# Back to Basics: Ohms Law

During last night’s Open Hack Night, while trying to explain things like how transistors and mosfets work, it was discussed that maybe taking a step back and outlining some of the basics of electronics would be beneficial to everyone.  So I decided to start with Ohms Law.

There are 3 things that Ohms Law deals with, and they are all related to each other.  Those things are Voltage, Current and Resistance.

Voltage is measured in… well Volts of course!

Current is measured in Amperes.

Resistance is measured in Ohms!  Named after this guy who put a bunch of work in discovering resistive properties electricity, and apparently came up with this relationship known as Ohms Law.  Anyhoo.

I like analogy’s to help me relate things that I know, against things I don’t know.  So everyone’s favorite analogy of electricity is water.  Some will argue this topic to death, but for my purpose I’m sticking to it.

Voltage — So Voltage is actually a potential concept, think of a large water tank on the top of a hill and a small pipe coming out of the bottom.  The difference between what is coming out of the pipe versus the pressure in the tank is the potential voltage.  So voltage is the pressure.

Current — This would be the volume of water pushed through that pipe by the voltage (pressure) and would refer to the quantity of water flowing through it.

Resistance — So this would be the pipe its self.  The larger the pipe (lower resistance), the easier current can flow through, thus having more current.  The smaller the pipe (higher resistance), the harder it is for the current to flow through the pipe, yielding less water.

So with the analogy you can start to formulate how the three things go together.  Large pipes are useless without enough pressure to push the water through.  Small pipes and high pressure still yield small volumes, etc.  Essentially if you change any 2 of the 3, the 3rd one is going to be affected.

The formula for calculating ohms law is quite simple!

Someone figured out an easy way of remembering this ratio by creating a Wheel or Triangle.  It was so cool we spray painted it on our wall.

V = I * R   ( Voltage (V) = Current (I) multiplied by Resistance (R) )

R = V / I  ( Resistance (R) = Voltage (R) divided by Current (I) )

I = V / R ( Current (I) = Voltage (V) divided by Resistance (R) )

So let’s do some examples.  Starting with the first one: V = I * R.  Given a circuit where disconnect a wire right before a 1.2k ohm resistor, I whip out my trusty multimeter, and measure 5 milliamps (or 0.005 amps or amperes) in series with this resistor.   I now have current, and resistance.  So V = I * R.  V = 0.005 * 1200.  V = 6 volts.  So there must be 6 volts going across this resistor.

Keeping the same numbers, if I measured 6 volts with my meter, and then 5 milliamps.  I could figure out the resistance.  R = V / I.  R = 6 / 0.005.  R = 1200 ohms.

And lastly if I measured 6 volts with my meter, and I can see its a 1.2k ohm resistor.  I = V / R.  I = 6 / 1200.  I = 0.005 amps.

So how does this pan out in a real world scenario ?   Imagine installing an exhaust fan 100 feet away from its power source.  The fan will operate at 115 volts at 100 amps, or 230 volts at 50 amps.    One option will require a much thicker wire than the other because of the wire resistance.  Or take a look at the dimmer switch on your wall at your house.  When you turn the dimmer switch, the resistance goes up or down lowering the current going across the bulb’s filament causing it to glow more or less.  