Ohm's law
Resistance is how strongly a part holds back the flow of charge, measured in ohms. Ohm's law ties it to voltage and current: V = I × R. Turn a battery up to 9 V across a 450 Ω resistor and watch the meter read 20 mA, double the resistance and see the current halve, find out why resistors get warm (P = V × I), and work out the 150 Ω resistor that keeps an LED safe on a 5 V supply.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01A 9 V battery is connected across a 450 Ω resistor. What current flows?
I = V ÷ R = 9 V ÷ 450 Ω = 0.02 A, which is 20 milliamps.
02You keep the battery at 9 V but swap the 450 Ω resistor for a 900 Ω one. What happens to the current?
With the same voltage, doubling the resistance halves the current: 9 V ÷ 900 Ω = 0.01 A = 10 mA.
03A resistor has 9 V across it and 0.02 A through it. How much power does it turn into heat?
P = V × I = 9 V × 0.02 A = 0.18 W. That energy leaves the resistor as heat, which is why it gets warm. A common ¼ W (0.25 W) resistor can handle it.
04A red LED drops about 2 V and should run at 20 mA. Which series resistor suits a 5 V supply?
The resistor takes the rest of the voltage: 5 V − 2 V = 3 V. R = V ÷ I = 3 V ÷ 0.02 A = 150 Ω.
05Why does an LED need a resistor in series, when a plain resistor does not?
An LED is a nonohmic part: past about 2 V its current rises very steeply, so on its own it would draw too much current and burn out. The resistor in series sets the current using Ohm's law.
Glossary
- Resistance
- How strongly a part holds back the flow of charge. Measured in ohms (Ω).
- Ohm (Ω)
- The unit of resistance. One ohm lets one amp flow when one volt is applied: 1 Ω = 1 V/A.
- Ohm's law
- Voltage equals current times resistance, V = I × R. It holds for ohmic parts such as resistors.
- Milliamp (mA)
- A thousandth of an amp: 20 mA = 0.02 A.
- Power
- How fast energy is used or turned into heat, in watts (W): P = V × I.
- Watt (W)
- The unit of power: one joule of energy every second.
- LED
- Light-emitting diode: a small lamp that conducts in one direction and needs a resistor to limit its current.
- Current-limiting resistor
- A resistor in series with a part such as an LED, chosen with Ohm's law to set a safe current.
Notes and sources
- The board is a teaching model: the battery has no internal resistance, and the wires and the meter are ideal (no resistance), so the whole battery voltage sits across the resistor.
- Ohm's law holds for ohmic parts such as resistors, where current is proportional to voltage. Diodes, including LEDs, are nonohmic (OpenStax University Physics 2, section 9.4).
- The LED is drawn as a fixed 2.0 V drop once it conducts, a common first approximation. Kingbright's WP7113ID red LED lists 1.9 V typical at 10 mA (2.3 V maximum) and reaches 20 mA at about 2.0 V; its steady-current maximum is 30 mA.
- P = V × I = I² × R = V² ÷ R; in a resistor this energy leaves as heat (OpenStax, section 9.5). SparkFun notes that standard through-hole resistors usually come in ¼ W or ½ W ratings.
Quiz Unlocks after the last step.