Page 46 - Teach Yourself Electricity and Electronics

26 Electrical units

The ampere

Current is a measure of the rate at which charge carriers flow. The standard unit is the
ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers
per second past a given point.
An ampere is a comparatively large amount of current. The abbreviation is A. Often,
current is specified in terms of milliamperes, abbreviated mA, where 1 mA 0.001 A
or a thousandth of an ampere. You will also sometimes hear of microamperes (µA),
where 1 µA 0.000001 A 0. 001 mA, a millionth of an ampere. And it is increasingly
common to hear about nanoamperes (nA), where 1 nA 0. 001 µA 0.000000001 A (a
billionth of an ampere). Rarely will you hear of kiloamperes (kA), where 1 kA 1000 A.
A current of a few milliamperes will give you a startling shock. About 50 mA will jolt
you severely, and 100 mA can cause death if it flows through your chest cavity.
An ordinary 100-watt light bulb draws about 1 A of current. An electric iron draws
approximately 10 A; an entire household normally uses between 10 A and 50 A,
depending on the size of the house and the kinds of appliances it has, and also on the
time of day, week or year.
The amount of current that will flow in an electrical circuit depends on the voltage,
and also on the resistance. There are some circuits in which extremely large currents,
say 1000 A, flow; this might happen through a metal bar placed directly at the output of
a massive electric generator. The resistance is extremely low in this case, and the gen-
erator is capable of driving huge amounts of charge. In some semiconductor electronic
devices, such as microcomputers, a few nanoamperes will suffice for many complicated
processes. Some electronic clocks draw so little current that their batteries last as long
as they would if left on the shelf without being put to any use at all.

Resistance and the ohm

Resistance is a measure of the opposition that a circuit offers to the flow of electric
current. You might compare it to the diameter of a hose. In fact, for metal wire, this is
an excellent analogy: small-diameter wire has high resistance (a lot of opposition to
current flow), and large-diameter wire has low resistance (not much opposition to
electric currents). Of course, the type of metal makes a difference too. Iron wire has
higher resistance for a given diameter than copper wire. Nichrome wire has still more
resistance.
The standard unit of resistance is the ohm. This is sometimes abbreviated by the
upper-case Greek letter omega, resembling an upside–down capital U (Ω). In this book,
we’ll just write it out as “ohm” or “ohms.”
You’ll sometimes hear about kilohms where 1 kilohm 1,000 ohms, or about
megohms, where 1 megohm 1,000 kilohms 1,000,000 ohms.
Electric wire is sometimes rated for resistivity. The standard unit for this purpose
is the ohm per foot (ohm/ft) or the ohm per meter (ohm/m). You might also come
across the unit ohm per kilometer (ohm/km). Table 2-1 shows the resistivity for vari-
ous common sizes of wire.
When 1V is placed across 1 ohm of resistance, assuming that the power supply can