Electronics
What is resistance?
Resistance is how strongly something opposes the flow of current. Its symbol is R and its unit is the ohm (Ω). Push the same voltage through more resistance and less current flows; through less resistance, more flows.
A squeeze on the flow
Pinch a hose and less water gets through, even with the same pressure behind it. Resistance is that pinch. Copper wire has very little, so in most circuits the resistance comes from the parts you put in.
A resistor is a part made to have a precise resistance. It turns the electrical energy it takes into a little heat, and it works the same either way round.
Live experiment
Slide the squeeze
A 9 V battery, a resistor and Pip, an LED. Slide the resistance from 0 Ω to 10 kΩ: every dot round the loop slows as the squeeze grows and Pip dims. Go too low and Pip burns out; 330 Ω gives about 21 mA.
“Resistance: the squeeze” is lesson 4 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Reading resistor values
Values use the same prefixes as other units:
- k (kilo) = × 1,000, so 4.7 kΩ = 4,700 Ω
- M (mega) = × 1,000,000, so 1 MΩ = 1,000,000 Ω
Resistors come in standard values like 100, 220, 330, 470, 1k, 2.2k, 4.7k and 10k Ω, and you usually pick the one closest to what you calculate (for an LED, the next one up). Small through-hole resistors mark their value with coloured bands.
Why LEDs need a resistor
An LED only conducts one way, and it behaves nothing like a resistor. Below its forward voltage, about 2 V for a red LED, almost no current flows. Above it, the current shoots up with only a small rise in voltage.
Nothing inside the LED holds the current back, so connected straight to a battery it burns out. A series resistor sets a safe current. On 9 V, a red LED with 330 Ω in series gets about 21 mA:
Where resistance comes from
A material's resistance depends on what it's made of and its shape: R = ρL over A, where ρ is resistivity, L is length and A is cross-sectional area. Longer wire resists more, thicker wire less. Copper's resistivity is so low that 1 m of 0.5 mm² wire is only about 0.034 Ω, which is why circuit diagrams treat wires as perfect.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Resistance: the squeeze” is lesson 4 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Or start from the very first lesson, no account neededWork it out
- Ohm's law calculatorAny two of V, I and R give the third, and the power.
- Electrical power calculatorWatts from any two of V, I and R, and the resistor rating that stays cool.
- Resistor colour code calculatorBands to ohms, or ohms to bands, on 4- and 5-band resistors.
- Ohm's law practice problemsFind V, I or R, then the power, with the working for each.
- Units and prefixes practice problemsConvert mA, kΩ, µF, nF, pF, ms and mV, with the working for each.
- Standard resistor values chartThe E12, E24 and E96 series, decade by decade, and which one to reach for.
- SMD resistor code chartTurn 472, 4702, 4R7 or 01C on a tiny resistor into ohms.
- Electronics glossaryFrom AC to Zener diode: the words of electronics, each in a sentence or two.
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: How strongly something squeezes the flow. Resistance is the squeeze. Same push, more squeeze, less current gets through.
- Why not “How long the wire is”? A longer wire does have more resistance, but length is only one thing that sets it. Resistance itself is how strongly the flow is squeezed, in ohms.
- Why not “How hard the battery pushes”? How hard the battery pushes is voltage. Resistance is what that push works against.
- Why not “How much charge goes past each second”? Charge going past each second is current, measured in amps. Resistance decides how much current a given push can drive.
Show the answer
Answer: 4,700 Ω. k (kilo) means × 1000, so 4.7 kΩ = 4.7 × 1000 = 4,700 Ω.
- Why not “47,000 Ω”? 47,000 Ω is 47 kΩ, ten times too big. 4.7 × 1000 = 4,700.
- Why not “47 Ω”? 47 Ω is a hundred times too small. Kilo means × 1000, so 4.7 kΩ is 4.7 × 1000.
- Why not “470 Ω”? 470 Ω is 0.47 kΩ, ten times too small. Going from kΩ to Ω multiplies by 1000, not 100.
Show the answer
Answer: Burns out. Nothing inside an LED holds the current back. With no squeeze, far too much current rushes through and it burns out. That's why it needs a resistor.
- Why not “Stays dark”? An LED across 9 V gets more than enough push to light. The trouble is the opposite: with no resistor, too much current flows.
- Why not “Glows dimly forever”? Without a resistor the current isn't limited, so the LED can't settle at a gentle glow. It overheats and burns out quickly.
- Why not “Glows gently”? Nothing inside an LED holds its current back. Straight across 9 V, far too much current flows and it burns out at once.
“Resistance: the squeeze” is lesson 4 of 6 in Copper Flats, Electronics World 1. Free with an account. Lessons unlock in order, so you’ll do the 3 before it first.
Questions people ask
What is resistance in simple terms?
Resistance is how strongly something opposes the flow of current. Its symbol is R and its unit is the ohm (Ω). For the same voltage, more resistance means less current.
What does a resistor do?
A resistor is a part made to have a precise resistance, so it sets how much current flows. It turns the electrical energy it takes into a little heat, and it works the same either way round.
Why does an LED need a resistor?
Above its forward voltage, about 2 V for a red LED, an LED's current shoots up with only a tiny rise in voltage, and nothing inside it limits the current. Connected straight to a battery it burns out. A series resistor sets a safe current.
What does 4.7k mean on a resistor?
The k means kilo, times 1,000, so 4.7 kΩ is 4,700 Ω. M means mega, times 1,000,000, so 1 MΩ is one million ohms. Values are often written 4k7, with the letter where the decimal point goes.
Related topics
- Ohm's law explainedV = I × R: set any two of voltage, current and resistance and the third follows.
- What is electric current?How much charge flows past a point each second, in amps, and why it's the same all round a loop.
- Series vs parallel circuitsOne path or many: what stays the same, what adds up, and how to combine resistors.