No. If you double the voltage across a resistor, the current doubles too and the resistance stays the same. With R is U divided by I you calculate the resistance, but it does not depend on them: it is a property of the component itself. An ohmic resistor gives a straight line through the origin in an (I,U) graph.
This video can also be listened to in العربية, English, Français and Türkçe. Choose your language under the gear icon in the player, at Audio track and Subtitles.
Put twice the voltage across a resistor: does its resistance become twice as big? No. The current doubles with it, and the resistance stays what it was.
Resistance says how hard it is for charge to get through a component. In a wire, free electrons move between the positive ions, and those ions get in the way. The symbol is R, the unit the ohm, written as the Greek letter omega. Conductors such as copper, silver, aluminium and salt water have a small resistance; insulators such as plastic, porcelain and dry air a large one.
Ohm's law says how voltage, current and resistance are related: U is I times R, with U in volts, I in amperes and R in ohms. Measure the same resistor three times, at 2.0, 4.0 and 6.0 volts, and 20, 40 and 60 milliamperes flow. Convert the milliamperes to amperes first and work out R is U divided by I: three times 100 ohms. With R is U divided by I you calculate the resistance, but it does not depend on them: the resistance is a property of the component itself.
Plot those measurements in an (I,U) graph and you get a straight line through the origin. Such a component is called an ohmic resistor. A filament lamp doesn't do that: its line bends, because the filament gets hot, and a hot metal has a greater resistance than a cold one.
For upper secondary physics, on electricity.
In this video
The full explanation, in writing
Twice the voltage
0:00U is I times R, so R is U divided by I. Put twice the voltage across a resistor, and its resistance becomes twice as big.
What resistance is
0:16In a wire, free electrons move between the positive ions, and those ions get in the way. The harder it is for the charge to get through, the greater the electrical resistance. The symbol is R, and the unit is the ohm, written as the Greek letter omega.
0:34Materials that charge passes through easily are called conductors: metals such as copper, silver and aluminium, but also salt water. Their resistance is small. Materials that charge hardly gets through are called insulators: the plastic sheath of your charging cable, porcelain, and dry air.
0:56Their resistance is large.
Three measurements on one resistor
0:58How voltage, current and resistance are related is given by Ohm's law: U is I times R. U in volts, I in amperes, R in ohms. Take a resistor, and measure three times. Two point zero volts across it: twenty milliamperes flow. Four point zero volts: forty milliamperes.
1:17Six point zero volts: sixty milliamperes. Now work out the resistance for each measurement, and convert the milliamperes to amperes first. Two point zero divided by zero point zero two zero is a hundred ohms. Four point zero divided by zero point zero four zero: a hundred ohms.
1:36And six divided by sixty milliamperes: a hundred ohms again. The voltage doubles, the current doubles with it, and the resistance stays what it was. R is U divided by I tells you how to calculate the resistance, not what it depends on. The voltage determines the current; the resistance is a property of the component itself.
1:57Your turn. If you plot those three measurements in a graph, with the voltage horizontal and the current vertical: what kind of line do you get?
A straight line, and a lamp that bends
2:09A straight line through the origin. Zero volts, zero current, and every step in voltage gives the same step in current. That is an (I, U) graph, and a component with a straight line like that through the origin is called an ohmic resistor: its resistance is constant.
2:25A filament lamp doesn't do that. At a low voltage its line rises steeply, and the higher the voltage, the more it bends. Each extra volt gives less and less extra current, so the resistance of the lamp increases. And that is not because the voltage makes the resistance bigger.
2:43The filament in the lamp gets hot, and a hot metal has a greater resistance than a cold one. So a filament lamp is not an ohmic resistor.
Three things to remember
2:53Three things. Resistance says how hard charge gets through a component: symbol R, unit ohm. Conductors have a small resistance, insulators a large one. Ohm's law: U is I times R. With U divided by I you calculate the resistance, but it doesn't depend on them.
3:12And in an (I, U) graph, an ohmic resistor gives a straight line through the origin. A filament lamp bends, because the filament gets warm.
Two questions
3:23Two questions. One. There are nine point zero volts across a resistor, and forty-five milliamperes flow through it. How large is the resistance? Two. You put three times the voltage across an ohmic resistor. What happens to the current, and what to the resistance?
The answers
3:43The first: forty-five milliamperes is zero point zero four five amperes. R is U divided by I, so nine point zero divided by zero point zero four five. That is two point zero times ten to the second ohms. The second: the current becomes three times as large, and the resistance stays the same.
What the resistance of a wire depends on
4:04Why a long, thin wire has more resistance than a short, thick one, and what the material has to do with it: there is a separate video on the channel about resistivity.
Frequently asked questions
What is Ohm's law?
Ohm's law is U is I times R: the voltage in volts is the current in amperes times the resistance in ohms. For an ohmic resistor R is constant, so the current is directly proportional to the voltage.
How do you calculate resistance from voltage and current?
With R is U divided by I. Convert the current to amperes first. If there are 9.0 volts across a resistor and 45 milliamperes flow, that is 9.0 divided by 0.045 amperes, which is 2.0 times ten to the second ohms.
Does resistance increase if you increase the voltage?
Not for an ohmic resistor: double the voltage and the current doubles while the resistance stays the same. For a filament lamp the resistance does increase at a higher voltage, but that is because the filament gets hot, not because of the voltage itself.
What is the difference between a conductor and an insulator?
Charge passes through a conductor easily, so its resistance is small: metals such as copper, silver and aluminium, but also salt water. Charge hardly gets through an insulator, so its resistance is large: plastic, porcelain and dry air.
Why is a filament lamp not an ohmic resistor?
Its line in an (I,U) graph is not a straight line through the origin but bends: each extra volt gives less and less extra current. The filament gets hot, and a hot metal has a greater resistance than a cold one.
Before this came how to connect an ammeter and a voltmeter: exactly the measurement you see here. Next: why a long, thin wire has more resistance than a short, thick one, and what the material has to do with it, with resistivity.
This video carries audio and subtitles in several languages. Pick yours under the gear icon in the player, at Audio track and at Subtitles.
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.