Electronics and projects
Resistor colour code
Read 4-band and 5-band resistors, or find the colours for any value, on a drawn resistor.
- Colours to value
- Value to colours
Class 6 to 12, diploma, makers
Electronics and projects
Ohm's law calculator
Type any two of voltage, current, resistance and power. The calculator finds the other two and shows every step, ready to copy into a practical file or check a numerical.
- Made forClass 10 to 12, diploma, makers
- Shows the workingEvery step, ready for your practical file
- Instant answersResults change as you type
Ohm's law and power
Fill in any two boxes and leave the other two empty. Pick the unit beside each box (mA, kΩ and so on).
Current and power9 mA and 81 mWFrom voltage 9 V and resistance 1 kΩ
- Voltage
- 9 Vyou entered
- Current
- 9 mAcalculated
- Resistance
- 1 kΩyou entered
- Power
- 81 mWcalculated
Working
- I = V ÷ R = 9 V ÷ 1 kΩ = 9 mA
- P = V² ÷ R = (9 V)² ÷ 1 kΩ = 81 mW
- Tip: If R is a resistor, it turns 81 mW into heat. Use one rated for at least twice that: ¼ W (0.25 W), the common size.
What it works out
Understanding Ohm's law
Ohm's law says that the current through a resistor is proportional to the voltage across it: V = I × R. Put together with the power formula P = V × I, any two of the four quantities fix the other two.
It is the formula used most in school electronics, from the electricity chapter in Class 10 to Class 12 practicals and every Arduino project, because it tells you how much current a circuit will draw and how much heat a resistor has to get rid of.
Formula and worked example
Working out Ohm's law by hand
The calculator uses exactly these formulas. Write them in your notebook and check one answer by hand: it is the surest way to remember them.
The formula
- V = I × R
- I = V ÷ R and R = V ÷ I
- P = V × I = I² × R = V² ÷ R
- V
- voltage in volts (V)
- I
- current in amperes (A); 1 mA = 0.001 A
- R
- resistance in ohms (Ω); 1 kΩ = 1,000 Ω
- P
- power in watts (W); 1 mW = 0.001 W
Worked example
A 9 V battery across a 1 kΩ resistor
- Convert to base units: 1 kΩ = 1,000 Ω.
- I = V ÷ R = 9 ÷ 1,000 = 0.009 A, which is 9 mA.
- P = V × I = 9 × 0.009 = 0.081 W, which is 81 mW.
- A common ¼ W (0.25 W) resistor handles that with room to spare.
AnswerI = 9 mA and P = 81 mW
In projects and exams
How students use it
Enter any two of voltage, current, resistance and power and get the other two, with the working. Here is where it earns its place in a school year.
Numericals in board exams
Check your answers to electricity numericals, including the unit conversions that cost the most marks.
V-I practicals
Predict the current before you switch on, then compare it with the ammeter reading in your observation table.
Will it overheat?
Find the power a resistor, bulb or motor turns into heat, and choose parts rated well above it.
Battery planning
Knowing the current a circuit draws is the first step to working out how long its battery will last.
Check your work
Common mistakes to avoid
These are the slips that cost marks in exams and burn out parts on the bench.
Forgetting to convert mA and kΩ
9 V ÷ 1 kΩ is 9 mA, not 9 A. Convert to volts, amperes and ohms first, or pick the unit beside each box.
Using the wrong voltage
Use the voltage across the part you are calculating, not the whole supply, when other parts share the circuit.
Applying it to LEDs and bulbs
An LED or a filament bulb does not have a fixed resistance. For an LED, calculate the resistor with the LED resistor calculator instead.
Ignoring the power rating
A resistor can have the right value and still burn out. Check P and choose a rating at least twice as large.
Questions
Questions about Ohm's law
Stuck on a project or a sum? Ask the institute on WhatsApp.
Ask on WhatsAppDoes Ohm's law work for an LED?
Not directly: an LED drops a nearly fixed voltage and its current rises very steeply above it, so it has no single resistance. Use Ohm's law on the resistor in series with it, which is what the LED resistor calculator does.
How do I measure current with a multimeter?
Break the circuit and connect the meter in series, on the mA or A range with the probe in the matching socket. Never connect a meter set to current directly across a battery: that is a short circuit and can blow the meter's fuse.
Why is a bulb's resistance lower when it is cold?
A filament's resistance rises a lot as it heats up, so a multimeter reading on a cold bulb is far lower than its resistance when lit.
What is 1 kΩ and 1 MΩ in ohms?
1 kΩ is 1,000 Ω and 1 MΩ is 10,00,000 Ω (one million ohms).
Free ebooks
Read more about Ohm's law
Free PDF ebooks in easy English: download, print and share them.
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