UNDERSTANDING & CALCULATING SERIES CIRCUITS

 

BASIC RULES

A series circuit has certain characteristics and basic rules summized here: 

1. The same current flows through each part of a series circuit. 
2. The total resistance of a series circuit is equal to the sum of individual resistances.  
3. Voltage applied to a series circuit is equal to the sum of the individual voltage drops. 
4. The voltage drop across a resistor in a series circuit is directly proportional to the size of the resistor. 
5. If the circuit is broken at any point, no current will flow. 

Let's look at each of these closer to gain an understanding of series circuits. 

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"1. The same current flows through each part of a series circuit." 
 

In a series circuit, the amperage at any point in the circuit is the same.  This will help in calculating circuit values using Ohm's Law. 
 
 

You will notice from the diagram that 1 amp continually flows through the circuit.  We will get to the calculations in a moment.


 

"2. The total resistance of a series circuit is equal to the sum of individual resistances." 
 

In a series circuit you will need to calculate the total resistance of the circuit in order to figure out the amperage.  This is done by adding up the individual values of each component in series. 

In this example we have three resistors.  To calculate the total resistance we use the formula: 

RT = R1 + R2 + R3 

5 + 5 + 10 = 20 Ohms 

R total is 20 Ohms
Now with these two rules we can learn how to calculate the amperage of a circuit. 

Remember from Ohms Law that I = V / R. Now we will modify this slightly and say I = V / R total. 

Lets follow our example figure: 
RT = R1 + R2 + R3 
RT = 20 Ohms 

I = V / RT 
I = 20V / 20 Ohms 
I = 1 Amp 

If we had the amperage already and wanted to know the voltage, we can use Ohm's Law as well. 

V = I x R total 
V = 1 A x 20 Ohms 
V = 20 V
"Voltage Drops" 
Before we go any further let's define what a "voltage drop" is. A voltage drop is the amount the voltage lowers when crossing a component from the negative side to the positive side in a series circuit.  If you placed a multimeter across a resistor, the voltage drop would be the amount of voltage you are reading.  This is pictured with the red arrow in the diagram.

Say a battery is supplying  10 volts to a circuit of two resistors; each having a value of 5 Ohms.  According to the previous rules we figure out the total resistance.:

RT = R1 + R2 = 5 = 5 = 10 Ohms 
Next we calculate the amperage in the circuit:

I = V / RT = 10V / 10 Ohms = 1 Amp

Now that we know the amperage for the circuit (remember the amperage does not change in a series circuit) we can calculate what the voltage drops across each resistor are using Ohm's Law (V = I x R). 

V1 = 1A x 5 Ohms = 5 V 
V2 = 1A x 5 Ohms = 5 V

Now we get to the next rule.
"3. Voltage applied to a series circuit is equal to the sum of the individual voltage drops."

This simply means that the voltage drops have to add up to the voltage coming from the battey or batteries. 

V total = V1 + V2 + V3 ...

In our example above, this means that 5V + 5V = 10V.
"4. The voltage drop across a resistor in a series circuit is directly proportional to the size of the resistor."

This is what we described in the Voltage Drop section above.

Voltage drop = Current times Resistor size.
"5. If the circuit is broken at any point, no current will flow."
The best way to illustrate this is with a string of light bulbs. If one is burnt out, the whole thing stops working.