Physical Laws: Volts, Amps, Coulombs, Joules, Watts

CS 641 Lecture, Dr. Lawlor

Physical reality places surprisingly pressing limitations on modern computer performance.

One Watt is a Joule per second (W=J/s).  For example, the Fukishima Daichi nuclear plant was rated at 4.7 gigawatts, and could power a medium-sized city, but the cooling system is absolutely life critical.  My ordinary 140hp four cylinder passenger car produces over 100,000 watts, and needs a dedicated liquid cooling system to keep from melting down.  My big desktop replacement laptop burns 80W at idle, and over 130W when the graphics card is cranking, so can get by with a copper finned heatsink and dedicated fan.  A cellphone might burn 2W at maximum usage, so needs no fan.  A single LED burns a few milliwatts.  Generally:
One amp is a coulomb of electrons (6.24x1018 e-) per second.  Typical microcontroller signals are milliamps, typical wall plug currents are up to a few dozen amps, and typical arc welding current is about a hundred amps.

In order of delivered amps at 12 volts:
An electrical arc is actually a fairly useful component:
A surprising variety of electrical components can be constructed from ordinary wire:

A 1 ohm resistor will conduct no more than 1 amp of current at 1 volt.  Ohm's law (V=I R or volts = amps * ohms) is more of a guideline, an assumption of linearity that is only valid for resistive materials (OK for most metals, poor for most insulators, liquids, or semiconductors). 

Power law: P = I V, or watts = volts * amps.  A 0.1 ohm piece of wire will drop 1 volt if you push 10 amps through it, which takes 10 watts: the wire might get fairly warm, but will still be there.  The same wire taking 100 amps will drop 10 volts, so must dissipate 1000 watts: the wire is going to feel some serious heat.

For example, a current-signaling network might represent individual byte values as groups of 0 to 255 electrons.  255 electrons per sample at 100 million samples per second is 25.5x109 Ge-/s.  One Coulomb is 6.24x1018 e-, so that's a current of 4x10-9 C/s, or 4 nano-amps.  At 1V signal strength, a one-ohm wire will lose 4 nano-volts.  Not much!  Of course, in practice we usually use voltage-signaled networks, and single electrons have a bad habit of obeying only the funky quantum laws instead of ordinary classical dynamics, which makes things much more complicated.