SI Units

What is current and voltage? To answer this question we must first review some basic concepts from physics. Three different quantities will be of interest to us. These quantities are force, work or energy, and power. The standard measure of force in the international system (SI) system of units is the newton (abbreviated as $N$). A newton is the force required to accelerate a one kilogram mass by 1 ${\rm meter}/ {\rm sec}^2$. The standard measure of work or energy in the SI system is the joule (abbreviated as $J$). A joule is the work done when a one newton force is applied over a distance of 1 meter. One joule, therefore, equals 1 newton-meter. Finally, power measures the rate at which work is done. The standard measure of power in the SI system is the watt. One watt of power is generated when one joule of work is done in one second. One watt, therefore, equals 1 joule/second.

In circuit analysis, we often encounter values of power that are either much larger (as in the case of power distribution systems like our national power grid) or much smaller (as in the case of the power dissipated by integrated circuits) than a single watt. There is a standard set of prefixes that we use in such situations. These prefixes are listed below:

	prefix	symbol
$10^9$	giga	G
$10^6$	mega	M
$10^3$	kilo	k
$10^{-3}$	milli	m
$10^{-6}$	micro	$\mu$ or u
$10^{-9}$	nano	n
$10^{-12}$	pico	p

So if we have a device that dissipates $10$ MWatts , then we mean it dissipates $10^7$ watts. This level of power dissipation is consistent with large power systems. On the other hand, a small integrated circuit may dissipate somewhere on the order of 10 m-watts. This, of course, corresponds to a dissipation of $10^{-4}$ watts.