You may only add up resistors that are in series. In a series-parallel circuit you first replace the parallel part with one resistor: 40 ohms and 60 ohms in parallel make 24 ohms together. That is in series with 16 ohms, so the total resistance is 40 ohms and not 116 ohms.
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Three resistors in one circuit: 16, 40 and 60 ohms. Add them up and you get 116 ohms and a small current. The ammeter shows almost three times as much.
If part of a circuit is in series and part in parallel, it is called a series-parallel circuit. You may only add up resistors that are in series. Here the 40 and the 60 ohms are side by side, each in its own branch, and the 16 ohms is in series with those two together.
You solve it by simplifying the circuit step by step. First replace the parallel part with one resistor: 40 ohms and 60 ohms in parallel make 24 ohms together. That is in series with the 16 ohms, and together that is 40 ohms. At 12 volts that gives 0.30 amperes, exactly what the meter shows.
Then you work back. Across the 16 ohms there is 4.8 volts, so 7.2 volts is left for the parallel part, and that voltage is across both branches. Through the 40 ohms goes 0.18 amperes, through the 60 ohms 0.12 amperes, together 0.30 again.
For upper secondary physics, on electricity.
In this video
The full explanation, in writing
Adding up that doesn't work
0:00Three resistors in one circuit: sixteen, forty and sixty ohms. I add them up, a hundred and sixteen ohms, and work out the current. The ammeter shows almost three times as much.
Series and parallel mixed together
0:18If part of a circuit is in series and part in parallel, it is called a series-parallel circuit. You may only add up resistors that are in series. Here the forty- and sixty-ohm resistors are side by side, each in its own branch, and the sixteen-ohm resistor is in series with those two together.
0:37You solve it by simplifying the circuit step by step. First replace the parallel part with one resistor that does exactly the same. Then an ordinary series circuit is left. Your turn. The resistor that replaces the forty- and sixty-ohm ones together: is it larger than forty ohms, or smaller?
From three resistors to one
1:02Smaller, because in parallel the total resistance is always smaller than the smallest. One divided by forty plus one divided by sixty is five over a hundred and twenty, and that is one divided by twenty-four. So the parallel part is twenty-four ohms.
1:19That twenty-four ohms is in series with the sixteen ohms, and together that is forty ohms: the total resistance of the whole circuit. The source delivers twelve volts, so the current is twelve divided by forty, zero point three zero amperes.
And back again
1:36Now the other way. That zero point three zero amperes goes through the sixteen ohms, so the voltage across it is zero point three zero times sixteen: four point eight volts. That leaves seven point two volts for the parallel part, and that voltage is across both branches.
1:53Through the forty ohms goes seven point two divided by forty, zero point one eight amperes. Through the sixty ohms zero point one two amperes. Together zero point three zero again: the current rejoins further on.
Three things to remember
2:08Three things. A series-parallel circuit has a part in series and a part in parallel, and you may only add up resistors in series. You simplify it step by step: first replace the parallel part with one resistor, and add it to whatever is in series with it.
2:25And then you work back: with the current from the source you find the voltage across each part, and with that voltage the current through each branch.
Two questions
2:35Two questions. One. A resistor of five point zero ohms is in series with two resistors of ten ohms that are in parallel. What is the total resistance? Two. The source delivers six point zero volts. What is the current through one of the two ten-ohm resistors?
The answers
2:56The first: two equal resistors of ten ohms in parallel together make half, five point zero ohms. With the five point zero ohms in series, the total resistance is ten ohms. The second: the current from the source is six point zero divided by ten, zero point six zero amperes.
3:15It splits over two equal branches, so through each ten-ohm resistor goes zero point three zero amperes.
Resistors that change by themselves
3:23How a resistance changes by itself with temperature or with light: on the channel there is a separate video about the NTC, the PTC and the LDR.
Frequently asked questions
What is a series-parallel circuit?
A circuit in which one part is in series and one part in parallel. You may then only add up the resistors that are in series; for the parallel part you first work out one equivalent resistor.
How do you calculate the total resistance of a series-parallel circuit?
Step by step. First replace the parallel part with one resistor, using one over R equals one over R one plus one over R two. Then add it to whatever is in series with it. For 16 ohms in series with 40 and 60 ohms in parallel, that is 16 plus 24, together 40 ohms.
Is the equivalent resistance of a parallel part larger or smaller?
Smaller than the smallest resistance in that part. Forty ohms and sixty ohms in parallel make twenty-four ohms together, smaller than the forty.
How do you find the current through each branch of a series-parallel circuit?
Work back from the current from the source. With that current you find the voltage across the part in series; what is left of the source voltage is across the parallel part, equally large across each branch. Divide that voltage by the resistance of a branch and you have the current through that branch.
What is the total resistance of 5.0 ohms in series with two 10 ohm resistors in parallel?
Ten ohms. Two equal resistors of ten ohms in parallel make half together, five point zero ohms. With the five point zero ohms in series, the total resistance is ten ohms. At six volts, 0.60 amperes then flows, and through each ten-ohm resistor half of that, 0.30 amperes.
Before this came the series circuit and the parallel circuit. Next: resistors that change by themselves with temperature or with light, the NTC, the PTC and the LDR.
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Physics with Thomas: physics explained with pictures where they add something, and without them where they distract.
In this series
Electric current6:12
Voltage and capacity4:43
Connecting ammeter and voltmeter4:10
Resistance and Ohm's law4:21
Resistance of a wire4:40
Variable resistor, NTC, PTC and LDR3:40
Series circuit3:32
Parallel circuit3:56
Series-parallel circuit3:37
Circuits practical test5:13
Power2:58
Kilowatt-hour3:05
About me
My name is Thomas Schuurmans, and I have a passion for physics and for understanding why things work the way they do. What I want is to get young people asking “why is that?” and “how does that work?” more often, and to hand them the tools of physics in a way that is simple and that you can see.
I graduated in applied physics at Delft University of Technology in 2003. After that I spent more than twenty years outside education: first at TNO, the Dutch applied research institute, then at a design agency, and eventually founding Proportion Global, through which I work on innovation questions in Africa, Latin America and South Asia. That work resembles physics more than you would expect: don't start from a solution, first understand what is going on, try something, be wrong, and look again.
Since 2026 I have been teaching physics to both lower and upper secondary classes, and I started a master's at the University of Amsterdam for my full teaching qualification. That is where I learned that a secondary school pupil's real attention for new material lasts about seven minutes. And my own weakness happens to be telling too many side stories.
That is where the idea came from: videos that explain one topic sharply and visually, inside those seven minutes. I make them for my own pupils. Then I publish them, because good explanation should be within reach of anyone who needs it, wherever you live and whatever language you think and speak in.