Foundational Physics

Short Answer: Adding a resistor in series increases equivalent resistance because the current must pass through an additional resistor on its only path. Adding a resistor in parallel decreases equivalent resistance because it provides another path for current. At the same applied voltage, more total current can flow, so the combination has a smaller equivalent resistance.

Why does series resistance increase?

Imagine a battery connected to one resistor. Charge has a single path through the circuit. Now put a second resistor in that same path.

The current must pass through both resistors. With the same battery voltage, the added resistance makes less current flow through the loop. That is why the equivalent resistance increases.

For two ordinary resistors in series, their resistance values add: R_eq = R₁ + R₂.

Why does parallel resistance decrease?

Now place the second resistor in a new branch that connects across the same two points as the first resistor.

Charge can move through either branch. The voltage across the first resistor has not changed, so it still carries the same current as before. But the new branch carries additional current.

For the same battery voltage, the source now supplies more total current. Since equivalent resistance compares source voltage with total source current, the equivalent resistance decreases.

A quick comparison with two resistors

Start with a 6 Ω resistor. Connect another 6 Ω resistor:

In series: the equivalent resistance becomes 12 Ω. At 12 V, the source current is 1 A.

In parallel: the equivalent resistance becomes 3 Ω. At 12 V, each branch carries 2 A, so the source supplies 4 A in total.

Nothing mysterious happened to either resistor. The difference is how the paths are connected.

What is the idea to remember?

Series adds another resistance along the same route. Parallel adds another route. That is why the total opposition to current increases in series but decreases in parallel.

When This Breaks

This comparison assumes ordinary positive resistances connected to an ideal voltage source, with the circuit in steady operation. A real source may have internal resistance or current limits, so adding a parallel branch may also change the terminal voltage. Non-ohmic devices can behave differently as their operating conditions change.

But for standard resistors, adding a positive resistor in series increases equivalent resistance; adding a new positive-resistance path in parallel decreases it.

Keep Exploring: Current, Resistance, and Ohm’s Law

Want to see how voltage, current, and resistance work together before exploring more circuit combinations? Choose your course and the amount of review you need.

Reviewed by Physics Sensei

Still stuck? Get unstuck. Choose the review that matches your course and the time you have.