Resistors in Series
Series resistances add directly.
Formula
Variables
Example
10 + 20 = 30Ω
How it works
When resistors are connected end-to-end in a single path, the same current must pass through each one. Each resistor creates its own voltage drop, and the total drop across the chain is the sum of those individual drops, so the circuit behaves like one larger resistor with resistance equal to the total of all the parts. This is why you add the resistance values directly.
When to use it
Use this formula whenever resistors are placed in one continuous line with no branches, such as in simple current-limiting circuits or when finding the total resistance of a series network. It is also useful as a first step before calculating current or voltage in basic DC circuits.
Worked example, step by step
Suppose three resistors are connected in series: R1 = 4 Ω, R2 = 6 Ω, and R3 = 15 Ω. R_eq = R1 + R2 + R3 R_eq = 4 + 6 + 15 R_eq = 25 Ω So the equivalent resistance of the series combination is 25 Ω.
Common mistakes to avoid
- Adding resistors in series when the circuit actually has branches
- Forgetting to include every resistor in the path
- Mixing up series and parallel rules and trying to use reciprocals
Did You Know?
Resistors in series simply add up — the current has only one path through them all.
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More in Electronics & Circuits
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BasicVoltage equals current times resistance.
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BasicPower dissipated by a component.
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BasicPower lost as heat in a resistor.
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Resistors in Parallel
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Capacitors in Parallel
IntermediateParallel capacitances add.