Rev. 11/10/00
2
Current
Current is the rate of flow of electricity through a circuit. In simple terms, it is
the number of electrons that flow past a specified point in one second of time.
Current is measured in amperes using the letter A in the measurement and the
letter I in formulas. A typical circuit breaker for a house is rated at 15A or 20A.
Current flow in telephone circuits and electronics is usually in the milliampere
(mA) range. A milliamp is 1/1,000
th
of an ampere (1mA = .001A).
Resistance
Resistance is the opposition to the flow of electricity in a circuit. The length
and diameter of a wire or cable pair affects the total resistance in a circuit. Other
factors like temperature and humidity have some affect on resistance as well.
Resistance increases as the wire diameter decreases. Resistance is measured in
Ohms and is identified by the omega (
Ω
) symbol. Formulas use the letter R.
Different materials offer different amounts of resistance. Conductors offer little
resistance while insulators offer high resistance. A multi-stranded wire has less
resistance than a solid single-strand wire because the multi-stranded wire has more
surface area for the electrons to travel on than a solid wire.
Ohms Law
Ohms Law is a formula used to calculate voltage, current, and resistance when
any two of these values are known. The formula is expressed in three different
variants depending upon which values are known and which value is to be
calculated. The variants are: E = IR, I = E/R and R = E/I. This means that
voltage is directly proportional to the current multiplied by the resistance. Current
is calculated by dividing the voltage by the resistance; and resistance is calculated
by dividing the voltage by the current. Therefore, we can observe that the higher
the voltage, the higher the current flow; and the higher the resistance the lower the
current flow.
Capacitance
A capacitor is a device that stores an electrical charge and usually consists of
two conductive plates separated by an insulator. A twisted pair of wires with
insulation acts much like a capacitor when voltage is applied to the pair. A
capacitor blocks DC voltage. Capacitance is referred to as the ability to hold an
electrical charge and is present on telephone cables. The unit of measurement for
capacitance is the farad, although micro and pico-farad measurements are most
commonly encountered. Larger gauge wires and longer runs of wire have more
capacitance on them. The greater the capacitance there is on a twisted pair, the
greater the voice signal loss. Insulation, thickness, and spacing of wires affect the
capacitance in a cable.
