The
three basic elements of electrical engineering are resistor, inductor and
capacitor.
Resistor
coverts electrical energy into heat energy when current is forced through a
material.
Inductor
and capacitor store in the positive half cycle and give away in the negative
half cycle of supply the magnetic field and electrical field energies
respectively.
INSTANTANEOUS
POWER
The
power supplied to a circuit is the product of instantaneous of voltage and
instantaneous current and it is measured in watts irrespective of the type of
circuit used.
The
instantaneous power may be positive or negative.
Positive
value means that power flows from the source to the load.
Negative
value means that power flows from the load to the source.
APPARENT
POWER (S)
The
apparent power is the power that appears to be present when the voltage and
current in a circuit are measured separately.
The
apparent power, then, is the product of voltage and the current regardless of
the phase angle θ.
Apparent
power is denoted as S
S
= VI its unit is volt-ampere (VA) and its bigger units are kVA and MVA.
Apparent
power can be measured by using voltmeter and ammeter.
ACTIVE
POWER OR TRUE POWER
The
power which actually consumed in the circuit is called true power or active
power. A wattmeter is constructed so that it takes into account any phase
difference between current and voltage.
Active
power is denoted as P and its unit is watts and its bigger units are kW and MW.
P
= Voltage x component of total current in phase with voltage.
P
= V I cosθ in watts
True
power can be measured by using wattmeter.
True
power is used for producing torque in motors and supply heat, light etc. The
used true power cannot be recovered.
True
power does useful work in the circuit.
REACTIVE
POWER (OR) WATTLES POWER
This
power that flows back and forth in both directions in the circuit or reacts
upon itself. Hence it is called as reactive power.
The
product of voltage (V) and component of total current 90º out of phase with
voltage (I sinθ) is equal to reactive power.
Reactive
power is denoted as Q and its unit is VA and its bigger units are kVA and MVA.
Q
= Voltage x component of total current 90º out of phase with voltage.
Q
= V I sinθ in volt-amperes
I
sinθ is called the reactive component or wattles component.
A
wattmeter does not measure the reactive power.
Reactive
power does no useful work in the circuit and merely flows back and forth in
both directions in the circuit. 
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