Ohm’s Law, Power and Powers of Ten – AS Electronics Revision

Ohm's law triangle

Ohm’s law triangle (Photo credit: Wikipedia)

Ohm’s Law

  • V=IR
  • Voltage = Current * Resistance
  • It’s so useful that the guy who invented it got the unit for resistance named after him
  • Obviously, it can be rearranged to work out any of the three things, as long as you know two of them
  • It only works for ohmic conductors (conductors which follow Ohm’s law)
  • Resistors in circuit diagrams are ohmic, because they’re made from metal
  • Lamps, LEDs, thermistors and Light-Dependent Resistors are some of the many things that aren’t ohmic – you have to use ways of getting around that


  • Power is the rate of use/transfer/conversion/wasting of energy
  • The symbol for power is P
  • It’s measured in Watts (unit represented by the letter W)
  • One Watt is equal to one Joule being used per second (Joules are the units of energy)
  • Power = Energy / Time, but Electronics doesn’t use that formula
  • P=IV (Power = Current * Voltage) is more useful when you’re talking about electricity
  • Power is dissipated in resistors, because forcing electrons through resistors wastes energy as heat
  • The amount of energy converted to heat depends on the current being forced, and the resistance it’s being forced through, thus Power = Current squared * Resistance

    Licensed under Creative Commons Attribution-NonCommercial 2.5

    A relevant image from xkcd

  • Also, Power = Voltage squared / Resistance, because Current = Voltage / Resistance
  • If you remember V=IR (Ohm’s Law) and P=IV, you can figure out the other two formulae (the ones with the squaring) without needing to remember them

Powers of Ten

  • Electronic circuits often have small currents measured in milli-Amps, and large resistances measured in kilo-Ohms
  • A milli-Amp (I don’t know how you spell it, but they’re written as mA – the lower-case m is very important) is one thousandth of an amp
  • A kilo-Ohm (kΩ) is one thousand Ohms
  • This can make Ohm’s law calculations a bit more complicated
  • If you do a calculation with kΩ and regular volts it’ll come out as mA
  • If you do a calculation with regular volts and mA, it’ll come out as kΩ
  • You might want to convert things to regular units before doing your calculations
  • (Divide your kΩ by 1000 and multiply your mA by 1000) – that way, there’ll be no kilo or milli prefixes on whatever your result is (you can then convert it into a milli or a kilo whatever, if you want)
  • This also applies to power calculations – you could end up with an answer in mW (milli-Watts)… or even kW (kilo-Watts), I suppose, but that’s unlikely!
  • The safest way to do it (in my opinion) is to use the standard form button on your calculator (the times-ten-to-the-power-of-something button)
  • Also remember not to ohmify things that aren’t ohmic (as mentioned before)
  • There are a lot of these powers of three multipliers:
10 to the power of Prefix Letter Pronunciation
-12 p pico
-9 n nano
-6 μ micro
-3 m millli
0 (None)
3 k kilo
6 M Mega
9 G Giga
12 T Tera

About Matt

I like writing, filmmaking, programming and gaming, and prefer creating media to consuming it. On the topic of consumption, I'm also a big fan of eating.
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