The same through both. In a series circuit the charge has only one path: every electron that goes into the first lamp comes out again and goes through the second. No charge gets lost along the way, so the current is the same everywhere in the circuit. What a lamp converts is energy, not current.
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Two lamps one after the other in one circuit. Which lamp gets the most current: the first one, the second one, or both the same? Both the same.
If components are one after another in one circuit, with no branches, that is called a series circuit. There the charge has only one path: every electron that goes into the first lamp comes out again and then goes through the second. No charge gets lost along the way, so the current is the same everywhere: I total equals I one equals I two. What a lamp converts is not current but energy.
The voltage of the source, however, is shared. Each component gets part of it, the voltage across that component, and together those voltages equal the source voltage: U total equals U one plus U two. That is called voltage division. And the resistances add up: R total equals R one plus R two, because every component is one more obstacle on the same single path.
In the worked example, 20 ohms and 40 ohms are in series on 12 volts. Together that is 60 ohms, so the current is 0.20 amperes, through both. Across the 20 ohms there is 4.0 volts, across the 40 ohms 8.0 volts: the largest voltage is across the largest resistance.
For upper secondary physics, in electricity.
In this video
The full explanation, in writing
Two lamps one after the other
0:00Two lamps one after the other in one circuit. Which lamp gets the most current: the first one, the second one, or both the same? No calculating, just choose.
One path for the charge
0:17If components are one after another in one circuit, with no branches, that is called a series circuit. There the charge has only one path. Every electron that goes into the first lamp also comes out again and then goes through the second. No charge gets lost along the way.
0:35So the same current goes through both, and that holds everywhere in the circuit: I total equals I one equals I two. What a lamp converts is not current but energy. The electrons come out just as fast as they go in, only with less energy.
The voltage is shared
0:54The voltage of the source, however, is shared. Each component gets a part, the voltage across it, and together those voltages are equal to the voltage of the source: U total equals U one plus U two. This sharing is called voltage division. And the resistances add up: R total equals R one plus R two.
1:15Every component you add is one more obstacle on the same single path. Your turn. A resistor of twenty ohms and one of forty ohms are in series on twelve volts. Which of the two has the larger voltage across it?
Twenty and forty ohms
1:34Calculate the current with the total resistance, not with one of the two. Twenty plus forty is sixty ohms. The current is U divided by R: twelve volts divided by sixty ohms is zero point two zero amperes, and that current goes through both. Across the twenty ohms is zero point two zero times twenty: four point zero volts.
1:57Across the forty ohms: zero point two zero times forty, eight point zero volts. Four plus eight is twelve, exactly the voltage of the source. The largest voltage is across the largest resistance.
Three things to remember
2:09Three things. In a series circuit the current is the same everywhere, because the charge has only one path and loses nothing along the way. The voltage of the source is shared: the voltages across the components add up to the source voltage, and the largest voltage is across the largest resistance.
2:28And the resistances add up: R total equals R one plus R two.
Two questions
2:34Two questions. One. A resistor of ten ohms and one of fifty ohms are in series on six point zero volts. What is the voltage across the fifty-ohm resistor? Two. You add a third resistor to a series circuit. What happens to the current?
The answers
2:55The first: the total resistance is sixty ohms, so the current is six point zero divided by sixty, zero point one zero amperes. Across the fifty ohms is zero point one zero times fifty: five point zero volts. The second: the total resistance gets larger, so at the same voltage the current gets smaller.
3:16And that smaller current is again the same everywhere in the circuit.
When the current splits
3:20What happens when the current can split: there is a separate video on the channel about the parallel circuit.
Frequently asked questions
Is the current the same everywhere in a series circuit?
Yes. The charge has only one path and loses nothing along the way, so the same current goes through every component: I total equals I one equals I two. A lamp converts energy, not current: the electrons come out just as fast as they go in, only with less energy.
How do you calculate the total resistance in a series circuit?
You add the resistances: R total equals R one plus R two. Every component you add is one more obstacle on the same single path. Twenty ohms and forty ohms in series make sixty ohms together.
What is voltage division?
In a series circuit the voltage of the source is shared between the components. Each component gets its own part, the voltage across it, and together those voltages equal the source voltage: U total equals U one plus U two. The largest voltage is across the largest resistance.
What is the voltage across each resistor in series?
First calculate the current with the total resistance, not with one of the two. With 20 ohms and 40 ohms on 12 volts that is 12 divided by 60, 0.20 amperes. Then the voltage across the 20 ohms is 0.20 times 20, 4.0 volts, and across the 40 ohms 8.0 volts. Together 12 volts again.
What happens to the current if you add a resistor in series?
It gets smaller. The total resistance gets larger, so at the same voltage less current flows. And that smaller current is again the same everywhere in the circuit.
Next: the parallel circuit, where the current does split and the total resistance actually gets smaller.
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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.