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

- 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
- 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 |

###### Related articles

- Electricity Basics – AS Electronics Revision (mattg99.wordpress.com)
- Eqivalent Resistances and Kirchoff’s Laws – AS Electronics Revision (mattg99.wordpress.com)
- Ohm’s Law, Power and Powers of Ten – AS Electronics Revision (mattg99.wordpress.com)

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