheid
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Feb5-13, 09:57 PM
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Quote Quote by Drakkith View Post
Could someone elaborate on how the battery increases the potential energy? I'm not clear on that. And potential energy of what? The charges, the battery, or something else?
A voltage is a difference in electric potential between two points. Key word emphasized. Electric potential is the potential energy per unit charge (measured in volts = joules per coulomb). So if you imagine a unit test charge of +1 C moving from an area of low electric potential (like the battery negative terminal) to an area of high electric potential (like the +terminal) it will gain an amount of potential energy in joules that is numerically equal to the voltage (potential difference) in volts.

Of course, in reality what happens is that negative charges move from an area of high electric potential to low, and in so doing, they gain potential energy (because they are negative, and so the electric field affects them in the opposite way).

The increase in electric potential energy occurs because the charge, when going through the battery, moves in the opposite direction of the electric force (from the field), and so the electric force does negative work on the charge (just like when a ball is thrown up, gravity does negative work on it and it gains gravitational potential energy).

Quote Quote by mmango View Post
Thanks for the fast response.

Another question I have refers to the idea of current forming from this voltage. How do the electrons begin flowing from the voltage? Do electrons start in the battery and flowing out from it or is it electrons from the material itself? And what causes the electrons to flow if the potential energy is used up at the resistor?
The answer is that electrons everywhere in the circuit start flowing all at the same time. So charges that are already in the wires start moving. (A conductor contains a whole bunch of free electrons.) The reason why this happens even though the source of the potential difference is at the battery is because if charges started flowing out of the battery, but the charges ahead of them in the wire didn't move as well, then there would be a pile up of charges. However, like charges repel each other. In other words, the pile up of charges would lead to an electric field that would tend to smooth out the pile up of charges. So, everywhere in the circuit, locally, there is an electric field that causes a steady net flow of charge.
Drakkith
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Feb6-13, 05:13 AM
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Quote Quote by cepheid View Post
Of course, in reality what happens is that negative charges move from an area of high electric potential to low, and in so doing, they gain potential energy (because they are negative, and so the electric field affects them in the opposite way).

The increase in electric potential energy occurs because the charge, when going through the battery, moves in the opposite direction of the electric force (from the field), and so the electric force does negative work on the charge (just like when a ball is thrown up, gravity does negative work on it and it gains gravitational potential energy).
How is the field doing negative work on the charges? Isn't the field doing the pushing on the charges in the first place?
mmango
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Feb6-13, 08:22 AM
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Quote Quote by Drakkith View Post
How is the field doing negative work on the charges? Isn't the field doing the pushing on the charges in the first place?
The field does the pushing when electric potential is decreasing, kind of like when a ball is being dropped. In the case of battery it isn't the field that is doing the pushing of the electons, but (from my understanding) an external source of energy (IE chemical energy from the battery or mechanical energy from a waterfall). In that case the energy is converted to electric potential which is then used up at some point at a circuit element (like a resistor).

I think you just have the context of where the field does the pushing mixed up.
mmango
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Feb6-13, 08:31 AM
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Quote Quote by cepheid View Post
The answer is that electrons everywhere in the circuit start flowing all at the same time. So charges that are already in the wires start moving. (A conductor contains a whole bunch of free electrons.) The reason why this happens even though the source of the potential difference is at the battery is because if charges started flowing out of the battery, but the charges ahead of them in the wire didn't move as well, then there would be a pile up of charges. However, like charges repel each other. In other words, the pile up of charges would lead to an electric field that would tend to smooth out the pile up of charges. So, everywhere in the circuit, locally, there is an electric field that causes a steady net flow of charge.
So from the point of view of a circuit. the battery provides potential potential energy physically moving the charges from a point of low to high potential. As the electrons move electrons around the rest of the circuit also begin to move due to what you just stated. When electrons pass a resistor they lose this potential energy, but they still physically moving because other electrons being gained potential energy at the source cause them to move. As long as there is a source of potential energy the electrons will the electrons keep on moving?

Also do the electrons begin flowing because they are physically moved by the energy provided at the battery or is it because of the potential drop at a resistor, or is it both? Really what I want to understand now is what causes the electrons to physically flow at the beginning.
DrZoidberg
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Feb6-13, 11:50 AM
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There are only two ways to "push" an electron. With an electric field or by quantum effects. A chemical reaction is a quantum phenomenon.
If you have an alkaline battery there is some zinc in there that dissolves. Zinc atoms at the surface of the metal give of electrons and turn into positively charged zinc ions. That means the zinc metal that's not dissolved yet gets negatively charged. There is an electric field inside the battery. The field lines point from the alkaline solution to the zinc metal. That field slows down the process of the zinc dissolving. When that field is strong enough the reaction stops completely. There is a second reaction taking place at the positive terminal of the battery which consists of manganese dioxide. In that reaction however electrons flow from the electrode into the solution to help form OH- ions. The voltage between the positive terminal and the solution plus the voltage between the solution and the negative terminal is usually equal to about 1.5V because at that voltage the electric fields inside the battery are strong enough to stop the chemical reactions.
Outside the battery the electrons flow from the negative to the positive terminal because they are being pushed by an electric field.


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