# Need help with understanding resistance

B4ssHunter
so i was studying resistance and there is this something that i can't really understand
imagine a circuit with Potential difference V and resistance R
it has like say 3 resistors , now How is the current intensity constant ?
if the resistors decrease the potential energy , after the 3rd resistor shouldn't electrons actually stop moving because there is no potential energy anymore ?? since all the potential energy has been used up by resistors ?
the only way to think of this is if the whole circuit design is not physically accurate , and that those resistors drawn are just symbols for the resistance across the whole wire not just those places , this would make sense . is this true ?

eigenperson
Potential and kinetic energy are different.

The electrons in the circuit are all moving at the same speed. If something, like a resistor, tries to slow them down, some of their potential energy has to be converted to kinetic energy to bring them back to the original speed. After the electrons leave the last resistor, they have no* potential energy remaining, but they still have the small amount of kinetic energy they need to keep moving at the right speed.

*In reality the wire has some resistance, so the electrons still have a little bit of potential left -- just enough to compensate for the energy they lose in the wire. But even if the wire were genuinely of zero resistance, then the electrons could simply coast back to the battery on their kinetic energy, so they wouldn't need any potential energy.

B4ssHunter
Potential and kinetic energy are different.

The electrons in the circuit are all moving at the same speed. If something, like a resistor, tries to slow them down, some of their potential energy has to be converted to kinetic energy to bring them back to the original speed. After the electrons leave the last resistor, they have no* potential energy remaining, but they still have the small amount of kinetic energy they need to keep moving at the right speed.

*In reality the wire has some resistance, so the electrons still have a little bit of potential left -- just enough to compensate for the energy they lose in the wire. But even if the wire were genuinely of zero resistance, then the electrons could simply coast back to the battery on their kinetic energy, so they wouldn't need any potential energy.

i have read some kind of an explanation , would you care to assure me that i am right ?
i have read that for an electron to move it has to push all the electrons in the wire , so it pushes electrons through the 3 resistors , that's why an electron at the beginning of the circuit faces the resistance of the resistors that it hasn't reached , now when it passes the 1st resistor , it has less speed , but it also has less resistance to push electrons through , and so on , in the end , when electrons have passed three resistors , they have no resistance anymore , so they can easily be pushed by electrons behind them who still have some potential difference * stating that the wire is super conducting *

so i was studying resistance and there is this something that i can't really understand
imagine a circuit with Potential difference V and resistance R
it has like say 3 resistors , now How is the current intensity constant ?
if the resistors decrease the potential energy , after the 3rd resistor shouldn't electrons actually stop moving because there is no potential energy anymore ?? since all the potential energy has been used up by resistors ?
the only way to think of this is if the whole circuit design is not physically accurate , and that those resistors drawn are just symbols for the resistance across the whole wire not just those places , this would make sense . is this true ?

The circuit design is physically accurate it just your idea of how electrical energy moves in a circuit is not accurate. The potential and kinetic energy of the circuit is not carried by the electrons but is in fields that surround the wires and resistors but for now let stick to basic circuits and the flow of charge.
http://www.physicsclassroom.com/class/circuits/u9l4a.cfm

Gold Member
Assume that as many electrons enter as leave the resistors. The current flow is constant throughout the circuit. The applied potential can force a limited number of electrons through the resistors, and so there is a potential drop distributed across each resistor (resulting in dissipated power). The two extremes would be an infinite number of resistors, or 1 resistor. There is no difference. The number of coulombs per second is constant at all points in the circuit and determined by the ratio of potential and resistance. The electrons don't really know what the voltage is at any point in the circuit. They just flow.

Be careful about the difference between electrons and current. Your model of an electron entering causing an electron to leave is somewhat correct. Like pingpong balls in a pipe (people here probably hate that analogy).

Resistors do not slow down electrons. Resistors just increase the potential required to force a given number of electrons per second (current) through a circuit.

B4ssHunter
Assume that as many electrons enter as leave the resistors. The current flow is constant throughout the circuit. The applied potential can force a limited number of electrons through the resistors, and so there is a potential drop distributed across each resistor (resulting in dissipated power). The two extremes would be an infinite number of resistors, or 1 resistor. There is no difference. The number of coulombs per second is constant at all points in the circuit and determined by the ratio of potential and resistance. The electrons don't really know what the voltage is at any point in the circuit. They just flow.

Be careful about the difference between electrons and current. Your model of an electron entering causing an electron to leave is somewhat correct. Like pingpong balls in a pipe (people here probably hate that analogy).

Resistors do not slow down electrons. Resistors just increase the potential required to force a given number of electrons per second (current) through a circuit.

wouldn't that mean that before the electron reaches the resistor , it would have infinite intensity since there is no resistance *in the hypothetical circuit where the resistance only exists in the resistors * ? i understand that its constant across resistors because as the resistance increase the voltage drop also increases , but what bothers me is not the resistor , but the length of wire where the electron travels before reaching the resistor . if there is no resistance there then how could electrons still travel with the same intensity unless the electrons really push each other as i said before ?
i also wonder why would the potential drop affect the current if the current is already traveling by the potential difference between the two terminals * which is the voltage of a battery * , it seems like a potential drop is only because the potential energy of the current was used due to resistance

Gold Member
Ok, I'm forced to use the water/hose analogy. Water flows in a hose, but the nozzle limits the flow. The more pressure, the more water flows out the nozzle. How does the water know there is a nozzle down stream. It's an analogy with flaws, but will help you think about it.

"unless the electrons really push each other as i said before ?" <--- That's a valid analogy

B4ssHunter
Ok, I'm forced to use the water/hose analogy. Water flows in a hose, but the nozzle limits the flow. The more pressure, the more water flows out the nozzle. How does the water know there is a nozzle down stream. It's an analogy with flaws, but will help you think about it.

"unless the electrons really push each other as i said before ?" <--- That's a valid analogy

well they know because it builds up pressure , the nozzle builds pressure on water next to it and the water next to it apply pressure to nearby water and so on , my analogy was right thank god :P

snorkack
Compare with water flow in a channel or pipe.

If you send water to sea, then even if you send huge amounts of water there, the sea will not rise noticeably. Sea generally away from the rivermouth will not rise much because the sea is big and it would take a lot of water to raise it; sea in the immediate vicinity of rivermouth will also not rise much, because being wide and deep it has very little resistance to flow - there will be slow current away from rivermouth in all directions, but the slope of water will be very small.

If you look at a channel where no water is lost or added, and the water flow is steady, then there is a constant rate of water flow. The linear speed of flow gets fast where the channel narrows or shallows, and slows down where channel is widened or deepened into ponds, but the amount is the same everywhere.

If the water flow is unsteady, like suddenly increased, then there might be different amounts of water flow along the channel, because some water is, e. g. filling ponds and not flowing on at the same rate. This applies mainly in case there is a lot of capacitance to fill. If you are dealing with circuit of little capacitance, like closed pipes full to the ceiling (even no air headspaces to compress) then the capacitance is very small - water is hard to compress, and attempting to do so causes a strong water hammer.

In electric circuits, wires do have small capacitance, but it is relatively small - piling up electrons anywhere causes strong electric field potentials. So, the electric current has to be mostly constant along a series. And if the current is steady then no capacitances are charging and discharging, so the current in series has to be constant even if there are appreciable capacitances present.