# Why does the voltage decrease across resistors in series?

I've read a lot of anologies about voltage and how is a type of "water pressure" but i can't understand why when 2 or more resistors are in series there is a voltage drop but when the 2 resistors are in parallel the voltage keeps the same.

• If you apply a voltage across something it has that voltage. putting more things across the same terminals does not change the applied voltage. – Trevor_G Oct 11 '17 at 14:58
• Voltage drops or keeps the same with respect to what? If you have a 12V power supply and put 2 resistors in series or parallel on it as load, its still gonna be 12V... – Wesley Lee Oct 11 '17 at 14:58
• The very reason you ask this question, is because you don't really understand voltage. But don't worry, most people , though applying KVL KCL everyday don't have a profound understanding of 'voltage' since the lack of appreciation on electromagnetic. The circuit you learn, it is called 'lumped circuit model', it is only an abstraction for ease of engineering, and far from the truth. – Eagle Shou Oct 11 '17 at 17:49
• The KVL and KCL, holds when certain conditions are met in Maxwell's Equation. Thus, the so-called resistor division rule, it is working in such condition, with the same assumptions. Don't spend too much time on thinking about the 'real meaning' behind this, it is just a model we use to describe things, only a mental picture. – Eagle Shou Oct 11 '17 at 17:58
• keeping with water, adding a 5gpm constrictor to a 10gpm pipe the flow is halved, but if you flow water through 2 side-by-side 5gpm constrictors, you would still be able to pass 10gpm... – dandavis Oct 11 '17 at 22:09

We think in ideals.

In the ideal circuit shown below, when you generate a voltage, with say a battery, then the applied voltage across R1 is the battery voltage. Further, your implication that there is no voltage drop across the resistors is incorrect. There is a voltage drop, it is equal to the entire applied voltage.

As you can see, attaching a second resistor will not change that voltage. simulate this circuit – Schematic created using CircuitLab

However, in reality nothing is ideal. In the real world the battery has some resistance as do the wires between the battery and the resistors as shown below. simulate this circuit

In a good circuit R_Source will be very low but is finite. As such there is some voltage drop across it. Now when you add R2, the total current through R_Source increases and the voltage across AB will indeed be lower.

The fundamental reason is KVL. The voltage around a loop must sum to zero.

You have a voltage source producing a fixed voltage. All the other branches in any loop containing this source must sum to this voltage (but going in the other direction along the loop).

If you can make a loop containing just one device and the source, then the voltage across that device must be equal to the source voltage, even if there are other loops that can be made using the source. This is why putting more devices in parallel doesn't change the voltage across the first device.

If you make a loop containing the source and several other devices, the voltages across the other devices must sum to equal the source voltage. This is why adding another device in series will generally reduce the voltage across the ones that were there before.