What is commonly defined as electricity is really just the
movement of electrons. So, let’s start at that point.
Current (I, Amps)
Current (as the name implies) is the movement or flow of
electrons (I) and is measured in units of Amperes. This is usually abbreviated
to Amp or, even shorter, A. The flow of electrons in an electrical current can
be considered the same as the flow of water molecules in a stream.
To get anything to move requires potential and the same
thing happens
to electrons.
Potential (V, Volts)
Potential is the force (called Electromotive Force or EMF)
that drives the electrons and has a measurement of voltage. This is abbreviated
as a unit of measurement to Volt or even further to V.
Resistance (R, Ohms)
Resistance is the property that resists current flow. It is
analogous to friction in mechanical systems. The unit of this is ohm (we have
to give some credit to the fellow who first named it). It is sometimes shown
with its official ohm mark (Ω) and the short form of the word resistance is
always R.
Resistance not only depends on the material used for the
conductor but also upon size and temperature. Increase in the cross-sectional
area will decrease the resistance Increase in the length will increase the
resistance. Increase in the temperature will increase the resistance (for most
materials that conduct electricity)
Capacitance (C, Farads)
Any two conductors separated by an insulating material form
a capacitor or condenser. Capacitance of a device is its capacity to hold
electrons or a charge. The units of measurement are farads. We commonly see it
in smaller amounts called microfarads μF and picofarads pF. Capacitance depends
on the construction.
Magnetic Flux (Unit of Measurement is Webers)
When current flows in a conductor, a magnetic field is
created around that conductor. This field is commonly presented as lines of
magnetic force and magnetic flux refers to the term of measurement of the
magnetic flow within the field.
This is comparable to the term current and electron flow in
an electric field. The following illustration shows the direction of magnetic
flux around a conductor and the application of the easily remembered
right-hand-rule. Mentally, place your right hand around the conductor with the
thumb pointing in the direction of current flow and the fingers will curl in
the direction of magnetic flux.
Magnetic Lines of Force (MMF)
Lines of magnetic force (MMF) have an effect on adjacent
conductors and even itself. This effect is most pronounced if the conductor
overlaps itself as in the shape of a coil.
Magnetic Self-Inductance
Any current-carrying conductor that is coiled in this
fashion forms an Inductor, named by the way it induces current flow in itself
(selfinductance) or in other conductors.
Inductance (L, Henrys)
Opposition to current flowing through an inductor is
inductance. This is a circuit property just as resistance is for a resistor.
The inductance is in opposition to any change in the current flow. The unit of
inductance is Henry (H) and the symbol is L.
Frequency (f, Hertz)
Any electrical system can be placed in one of two categories
direct current (dc) or alternating current (dc). In dc systems the current only
flows in one direction.
The source of energy maintains a constant electromotive
force, as typical with a battery. The majority of electrical systems are ac.
Frequency is the rate of alternating the direction of
current flow. The short form is f and units are cycles per second or Hertz
(short-formed to Hz).
Reactance (X, Ohms)
The opposition to alternating current (ac) flow in
capacitors and inductors is known as reactance. The symbol for capacitive
reactance is XC and for inductive reactance XL.
Although we will not go into the derivation of the values,
it can be shown that when f is the frequency of the ac current:
XL= 2 Πf L
XC=1/2ΠfC
Impedance (Z, Ohms)
The total opposition or combined impeding effect of
resistance plus reactance to the flow of alternating current is impedance. The
word impedance is short formed to Z and the unit is ohms.
Active Power (P Watts)
Instead of working directly with the term electrical energy,
it is normal practice to use the rate at which energy is utilized during a
certain time period. This is defined as power. There are three components of
power: active, reactive and apparent.
Active power or real power is the rate at which energy is
consumed resulting in useful work being done. For example, when current flows
through a resistance, heat is given off. It is given the symbol P and has the
units of Watts.
Reactive Power (Q, Vars)
Reactive power is the power produced by current flowing
through reactive elements, whether inductance or capacitance. It is given the
representative letter Q and has the units volt-amp-reactive (VAR).
Reactive power can also be considered as the rate of
exchange of energy between a capacitor or inductor load and a generator or
between capacitors and inductors.
Although it does not produce any real work, it is the
necessary force acting in generators, motors and transformers. Examples of this
are the charging/discharging of a capacitor or coil. Although this creates a
transfer of energy, it does not consume or use power as a resistor would.
Apparent Power (U, Volt Amps)
Apparent power is the total or combined power produced by
current flowing through any combination of passive and reactive elements. It is
given the representative letter U and has the units’ volt-amps (VA).
Power Factor (PF)
Real power/ apparent power
Power Factor is the comparison of Real power to apparent
power.
• For a resistor, there is no reactive power consumed. Thus
apparent power used is totally real. The power factor would be 1 or often
referred to as unity power factor
• For a pure inductor or capacitor, the apparent power
consumed is entirely reactive (real power is nil). The power factor would then
be 0.
• For power consumed by impedance consisting of resistance,
inductance and capacitance the power factor will of course vary between these
two limits. The most efficient use or consumption of power is obtained as we
approach unity power factor.
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