# Confusion regarding current and resistance in series circuit

Say we have a series circuit (containing a battery) with charges flowing through it.

1. We add a resistor to the circuit. The current should then decrease. If we add a 2 ohm resistor, the current would become lower than if we added a 1 ohm resistor.

2. Then say we add another resistor of 2 ohms. Based on what I have read, current is the same anywhere in a series circuit containing various resistors. So if we add a second 2 ohm resistor, it would have the same affect on the current as adding a 1 ohm resistor.

But how can both things be true? It seems that the first one is saying that the more resistance there is, the lower the current is. The second is saying that the current is the same anywhere in a circuit no matter the resistance. I have never been so confused in my life.

The overall current will decrease but in a series circuit current through each resistor is same .For example-say before connecting 1 ohm resistor the total current was say 5 amp , now after connecting 1 ohm resistor the current will be less than 5 amp . Let's say that current comes out to be 4 amp .However 4 amp is the current that will be flowing through the entire circuit.

The second is saying that the current is the same anywhere in a circuit no matter the resistance. I have never been so confused in my life.

In a series circuit the current is the same everywhere no matter what the resistance is. But the magnitude of that current in the circuit depends on the total series resistance.

Say we have a 10 volt ideal battery (one with no internal resistance) across which we place a 2 Ohm resistance. From Ohm's law the current I in the resistor is

$$I=\frac{V}{R}=10/2=$$ 5 amperes

Now we add another 2 ohm resistor in series with the battery and the first resistor. The current flowing in both of the resistors is now

$$I=\frac{V}{R}=10/(2+2)=$$ 2.5 amperes.

This current of 2.5 amperes is flowing through each of the two 2 ohm resistors, but its magnitude is half of that if there were only a single 2 ohm resistor.

If we add another, say 1 ohm resistor, in series with the first two the current through all three of the resistors is

$$I=\frac{V}{R}=10/(2+2+1)=$$ 2 amperes

As we keep adding resistors in series the magnitude of the current goes down. But the current through each resistor is the same because all the current that flows into each resistor flows out and into the next resistor.

Thank you for your answer, @BobD! Based on what you're saying, the current stays the same once we stop adding resistors, which makes sense. But I feel like I don't fully understand how resistors work/affect current. Do they slow down charged particles in the circuit as they are flowing through the resistors? Is that how resistors decrease current?

It is a common misconception that resistors "slow down" (reduce the speed) of electrons. If that were the case then the current leaving each resistor in a series circuit would be less than the current entering the resistor, so that the speed of the electrons coming out of the last resistor in series would be less than the speed of the electrons entering the first resistor. If that happened then electrons would be "piling up" in the resistors in between the first and last. That does not occur. The reason is conservation of charge.

To put it in a somewhat simplified manner, what the resistors do is to dissipate the electrical potential energy provided by the battery in the form of heat. As electrons move through the resistor they collide with the atoms and molecules of the resistor material. That briefly causes them to slow down a bit (losing kinetic energy), but then they speed up again (gain kinetic energy) due to the energy supplied by the electric field of the battery. The loss of kinetic energy shows up as heat. This alternately slowing down and speeding up of the electrons produces a net constant average speed (current) in the circuit.

Hope this helps.

• Thank you for your answer, @BobD! Based on what you're saying, the current stays the same once we stop adding resistors, which makes sense. But I feel like I don't fully understand how resistors work/affect current. Do they slow down charged particles in the circuit as they are flowing through the resistors? Is that how resistors decrease current? Commented Jul 3, 2020 at 20:14
• If so, then say a group of particles has just completed the circuit their first time and are starting to loop around again, for a second time. When they encounter the first resistor again, based on what you're saying the current would stay the same as it was. But if resistors slow down particles, it seems current would decrease more... Commented Jul 3, 2020 at 20:14
• Do the resistors constrict flow, i.e. "squeeze" the particles in more? That would sort of fit... Commented Jul 3, 2020 at 20:24
• See my edit in reply Commented Jul 3, 2020 at 20:37
• Thank you, @BobD! I'm still a little confused though. You say that, while in a resistor, electrons slow down a bit but then speed back up again; so how does a resistor result in a decrease in current through this mechanism? Commented Jul 3, 2020 at 21:12