How does voltage work in a circuit and why does it drop through resistors?

[member 529879]

I have a few questions about how a series circuit works. I've attached a picture of a series circuit on this post. First of all I'm a little bit confused about voltage. If I'm correct, voltage is electrical potential. For example there is a voltage between two electrons being pushed close together. How is this also true for a battery? The positive end of a battery doesn't actually have an electric field associated with it, and the same applies to the negative end. Is it the chemical energy in the battery that somehow creates a potential difference? Also how do resistors work? What is it that allows them to slow down current? Finally what is voltage drop and why does it occur?

 

[Drakkith]

Is it the chemical energy in the battery that somehow creates a potential difference?

That is correct. A battery is composed of electrochemical cells. In each cell a chemical reaction creates the potential difference that drives the current through the circuit.

Let's consider a lead-acid battery, which is a standard car battery. Bear with me here, as I need to explain some chemistry stuff.

In each cell there are two electrodes and an electrolyte solution. One electrode is plain old lead, or Pb. The other is lead dioxide, or PbO₂. (Dioxide is two oxygens, or O's, hence the two in PbO₂) The electrolyte solution consists of sulfuric acid that has been mixed with water. The acid reacts with the water to form two charged species, hydrogen ions, H⁺ (a proton with a charge of plus one), and SO₄^2- ions (Known as 'sulfate'.The 2- is read as "two minus" and represents a charge of negative two), which are distributed throughout the water.

On the lead electrode the lead reacts with the sulfate in the electrolyte solution and forms lead sulfate, PbSO₄, which builds up on the electrode. But there's a problem here. The lead atom is initially neutral while the sulfate has a charge of 2-. So when the reaction occurs two electrons are given up by the lead to the electrode and neutral lead sulfate is formed.

On the lead dioxide electrode, lead dioxide reacts with sulfate to also form lead sulfate. However, the key difference here is that lead dioxide has two oxygen atoms. Oxygen likes electrons. Oxygen really likes electrons. It likes them so much that when the reaction occurs each oxygen atom takes 2 electrons more than in needs to stay neutral, resulting in 2O^2-. Then each of these oxygen atoms reacts with two protons in the electrolyte solution to form plain old water, H₂O. Meanwhile, the lead atom is left with 4 missing electrons. Two of these missing electrons are replaced by sulfate (since sulfate is 2-), but that still leaves a charge of 2+ that needs to be neutralized. So where do these 2 electrons come from?

The other electrode!

The reaction on the lead electrode causes the lead to give up electrons, which build up and create an electric field which creates a potential difference. On the lead dioxide electrode electrons are taken from the circuit and given to the lead sulfate, which also creates an electric field. The combination of the fields generated by the two electrodes is what drives the current through the circuit.

All batteries work this way. In each cell, one electrode reacts and gives up electrons while the other reacts and takes electrons.

Also how do resistors work? What is it that allows them to slow down current?

It boils down to two things: number of mobile charge carriers (free charges) and the interactions between these charges and the ions and electrons that aren't free.

In order to be a 'free charge', an electron must exist in what's known as the conduction band. The conduction band is a range of energy levels in a material where any electron in this band is 'shared' between the atoms of the material. In conductors, the conduction band overlaps with the 'valence band', which is the energy levels of their outermost electrons. This means that almost every atom in a conductor has at least one electron that is free to move about the material. Because of these free charges, a conductor responds to an applied electric field with lots of current flow and exhibits very little resistance.

In insulators the conduction band is separated from the valance band and does not overlap it. Electrons can be excited from the valance band into the conduction band even when they don't overlap, but this requires energy to do so, and if lots of electrons in an insulator are being excited into the conduction band that usually means that breakdown voltage has been exceeded and the insulator is simply breaking down. Note that even insulators have some free electrons which are free to move around, usually having been excited into the conduction band by thermal energy. However the number of free charges in an insulator numbers only a tiny fraction of those available in a conductor. Still, this is at least part of the reason that even insulating materials conduct electric current to a small extent, and explains how insulating materials can be used in resistors, which allow at least some current flow.

In addition to the number of free charges, one has to look at the interaction between these charges and the rest of the material. The simple explanation is that as free electrons move through the material they collide with the ions or non-free electrons, giving up some of their energy, which is then turned into heat. Hence why a resistor both resists current flow and heats up when current passes through it.

Also, don't think of current a 'slowing down' or 'speeding up'. Think of it as a number of charges passing through any point in the circuit over time. This can be either a few charges moving rapidly, or a large number of charges moving slowly, so knowing the amount of current tells you little to nothing about the velocity of the charges.

Finally what is voltage drop and why does it occur?

Voltage is defined as the difference in electric potential energy between two points. (Well, there's some other details in there, but for now just think of it as that)

Consider a single electron placed between two oppositely charged electrodes. If we place the electron near the positive electrode it will accelerate for a short time, gaining kinetic energy, and then be captured by the electrode. If we instead place the electron near the negative electrode it will accelerate for a much longer amount of time, gaining much more kinetic energy by the time it reaches the positive electrode. So we can say that each point we placed the electron has a certain amount of electric potential energy, with the point near the negative electrode having more electric potential energy than the point near the positive electrode. The difference between the two values is defined as the voltage between those two points. So we can say that between points A and B there is X amount of electric potential energy, with X being measured in volts.

Since voltage represents energy, and energy is lost as current passes through a resistive component, some of the voltage is lost as well. We call this loss of voltage, or loss of electric potential energy, voltage drop. There is less energy available to the circuit after a resistor than before the resistor, as some of the potential energy has been turned into heat by the resistor.

 

[phinds]

Gads you do go on !

Nice post. I admire your willingness to spend the time to give such extensive answers.

 

[Drakkith]

I'm just happy I was able to use what I learned in my chemistry class this semester to explain how a battery works!

 

[PFuser1232]

I'm curious, why was this moved to the Electrical Engineering section? Isn't circuit theory a part of Classical Physics as well?

 

[Drakkith]

I'm curious, why was this moved to the Electrical Engineering section? Isn't circuit theory a part of Classical Physics as well?

Electrical engineering is specifically focused on electrical circuits. Classical physics is broader.

 

[PFuser1232]

Electrical engineering is specifically focused on electrical circuits. Classical physics is broader.

So in a way, EE is a subset of Classical Physics (or another set with a pretty big intersection), excluding microelectronics, transistors, and semiconductor physics of course, right?
I'm curious because I am probably going to major in EE and I want to know to what extent it's related to physics.

 

[Drakkith]

EE isn't a subset of physics, it's a subset of Engineering, which according to wiki is: the application of scientific, economic, social, and practical knowledge in order to invent, design, build, maintain, research, and improve structures, machines, devices, systems, materials and processes.

EE uses both classical and quantum physics along with relativity. Hence engineering gets its own forum and subforums here a PF.

 

[member 529879]

Thanks for the answer. Why does voltage only drop through resistors? I would think that potential energy decreases as the electron moves at any point through the wire like the potential energy of an object falling would. Also is the formula v = ir exact or is it an approximation? For example, if you have 10 volts and 5 ohms of resistance, would you always have exactly 2 amps of current? Sorry if these are stupid questions, I don't know much on the topic.

 

[Drakkith]

Thanks for the answer. Why does voltage only drop through resistors? I would think that potential energy decreases as the electron moves at any point through the wire like the potential energy of an object falling would.

Voltage drop occurs anywhere that energy is taken from the circuit. This includes all wires, resistive loads, and even non-resistive loads like a transformer.

Also is the formula v = ir exact or is it an approximation? For example, if you have 10 volts and 5 ohms of resistance, would you always have exactly 2 amps of current? Sorry if these are stupid questions, I don't know much on the topic.

For normal, everyday circuits you can treat it as exact. Things change once the energy scale or size scale changes to the very, very large or very, very small.