How to write formulas on this site
Write maths in plain words and symbols and the site typesets it. Every example on this page, with what you type and what the reader sees.
Ohm’s law says the current through a resistor is the voltage across it divided by its resistance. In symbols, I = V / R, so for a 9 V battery and a 470 ohm resistor:
I = V / R = 9 / 470 = 0.019 A
To pick a resistor for an LED you need the supply voltage, the LED’s forward voltage and the current you want:
R = (Vs - Vf) / I
With a 9 V supply, a 2 V LED and 20 mA:
R = (9 V - 2 V) / 0.02 A = 350 ohm
The nearest standard value is 390 ohm, which gives I = 7 / 390 = 18 mA, close enough.
The notation
| You write | Reader sees | Note |
|---|---|---|
[maths]V = I R[/maths] | V = I R | Letters next to each other are multiplied. Single capitals are variables. |
[maths]R = (Vs - Vf) / I[/maths] | R = (Vs - Vf) / I | A capital with lowercase letters gets a subscript. Brackets around a fraction's top or bottom disappear into the fraction. |
[maths]P = V^2 / R[/maths] | P = V^2 / R | ^ is a power. "V squared" and "V cubed" work too. |
[maths]f = 1 / (2 pi sqrt(L C))[/maths] | f = 1 / (2 pi sqrt(L C)) | Greek letters by name. sqrt() and cbrt() for roots, or "square root of". |
[maths]I = 20 mA = 0.02 A[/maths] | I = 20 mA = 0.02 A | A unit written after a number is set upright, including prefixes like k, m, u and n. |
[maths]R = 4.7 kohm[/maths] | R = 4.7 kohm | ohm becomes the omega sign. |
[maths]power = voltage times current[/maths] | power = voltage times current | Words are set as words. "times", "divided by", "plus", "minus" and "equals" all work. |
[maths]x sub 1 + x sub 2[/maths] | x sub 1 + x sub 2 | "sub" makes a subscript; so does an underscore. |
[maths]a is approximately 3.14[/maths] | a is approximately 3.14 | Comparisons in words: approximately, less than, greater than, not equal to, proportional to. |
[maths]T = 25 deg C[/maths] | T = 25 deg C | "deg" or the degree sign, with C, F or K. |
[maths]"total cost" = miles / mpg * 4.546 * price[/maths] | "total cost" = miles / mpg * 4.546 * price | Quote a phrase to keep it as text. Lowercase words are text anyway. |
[maths]n! / (k! (n - k)!)[/maths] | n! / (k! (n - k)!) | Factorials, percentages (%), plus or minus (+-) and infinity (inf) are understood. |
[maths]Ctotal = C1 + C2 + C3 + ...[/maths] | Ctotal = C1 + C2 + C3 + ... | Commas make a list and three dots are the "and so on" sign. |
[maths]f = 1.44 / (3000 ohm times 100 times 10^-6 F)[/maths] | f = 1.44 / (3000 ohm times 100 times 10^-6 F) | Scientific notation, with the unit after it set upright. |
Use [maths] … [/maths] for a formula on its own line and [m] … [/m] for one inside a sentence. Anything the converter cannot express can be written as LaTeX inside [tex] … [/tex].