# Why doesn’t current decrease when it passes through a resistor?

As far as I know current is the amount of charge passing per unit time.

$$I=q/t$$

When current passes through a resistor, the resistor resists the flow of current so the amount of charge going to the resistor and leaving the resistor is the same but the time component increases, i.e. it takes more time for the charge to travel through the circuit.

Maybe I'm missing something here.

My question is related to If the current is increased, is there more charge flowing or is it moving quicker?

If the current is larger more charge flows through the resistor per second.

What you are missing is the fact that the current through your resistor is controlled not only by the resistance of your resistor but the circuit elements of a complete electrical circuit of which your resistor is one part.

Your resistor not only has an effect on the current passing through it but also the currents passing through other circuit elements to which it is connected.
In the end as there is no source or sink of charge within your resistor, the current entering your resistor coming from the circuit to which it is connected to is equal to the current leaving the resistor and that current is going into the circuit to which your resistor is connected to.

This is what you are missing here:

Let's model your system as a water hose where a small length of it is constricted in diameter. We'll consider that constricted length to be our resistor.

The flow of water through the hose is analogous to the flow of electrical charge through a circuit and the pressure responsible for squeezing the water through the resistor in the hose is analogous to the voltage that is pushing the charge through the circuit.

Since our hose has no leaks, every bit of the water flowing into one end of the constriction has to flow out the other end. Analogously: charge is conserved, so all the charge flowing into one end of the resistor in the circuit has to come out the other end.

This is the reason that a resistor does not make the current flowing through it disappear.

We know that charge on the conducting material(resistor) is zero. Then amount of charge flowing into= amount of charge flowing out of resistor. Hence rate of charge flowing in = Rate of charge flowing out. Therefore current doesn't change.

• I have mentioned the same , the charge does not stay there but it moves slowly due to restriction by resistor. Causing time component to increase – Chemist Jan 13 at 7:29
• If we are able to store the charge continuously then current is not same on both sides. – Naga Sandesh Goli Jan 13 at 7:30
• I agree that the charge does not stay there but it moves slowly – Chemist Jan 13 at 7:31
• We know that i=neav (v=drift velocity). In resistor 'v' decreases, 'n' increases. All the variables adjust themselves. – Naga Sandesh Goli Jan 13 at 7:33
• So you are saying the decrease in speed is compensated by increase in number of electrons ... but how did they suddenly appear there – Chemist Jan 13 at 7:35

Imagine water draining a tank though a hose.

The amount of water in the hose is constant, so what goes in comes out.

That’s true whether the hose is large and free flowing with a lot of water moving, or small and constructed with only a little flowing.

The flow of water (analog to the current)does not decrease from the input to the output of the hose.

• See charges going inside are coming out I totally agree with that, but current is not an object it’s measure of rate of flow of charge so my argument is that rate of flow of charge decreases due to resistance – Chemist Jan 13 at 7:52

The image below, courtesy of Professor Richard Fitzpatrick, from the University of Texas at Austin, shows a resistor R connected in a simple circuit to a battery, which has internal resistance r. The battery supplies an (almost) constant voltage E, and by Ohms's law, the current I in the circuit is given by $$\mathrm{I=\frac E {R+r}}$$

If the resistance of R is increased the current in the circuit decreases, and vice versa. But for a given resistance the current is constant over the whole loop. That's because charge is conserved, so the charge flowing into any point of the loop must equal the charge flowing out of that point.

So yes, the resistor restricts the current, but it does so over the entire loop. If the current leaving the resistor were lower than the current entering the resistor, that would mean that charge was somehow leaking out of the resistor (or being destroyed by it!), and leaving the circuit. The resistor loses energy, by emitting heat, but it doesn't lose charge.

Maybe this could help. As a first approximation consider the Drude/free electron model

Conductivity = $$n e^2 \tau / m$$

$$n$$ is electron density. That changes from material to material. $$e$$ and $$m$$ are electron charge and mass which are universal constants $$\tau$$ is the mean time between collisions. This could change with temperature, addition of impurities etc.

When you have a higher resistance you have lower current . It could be because it has a lower $$n$$ or smaller time between collisions.

When you say less charge or more time you have to look at exactly the situation. Compared to what? If you are replacing a carbon resistor with another of higher value, actually neither $$\tau$$ nor $$n$$ change. Only dimensions. If you are changing the temperature, ( to a first approximation) $$n$$ does not change. If you replace carbon with copper both change.