Basic Electricity: Series Circuits

One of the easiest ways I remember series circuits is:

Series = same current, voltage divides, resistance adds.

That simple phrase covers the three big things you need to know.

A series circuit has only one path for current to flow. That is the key idea. Since there is only one path, the same current must pass through every component in the circuit.

Think of it like water flowing through one single pipe. Everything in that pipe gets the same flow.

1. Current stays the same

In a series circuit, the current is the same everywhere in the circuit.

For example, if a circuit has a battery and three resistors connected in series, the same amount of current flows through each resistor.

Current through R1 = 2 amps
Current through R2 = 2 amps
Current through R3 = 2 amps

It does not split because there are no branches.

That is the big difference between series and parallel circuits.

In a parallel circuit, current divides between branches.

In a series circuit, current stays the same all the way around.

2. Voltage divides

In a series circuit, the source voltage is divided across the components.

Each resistor uses up part of the total voltage. These are called voltage drops.

For example, if a 12-volt battery is connected to three resistors in series, the voltage drops across the resistors must add back up to 12 volts.

Battery voltage = 12 volts

Voltage drop across R1 = 2 volts
Voltage drop across R2 = 4 volts
Voltage drop across R3 = 6 volts

Total voltage drops = 12 volts

The voltage drops do not have to be equal unless the resistors are equal.

A larger resistor will have a larger voltage drop.

A smaller resistor will have a smaller voltage drop.

3. Resistance adds

In a series circuit, total resistance is found by adding the resistors together.

The formula is:

RT = R1 + R2 + R3

For example:

R1 = 2 ohms
R2 = 4 ohms
R3 = 6 ohms

So:

RT = 2 + 4 + 6
RT = 12 ohms

The total resistance is 12 ohms.

This is easier than parallel resistance because you simply add the values.

4. Series circuit example

Let’s say we have a 24-volt battery and three resistors in series.

Battery voltage = 24 volts

R1 = 2 ohms
R2 = 4 ohms
R3 = 6 ohms

First, find total resistance:

RT = R1 + R2 + R3
RT = 2 + 4 + 6
RT = 12 ohms

Now use Ohm’s Law to find total current:

I = E / R
I = 24 / 12
I = 2 amps

Since this is a series circuit, that same 2 amps flows through every resistor.

Current through R1 = 2 amps
Current through R2 = 2 amps
Current through R3 = 2 amps

Now find the voltage drop across each resistor.

For R1:

E = I × R
E = 2 × 2
E = 4 volts

For R2:

E = I × R
E = 2 × 4
E = 8 volts

For R3:

E = I × R
E = 2 × 6
E = 12 volts

Now check the voltage drops:

4 volts + 8 volts + 12 volts = 24 volts

That matches the battery voltage.

5. What happens if one component opens?

In a series circuit, if one component opens, the whole circuit stops working.

That is because there is only one path for current.

If that path is broken anywhere, current cannot flow.

Battery → R1 → R2 → R3 → back to battery

If R2 opens, the path is broken.

Battery → R1 → open circuit → R3

No current flows.

This is why old-style Christmas lights could all go out if one bulb failed. They were wired in series.

6. Easy way to remember series circuits

Here is the memory aid:

Series = same current, voltage divides, resistance adds.

Or even shorter:

Series:
Current stays the same
Voltage divides
Resistance adds

Compare that with parallel:

Parallel:
Voltage stays the same
Current divides
Resistance gets smaller

That comparison helps a lot on test questions.

7. Series vs. parallel quick comparison

This is one of the most important basic electricity ideas to understand for A&P testing.

If you can quickly identify whether a circuit is series or parallel, the rest of the question usually becomes much easier.

8. A&P test tip

When you see a series circuit question, look for these clues:

• Only one path for current
• Resistors connected end-to-end
• Total resistance is larger than any one resistor
• Same current through every component
• Voltage drops add up to source voltage

If the question asks for total resistance in series, just add the resistors.

If the question asks for current, find total resistance first, then use Ohm’s Law.

I = E / R

If the question asks for voltage drop across one resistor, use:

E = I × R

9. Practice question

A 24-volt battery is connected to three resistors in series.

R1 = 3 ohms
R2 = 5 ohms
R3 = 4 ohms

What is the total resistance?

Since the resistors are in series, add them:

RT = R1 + R2 + R3
RT = 3 + 5 + 4
RT = 12 ohms

Now find the current:

I = E / R
I = 24 / 12
I = 2 amps

Because this is a series circuit, the current is the same everywhere.

Current through R1 = 2 amps
Current through R2 = 2 amps
Current through R3 = 2 amps

Now find the voltage drops:

R1: E = I × R
E = 2 × 3
E = 6 volts

R2: E = I × R
E = 2 × 5
E = 10 volts

R3: E = I × R
E = 2 × 4
E = 8 volts

Check the answer:

6 volts + 10 volts + 8 volts = 24 volts

The voltage drops add up to the source voltage.

10. Common mistake

A common mistake is thinking voltage stays the same in a series circuit.

That is not correct.

Voltage stays the same in a parallel circuit.

In a series circuit, voltage divides.

Another common mistake is thinking current divides in a series circuit.

That is also not correct.

Current divides in a parallel circuit.

In a series circuit, current is the same everywhere.

Remember:

Series = same current
Parallel = same voltage

That one comparison can save you on a lot of test questions.

11. Series circuit formulas

Here are the main formulas to remember for a series circuit.

Total resistance

RT = R1 + R2 + R3

In series, resistance adds.

Current

I = E / R

Use total voltage and total resistance to find total current.

Since it is a series circuit, that current is the same through every component.

Voltage drop

E = I × R

Use this to find how much voltage is dropped across one resistor.

12. Series circuit checklist

When I see a series circuit, I ask myself:

1. Is there only one path for current?
2. Are the resistors connected end-to-end?
3. Do I need to add the resistors?
4. Do I need to find current using total resistance?
5. Do I need to find voltage drop across one component?

That keeps the problem from getting confusing.

Final thought

Series circuits are easier when you remember that everything is connected in one path.

No branches. No splitting current.

The current stays the same, the voltage gets divided, and the resistance adds together.

Series = same current, voltage divides, resistance adds.

That one sentence is the main thing to remember.