Voltage and Polarity in a Series Circuit

## Voltage and polarity in a series circuit

What voltage polarities should we expect in the circuit? If you look at the circuit you will see that when measuring the voltage across the resistors the red probe is placed nearest the positive terminal of the battery. How do we know this is the correct orientation? When conventional current enters a resistor there will be a back-up of voltage behind that resistor. You will get a higher voltage where conventional current enters the resistor and a lower voltage where the current exits the resistor.

 Higher voltage Lower voltage Conventional current
 The arrow points in the direction of conventional current. Voltage backs up as the current enters the resistor. Therefore, the voltage is positive where the current enters the resistor and negative where it exits.

The voltage will be positive where conventional current enters the resistor and negative where it exits.

Remember that negative voltage is not the opposite of positive voltage. A negative voltage is simply a voltage that is lower than some other voltage (review Polarity). Likewise a positive voltage is a voltage that is higher than some other voltage. Voltages are always considered by how they relate to other voltages. In this example we have two isolated voltages. We are not comparing them to voltages anywhere else in the circuit. We are only interested in how the two voltages relate to each other. In this case the higher voltage is considered positive and the lower voltage is considered negative.

A current source, such as a battery is opposite. Notice that the battery voltage is positive where the conventional current exits and negative where the current enters. A battery pushes current out the positive terminal so that is where the pressure is higher. It sucks the current back in the negative terminal so that is where the pressure is lower. With a resistor, electricity backs up where the current enters causing an increase in voltage at that point. Ther is a corresponding decrease where the current exits the resistor.

If you look at the diagrams for Kirchhoff's Voltage Law, you will see that the polarity across the battery is opposite to the polarity across the resistors.

 The polarity of the voltage across the battery is opposite to the voltage across the resistors.

Now we can make sense out of the proper way to state Kirchhoff's Voltage Law. Let's flip a coin and call the polarity across the battery positive and the polarity across the resistors negative (it will work just as well if we do it the other way around). That starts us out with positive 30 volts. Now we add the negative voltages across the resistors. Since adding a negative number is the same as subtracting a positive number we are actually subtracting the voltages across the resistors. Therefore, we start with 30 volts, subtract 10, subtract 15 then subtract 5 and we end up with 0 volts.

30 + (-10) + (-15) + (-5) = 0

or

30 – 10 – 15 – 5 = 0

We add all the voltages together and we get zero. There's Kirchhoff's Voltage Law.

Voltage Dividers