Voltage Drops across Resistive Elements

Ever studied KVL ?

Think from the angle of the moving component - the current.

As per KVL - the total voltage in the circuit is zero.

Talking of the resistive circuit and no branching (series cinnection)

You have total voltage equation as

E - V₁ + V₂ + .. +V_n) = 0

E - ( iR₁+iR₂+... iR_n) = 0

Where

V_x = Voltage drop across first resistor = iR_x .

As you can see, Larger is the resistance R_x more is the drop across it (iR_x)

It is obvious that yous are not one of the field or an elementary student in the field.

Think on mechanical terms

A ball has been given a motion from one side and passes through different zones (sand, gravel etc)

Each puts different friction (Resistance) to the ball

As it moves through, which zone will have the velocity reduction maximum ?

Obviously in the region where the friction is maximum.

Similar example is for a pipe line flow.

In this the Pressure is the voltage and the flow is the current.

The reduction in pressure (pressure drop) is maximum in obviously the pipe where the friction is maximum isn't it?

Same thing happens here.

Hope it clarifies.

PS: closed pipe line flow is one of the best analogies for the electrical DC circuits and vice versa (with the power source = pump ) It can even take care of the series and parallel connections.

Ok,you are walking through a series of doors. Each door has a spring load creating force against your attempt to open it. The amount of tension on each spring is different, some are stronger and some are weaker. As you pass through each door you must exert a different amount of pressure against that individual door to open and pass through.

Now imagine the doors are resistors of varying values and the person pushing them open is the voltage (electricity) passing through the circuit. The electricity must exhaust a different "electrical pressure" on each resistor or load in the circuit in order to pass through to the next.

Got it yet or do I need to simplify? Honestly, no insult intended but I explained this to my 6 year old son and he got it fairly quickly.

I understand everything you guys explianed, and I know it sounds like a stupid question but it actually is not. I am trying to visualize it on an atomic scale of collisions but it doesnt seem to make sense....

yes the math supports it...I am familiar with it all. But still I thought that an electron was subject to the entire electric field of the applied voltage and not just pieces of it like in KVL.

Thanks

Aaaarrrrgggghhhhh!!!!!
You've missed the whole point of visualizing something!!!!

It's to help you UNDERSTAND....
If it doesn't help you understand it's either, wrong, innapropriate..or a waste of time.

I suspect the latter in this case!!!

Del

No I understand and can visualize it ...for some reason I just don't see it working out that way.

Buddy,I don't think you stated your question properly to begin with.You made it seem as though you didn't understand why there are voltage drops across resistors. now you are going down to the atomic level.Why would you even need to know this?

Resistors cause a "back pressure" against the free movement of the electrons.Thus creating the voltage drop across the resistor. ON ANY LEVEL.

(I don't even drink coffee and I think I may be hallucinating this whole thread)

I guess I did need to simplify.

No I understand and can visualize it.

So you asked completely the wrong question, and lured me into wasting considerable time in answering it?

I'm afraid I shall unsubcribe to avoid being jerked about further.
Del (the extremely irritated Cat)

On the atomic and sub atomic level, the electron doesn't care... Quantium mechanics is a pretty messed up world as it is... lol

An electron - if it has a complete circuit - it will flow from one hole to the next and so on till it gets back to the power source. As for collisions - I don't believe that they happen, because current is said to 'flow' and from what I've learned and noted above - the electron 'hops' or 'flows' from one hole to the next.

Hole = Missing electron of an atom at the atomic (sub-atomic?) level.

Please revisit with the following.

A 1 ohm resistor has 1 "obstacle" for your quantum electron.

A 1k Ohm resistor has 1000 obstacles for your quantum electron.

A 1 meg resistor has 1000000 obstacles for your quantum electron.

In order to "pass" through the 1 ohm resistor, the electron only has to pass through one obstacle and it's then free to move on. (It thus uses only one unit of effort)

In order to pass through the 1 meg resistor, it has to pass through 1000000 obstacles before it's free to move on. (Just like there were 1000000 single 1 ohm resistors inside there and so uses much more effort.)

Thus if connected in series you will see less "effort" at the low value resistors and much more "effort" at the high value resistors.

I hope this helps you.

really helps thanks. now lets say there is not just one unit of effortt (source voltage) across one abstacle (one Ohm resistor) but rather two units of effort across one ohm. The result would be more current...similar if I was to reduce the obstacle in half.

What is the limit to how much current can flow per one unit of effort (voltage)? and what would be the voltage drop if there was 0 obstacles?

Depends on how the source voltage units are connected to each other and to ground.

well if the resistance is hyphothetical 0 the it would be voltage devided by 0 and that would mean a lot of smoke if you would not use a laberatory voltage generator

I have just re-read this post and see that this update to the original question was directed directly to my post.

The limit to the amount of current that can flow in any material relates to the physical properties of each material that is involved and the specific environment that it is in at that time.

Current flow through the material causes heating. (energy used as the electrons encounter obstacles in the atomic structure and then are re-accelerated by the electric potential) Some of that heat can be dissipated into the surrounding environment, while some will remain causing the conductor to heat up. (The exact application determines how this happens and that's why there are different cable specifications for whether cables are burried, loose in cable trays or bundled into multistrand arrangements.) As the conductor heats up, the resistance usually increases. (The particles in the material matrix are vibrating more and so there is greater chance of electron collision during transit through the material) Then, depending on the voltage applied one of two things happen. Either the conductor reaches a steady state condition where heating frm resistance equals heating loss to the surrounding environment (Like a light bulb filament), or the conductor melts and the system then physically fails (Like a fuse).

Thus, there is no single figure of "how much current can flow".

However, as others have stated, IFFFFF there was TRULY zero resistance, then there would be zero voltage drop. If you want more on this concept, then have a read on "superconductors" where they are trying to find materials with zero resistance properties for some interesting uses.

Keep it EASY !

Every book about electrics starts with OHM's Law. U = I x R

where U= voltage, I= current, R= resistance.

Believe it or not, but this law is universal, works on EVERY level of electronics, either if you talk about Mega volts or pico amps. It is valid on the bottom of the ocean, but also true on the moon. It was valid in the time of the dinosaures, and will be valid in the 2,000th century ... ALWAYS the same law.
When I went to school, our teacher for electrics, told us that we should know this law better than our own name ... it is the most important rule in electric/electronic technology.
And your atoms and molecules ? well : also those guys follow that rule. I think that if you are doing research on that level, you should start to try to understand this easy law of Ohm.

Ohm's "law" is a good approximation to current flow across a resistor. It is neither a true law nor is it universal. Think about a diode...

Just try to start more trouble?

For those who don't understand - ohms law is universal for its intended purpose and when used properly. A diode does not act like a simple resistance value so it will not work in the equation.

E = IR

The components are Voltage, Current and Resistance. The diode you mentioned is a semiconductor with a non-linear voltage to current relationship - clearly not a pure resistance.

Umm. No.

V = IR

or, if you prefer:

V = IZ

or:

Or:

Or any other form you want to put it in.

http://en.wikipedia.org/wiki/Ohms_law

If you're going to make bold statements like that, you had better post some math to back it up. (Like, a LOT of math). I would be very interested to see a proof where somehow your Voltage is no longer equal to your current times your impedance.

please read through my earlier thread (where there is an analogy) .

What you apply is the voltage (Pressure - some times I remember this word was used even in my old engg class also electromotive force (emf) )

What flows is the current.

so more obstacles do not change the current - it changes the pressure frop (ie the voltage drop) across obstacles.

Even that analogy is applicable in your atomic (or electron stage)

What is a resistor ? in an atomic stage ?

In a resistor there are less number of free electrons (what we call as conduction band) and most of them are in the valency band -

Assume it as the pipe line with lot of grit (or the artery choked with chlesterol ) now as the voltage (pressure) is applied the electrons have to flow, while flowing, it tries to pull out some from the valency band (relatively unbound) this energy to pull out is dissipated as heat in it but that is another story (jumping on and off from the conduction to valency band and vice versa)

Since the flow is getting restricted by gravel (remember it is a closed pipe flow and hence what ever pump give output in terms of quantity must reach teh end. Also it is a constant pressure pump so the pressure drop becomes distributed - not the flow (current) - more restriction - more pressure drop and less restriction less pressure drop.

The analogy works even in electron level.

Thats more like it.

1. So you can only have voltage drop when current is flowing right?

2. Whats the deal with a diode? why doesnt it act like a resistor? How many other kinds of circuit elements are there that do not follow Ohms Law?

3. And you guys still didn't answer what the maximum limit amount of current is per unit of "push" assuming 0 resistance...... If Ohms Law doesn't apply here...why? what does? and is there a voltage drop associated with this?

I get it but I really hate it when math is the explination, because math just describes what is happening not explain why.

A diode, unlike a resistor, is a non-linear resistance device, i.e. its apparent resistance varies, depending on the voltage across it, in an exponential manner whereas, the resistance of a resistor is linear with voltage. That is, a theoretical resistor's resistance does not vary, a real world resistor does vary some with power (I squared time R) and temperature. But let's try to keep this simple.

A diode is a complex resistance, unlike a resistor, both in theory and in the real world. It would be difficult to present a comprehensive explanation of a diode's curve in a short manner.

As explained a diode is another matter- the sor semiconductors, the firbidden gap (the energy required for an electron to leave its atom and become free) is very low.

These are non-linear devices and at a certain voltage across it, the electrons are simply shifted to the valency band (I will call it like fluidized bed - so at a certain pressure, the solid gravel or as we call in boilers coal) starts floating and acts like a fluid and no more a solid. Thus at that pressure drop across it, the diode is a conductor and this property is seen in all the semiconductors why only diode ?

And for Zero resistance (ie superconductor) there is no maximum limit of current - all the electrons available are now in a free state and are able to transmit current.

It may also be understood that the electrons entering a conductor are not the ones going out (just like incompressible fluid flow) only the numbers going in are the numbers going out.

In case of this there is no mathematics , it is just a physical phenomenon - a potential is applied across a line just like our home refrigeretaor or blood vessel (assuming - it is kidney isn't it ?) not filtering and rejecting the decomposed blood cells. So in a closed loop through the pump, the fluid (electrons) are circulating.

Only there are a few fellows like Diodes (got to think an equivalent but nothing striking) etc have the constant drop across it (may be a controlled orifice)

And where is limit on flow- the flow volume is only limited by the pipe line resistance (friction) - if there is no friction in the pipeline - no obstruction what so ever who is to stop it from carrying more and more fluid?

Just assume a pipe 1/2" dia - I pump 10lpm through it - you get a pressure drop that is build up by pump to say 0.2 bar

Now I try to push 20LPM and the pressure across it rises to 1.5 bar. I increase the pump pressure and it still pushes it thru.

As I increase it further and the pump can not build a pressure, the flow reduces.

But if the drop across pipe is zero (super conductor) - pump dowen't have to build the pressure (no pipeline friction, no viscous drag) and at the zero pressure i can pump in as much fluid as the pump has capacity to.

(PS: Are you an account ant ?)

You have received much correct information geared to engineering and mathematical theory. Also excellent non-electrical examples to compare for understanding - and still it does not answer your questions. Please realize that you will not completely understand these unless you eventually take some math, physics and/or engineering courses. While we can describe and predict what happens electrically, we really do not know why (sort of like what is life?). Math is just one way to predict what is expected, not why it happens.

That said, maybe this will help. Voltage is the measure of an electric field which has the potential to move (force) electrons from atom to atom which is defined as Current. This current is actually the flow of the holes (vacant electron sites in the atoms), because defined current flows from + to -, but electrons are - charged and are attracted to the + potential. To keep it simple, different materials have different atomic structures and therefore different abilities to allow (conduct) the movement of it's electrons when subjected to a voltage potential.

The simple "resistor" (as long as the temperature does not change it's ability to conduct) will allow the electrons to move in a way that is predictable by using Ohm's Law: The voltage is directly proportional to to the resistance and the current where Voltage (V)= Current(I) X Resistance(R). If resistance is a constant, and voltage is increased, the current will increase etc. You just have to accept this and understand.

Now to hopefully answer your questions:

1. You have a voltage drop whether current is flowing or not. If resistance is infinite, no current will flow, but the potential voltage can still exist. In the case of different simple resistors in series, the total current (I) that will flow with a given voltage applied is predicted by (Ohm's Law) and is limited by the total of all the resistors in series (added). As the same current(I) then flows through each resistor(R1, R2, etc.) in the series, the voltage measured across each resistor is therefore proportional to it's resistance (more resistance, more voltage needed to produce the same current).

2. The diode does not act like the resistor because it is a semiconductor junction of two materials which only conduct (forward) once some of the potential voltage (electric field) is used to free up some of it's electrons. Even after that, it does not follow Ohm's Law because the resistance increases as more voltage is applied. Other elements do not follow Ohm's Law, like inductors (store magnetic energy), capacitors (store electrical energy), and transistors (two diode junctions) which can switch the current off and on based on the current into the junctions. You will need to study these (physics course) to understand why these work the way they do.

3. If the resistance is 0 (or very low, known as a short circuit) and a voltage is applied, the current is instantaneously infinite (or very high). There is essentially no voltage dropped across the short, until something changes: (a) the potential electrical energy field is used up (dissipated as heat) and the voltage goes to 0. Like when lightning stops. (b) the resistor heats up and increases the resistance which in turn reduces the current flow, or overheats and melts, causing an open circuit where no current flows.

Hope this helps. If not, you will have to study more so you can understand more. It is that curiosity and determination which defines and engineer!

Thank You, Guest This really helps. And for the record I appreciate any other efforts made to help me understand. Although many of you believe to have superior knowledge and think my inquiries are a joke or a waste of time, you can go to hell because I really have a great understanding for things and just too curious of a mind that feels the need to dig and dig until there is nothing left, thats all. I have to satisfy this curious mind of mine or I will go crazy.

One thing still eludes me......You said that there is still voltage drop across an infinite resistance.....

How can this be if no current flows?

Doesn't Voltage drop really represents kinetic energy dissipated and not the potential?

Consider a voltage measurment across an infinite/OL resistance in a closed circuit...all you are really measuring is an open circuit right? all you are measuring is the potential of the supply terminals and no such voltage drop...

I think you are not listening to me

If I block the outlet of pump with the perfect sealing pump, there is no flow (current) - but is there or not a pressure diffce across the pump ?

Before trying out of course ensure there is a relief valve / inbuilt pump relief.

But you can always imagine it.

PS: your comment was uncalled for, do you not think, we are explaining or trying to explain the things as best as we can ? Sorry if we have given you the impressions.

The potential energy (voltage) exists whether current flows or not. Picture a battery that is not connected to anything. The voltage measured across it's terminals is just a measure of the electric field strength stored within the battery. This potential energy is dissipated as heat in the resistor (kinetic) only when current flows (resistor connected to the terminals). In the case of an open circuit, no current flows and the energy remains potential. In the case of a short circuit (very low resistance) a large current flows and the potential energy becomes kinetic. Unless the power dissipated (as heat) in the shorting element destroys it, it will continuously convert the potential energy into heat until the potential energy is used up or the voltage is removed (load resistor is disconnected).

Please keep in mind that we engineers don't think like most (no offense intended) and we sometimes forget this. It is easier for us to explain relationships with our most popular tool (math). Just as we both need to remember this, we both also need to develop the patience that someone had with us when they taught us what we have learned.

Best Regards and good luck with your quest for knowledge!

Hi friend, I just give u an example. Try to understand.

Suppose if we 10people attends the food party, then I would take more food than others due to my more hungryness / more capacity to do work / my basic body strategy just like resistor's resistance / filaments output heat / specific resistance.

Just like the above example, highly resistive element takes high voltage drop ( same current * more resistance).

Byeeeeeee

Guest...

India.