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voltage drops at the physical level

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faradayfan

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voltage drops

I don't really understand the concept of voltage drops in a resistor at the physical level. I was looking into voltage dividers and I was surprised to learn that it is not the value of the resistor in ohms that determines the voltage drop but the resistors proportion of the total resistance in the circuit. If power is dissipated in a resistor should it not be in proportion to how great the resistance is regardless of how much resistance there is in the rest of the circuit? Is it the fact that only the total resistance is what is instantly 'seen' by the electrons flowing have anything to do with this?

Also I don't understand how at the physical level voltage goes down after going through a resistor. The electrons hit atoms and thus are slowed down, but how does that translate into lower voltage? Isn't voltage at the physical level just a measure of the difference in the amount of electrons on each side of the resistor, and if that is true and the same amount of current flows out of the resistor as into it, shouldn't the voltage be the same on both sides?
 

how much voltage is lost through water

There is still the output impedance which forms a further divider with the load.

One way to think about this is that there is a total resistance across the source. This draws a current. The current through the resistor at the bottom produces a voltage.
 

voltage going through a resistor

Personally I think that for an absolute beginner, it'll be more easy to understand conductance instead of resistance.

If you think of voltage and currents as liquid through pipes, it might be more clear.

If two ideal pipes with liquid of different pressure where conected together with a long thin non-ideal hose, somewhere in the middle of the hose, the presure will be somewhere between the pressure in the main pipes. That again all depends of the resistance of the hose.

The resistance in the hose will depend on its length, friction and the thickness.
 

voltage drop after resistor

Is the following a true statement?

Voltage is dropped through the resistance because fewer electrons are exiting the resistor than originally entered it.

Added after 18 minutes:

I think the last statement is false. I don't understand though, if all the electrons get through the resistor why is a voltage drop registered? Where was the energy lost to, and how does this loss of energy manifest itself in a lower voltage? Are the electrons going slower?
 

physical level

Kirkoff's current law hold in all situations. Electrons go in complete paths and do not get stuck along the way.
 

why current increases when voltage drops

RULE 1: Law of conservation of energy-Energy can neither be created nor be destroyed can only be transformed from one form to another.

RULE 2: Current through a resistor in series is same.


faradayfan said:
I don't really understand the concept of voltage drops in a resistor at the physical level.

The voltage drop is nothing but reduction of voltage in a circuit. If we connect a resistor in series with a voltage source. We knw rule 2. And current is nothing but
dQ/dt... i.e no of electrn moving per unit time. now that speed is same in serirs connection. So when electrons enter the resistence it collides with the immobile electrons, Metal lattice, rough structure and repulsion from other electrons inside the resistance. However the moving electrons (constituting the current) has to move through the ckt as some positive force is acctracting them there(Voltage difference phenomemon). so they loose energy while traveling thru the resistor in form of heat. Rule 1 applies. Now by ohms law
V=IR
If the voltage between the conductor and a fixed reference point is measured at many points along the conductor, the measured voltage will decrease gradually toward the load. As the current passes through a longer and longer conductor, more and more of the voltage is "lost" (unavailable to the load), due to the voltage developed across the resistance of the conductor. That means since theres a large voltage developed across resistance, the
Total voltage minus this voltage across resistor is available after resistor... Thus voltage is said to be dropped across resistor
The local voltages along the conductor decrease gradually from the source to the load. If the load current increases, the voltage drop in the supply conductor also increases


I was looking into voltage dividers and I was surprised to learn that it is not the value of the resistor in ohms that determines the voltage drop but the resistors proportion of the total resistance in the circuit.

The source of electrons--battery releases as much electrons (responsible for current) depending upon the total impedence of the circuit.
this can be explained as:
If u had to operate a table fan and a celing fan on the same battery power, the table fan will run faster, as it will get more current to its motor(which makes it run fast) since the impedence of the table fan is small as compared to celing fan.
For example, an electric space heater may very well have a resistance of ten ohms, and the wires which supply it may have a resistance of 0.2 ohms, about 2% of the total circuit resistance. This means that 2% of the supplied voltage is actually being lost by the wire itself.
The total electrons(thus the current) are release by battery depending upon total impedence as i said b4. now consider that the main wire forges into 2 paths-path A(5 ohms) and B(15 ohms).
now current will be released upon the combinition of those equivalent resistance (depends if they r connected in parallel or deries or so...)
Now current (electrons) choose the path of minimun impedence (opposition)
Electrons going to some path will depend upon the ratio of the resistance offered by that path to the total resistance of the circuit. this ratio is because the electrons are unintellegent and will decide where to go depending upon the resistance and which has to be compared to something. that something has to be total resistance as the total electrons that came out were dependent on total resistance.
So electrons going to path A will be 5/(total impedence=5+15)



If power is dissipated in a resistor should it not be in proportion to how great the resistance is regardless of how much resistance there is in the rest of the circuit? Is it the fact that only the total resistance is what is instantly 'seen' by the electrons flowing have anything to do with this?
I hope even this is clear to you now.

Also I don't understand how at the physical level voltage goes down after going through a resistor. The electrons hit atoms and thus are slowed down, but how does that translate into lower voltage?
They are not slowed down if in series since I=dQ/dt=speed remains same. the voltage across resistor is high so linear addition makes the voltage at end less by the value it is lost at the resistor place.
in parallel I=dQ/dt=speed depends upon the resistance ratios


Isn't voltage at the physical level just a measure of the difference in the amount of electrons on each side of the resistor, and if that is true and the same amount of current flows out of the resistor as into it, shouldn't the voltage be the same on both sides?

Voltage is not the difference of the amount of electrons. Though the amount of electrons are same going in and out...voltage is something numerical which you get when current is multiplied with resistance.
In the water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage difference is likened to water pressure difference – the difference determines how quickly the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time (volumetric flow rate). The flow rate is determined by the width of the pipe (analogous to electrical resistance), and the pressure difference between the front end of the pipe and the exit is analogous to voltage. The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (amperes x volts = watts).
 

    faradayfan

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is voltage the same on both sides of a resistor

Woooow, congratulations to this bulky explanation !!
 

Thanks for that explanation. That's interesting that you said the electrons don't slow down, but if they are colliding with more atoms won't that make them slow down?

I understand the idea of conservation of energy and how if P=IxV and energy has been transformed into the heating of the resistor, then power must of have been dissipated and since the current is the same that leaves the voltage as the component that is less after going through the resistor.

I guess I'm having a problem picturing the nature of voltage. Using the water analogy, I would think that water going through some restrictions (resistors) would have the same pressure as it came around and was just about to go back into the pump (battery). I have a feeling though that water pressure is less after going through some restrictions although I'm not sure why that is either!
 

Thanks for that explanation. That's interesting that you said the electrons don't slow down, but if they are colliding with more atoms won't that make them slow down?

I'll try to explain you in some comical fashion.
Guess when there is a open ckt.... what would the electrons do.
Few gutsy electrons chat wid friend electrons and say..com'on lets rome. they set to go. they go...go...and...go.. and suddenly they discover that there is a broken wire JUST AHEAD. Nw can they decide to go back???

Okie for that matter lets consider going back... They were out of the -ve terminal, right? Now will that same -ve terminal co operate with them?? (I mean due to force of repulsion, -ve plate of battery doesnt take them inside again.

Now you tell me what to do in such situation???

Cant do nothing. So electrons are pre warned of everything. When i say electrons comin out of -ve term. depend upon total resistance, that means they are aware of thr resistances in their path. The current comin out is predecided.

You can argue with yourself that does a grp of electron decides how much to pass thru resistance when resistance is next to them or do they decide where to go when there is parallel connectn and stop at the junction???



Now lets understand the second part.
When i say current (electrons) flow through circuit, it is not that the same electron must flow through the same circuit.
A metal consists of a lattice of atoms, each with a shell of electrons. This can also be known as a positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.

Now when an electron comes from outside it adds to this sea and same amt of electrons are thrown out of the sea. Keeping the speed constant. But dissipiating heat.
Now if i say that the speed remains constant,where from this surplus energy in form of heat comes from. It comes from the battery.. That is why battery dies/drains one day.. It this was not the case, one, Law of conservation would be broken and two no battery would ever have drained.

Extend it to.. yourelf, walking on straight road and hill. On hill, u have more resistance, more energy, infact heat frm your body is disipiated. If you had to match th speed of walkin on hill and straight path, you would had to put in more energy and you will be quickly drained....!! Isnt it interesting.


Strech this idea to all what you knw in electronics.. coz even i have learned this all thru trials.. If in case something doesnt fit in this explanation let me know.


I understand the idea of conservation of energy and how if P=IxV and energy has been transformed into the heating of the resistor, then power must of have been dissipated and since the current is the same that leaves the voltage as the component that is less after going through the resistor.

I didnt get this.
 

    faradayfan

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