In electronics there are seven basic units to measure quantities which define what is going on in a circuit. These are (together with the letter symbols used as abbreviations):
Volts(V) - A measure of the potential difference, emf (electromotive force), or voltage in a circuit. For practical purposes, potential difference, emf and voltage all really mean the same thing.
Amps(A) - A measure of the current flowing in a circuit.
Watts(W) - A measure of the power developed by hte flow of current through a circuit.
The other four refer to the effect of components in the circuit, viz:
ohms(Ω) - A measure of the resistance or individual resistances in a circuit when the current flow is direct (dc).
Impedence(Z) - A measure of the effective resistance or individual resistances in a circuit when the current flow is alternating (ac).
Farads(F) - A measure of the capacitance present in a circuit or produced by individual components, i.e. Capacitors.
Henrys(H) - A measure of the inductance present in a circuit or produced by individual components such as coils.
Reactance(X) - The combined effect of inductance and capacitance in an 'ac' circuit.
Capital letters are also used as abbreviations for voltage and current. strictly speaking E (for emf) is the correct symbol for a voltage source, with V (for volts) in other parts of the circuit. Vs can be used instead of E for a sourse voltage. The capital letter I is used for current. In some circuits lower case letters are used to indicate voltages and currents flowing in different parts of a circuit, e.g. v and i, respectively. These may have a reference annotation attached, particularly in the case of transistor circuits, e.g. Ve describing emitter voltage.
Note: There are also other units employed which can be found in different pages of this site.
In practical circuits, numerical values of these units may be very large, or very small. Resistance values, for example, may run to millions of ohms. Capacitor values may be in millionths or even million-millionths of a farad. To avoid writing out such values in full, prefixes are used to designate the number of noughts associated with the particular value involved. Again the symbol rather than the full prefix is normally used:
mega(M) - meaning times 1,000,000
kilo(k) - meaning times 1,000
milli(m) - meaning divided by 1,000 (1/1,000)
micro(µ) - meaning divided by 1,000,000 (1/1,000,000)
nano(n) - meaning divided by 1,000,000,000 (1/1,000,000,000)
pica(p) - meaning divided by 1,000,000,000,000 (1/1,000,000,000,000)
For example, instead of writing out 22,000,000 ohms in full, this would be shown as 22 Mohms. Similarly a capacitor value of 0.000,000,000,220 farads would be shown as picaf, or more usually 220pF.
The multipliers (M or k) are most commonly associated with values of resistors, and for specifying radio frequencies. The lowest divisor(m) is most usually associated with the values of 'current' typical of transistor circuits, etc. It is also used to specify most practical values of inductances. The larger divisors (µ, n and p) are most commonly associated with capacitor values.
Single capital letter abbreviations are also used for components. The main ones are:
C - capacitors
D - diodes
L - for coils
R - for resistors
These are all standard and universally accepted abbreviations. With other components this is not always the case. Thus 'transistors' may be designated T, TR, Tr, VT or even Q on circuits originating from different sources. The use of TR (or Tr) is preferred, leaving the letter T as the abbreviation for transformers. But note the abbreviation FET (or fet) is always used for 'field effect transistor'.
In practical circuits more than one of the same type of components are normally used. Individual components of the same type are then designated by numbers (usually reading from left to right across the circuit) associated with the component symbol. Thus resistors would be designated R1, R2, R3, etc; capacitors C1, C2, C3, etc...and so on. There is no 'correct' or specific sequence in which such numbers are allocated. They are there only to identify a particular component.
Here are some other general abbreviations which are widely used, although again they may be shown in various different ways - capital letters, or lower case letters in upright or italic, with or without full stops. Thus the abbreviation of alternating current may appear in three different ways:
AC a.c. ac
The general preference is that all such abbreviations should be in lower case italic without full stops and so the following abbreviations are shown that way:
ac - alternating current
af - audio frequency
agc - automatic gain control
am - amplitude modulated (or amplitude modulation)
dc - direct current
eht - extra high tension
fm - frequency modulated (or frequency modulation)