Voltage Drop

As current flows through a resistor it creates a voltage drop. This means that the voltage drops from a high number to a lower number. You can calculate the voltage drop across each resistor by using Ohms Law. We'll use the same example as before.

We will use the basic form of Ohms Law to determine the voltage dropped by each resistor.

You can apply this law to each resistor in the series. Use I = 0.10 for both resistors because the same current is flowing through them both. We will use Vr1 to mean the voltage dropped by R1 (the 80 ohm resistor) and Vr2 to refer to the voltage dropped by R2 (the 20 ohm resistor). You will notice the green A, B and C added for reference in the next paragraph.

We have applied Ohms Law to both R1 and R2 to determine how much voltage is dropped by each. From the math, R1 drops 8.0 volts while R2 drops 2.0 volts.

Now think about point A (in green) for a second. The voltage at that point is +10V because it is connected directly to the +10 Volt battery. Resistor R1 is going to drop 8.0 volts as current I flows through it. That means that point B (in green) is 10 - 8, or 2 volts.

We have 2 volts at point B. Resistor R2 is going to drop 2.0 volts as current I flows through it, leaving us with 2 - 2 = 0 volts at point C. This makes sense, too, because point C is the negative terminal of the battery, which we tend to always associate with 0 volts.

It turns out that the voltage dropped by R1 + R2 will always add up to the value of the battery.