# Basic radio theory, circuits and calculations

### Electrical units

Current
The unit of current flow is called the 'ampere' (amp) and the strength of a current is said to be 'x amperes'. Currents are denoted in formulae by the symbol I, the symbol for the unit of current (ampere) is A. The currents used in radio are often very small fractions of an ampere and for convenience the two small units 'milliampere' (10-3A) and microampere' (10-6A) are used. Thus a current of 0.003 ampere is written as 3 milliamperes. See Table 2.2 for abbreviations.

Voltage
In order to make a current flow through a circuit, it is necessary to have some device which can produce a continuous supply of electrons. This may be a battery, in which the supply of electrons is produced by chemical action, or a dynamo or generator in which mechanical energy is turned into electrical energy. The battery or generator produces an 'electromotive force' (EMF symbol E), which may be used to force a current through a circuit. The unit of electric potential is the 'volt', and voltages are usually denoted in formulae by the symbol V.

Resistance
The ease with which an electric current flows through a wire depends on the dimensions of the wire and the material from which it is made. The opposition of a circuit to the flow of current is called the 'resistance' (R) of the circuit. The resistance of a circuit is measured in 'ohms'( ). For convenience, because the resistances used in radio equipment may be up to 10,000,000 ohms, two larger units called the 'kilohm' (1,000 ) and the 'megohm' (1,000,000 ) are used. Thus 47,000 may be abbreviated to 47k .

It should be noted that it is common to find values in more than one format. For resistors 1R2 could be used to indicate 1.2; 1k3 to indicate 1.3k; 2M1 to indicate 2.1M. In the case of voltage 5V6 would indicate 5.6V.

Table 2.2. Units and Symbols

Quantity | Symbol used in formulae | Unit | Abbreviation
current | I | ampere | A
EMF | E | volt | V
electric potential | V | volt | V
time | t | second | s
resistance | R | Ohm |
capacitance | C | farad | F
mutual inductance | L | henry | H
power | W | watt | W
frequency | f | hertz | Hz
wavelength |  | meter | m

Abbreviations for multiples and sub-multiples

G | giga | 109 | 1,000,000,000
M | mega | 106 | 1,000,000
k | kilo | 103 | 1,000
c | centi | 10-2 | 0.01
m | milli | 10-3 | 0.001
| micro | 10-6 | 0.000001
n | nano | 10-9 | 0.000000001
p | pico | 10-12 | 0.000000000001

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