Power
- It is defined as the rate of doing work.
OR
- It is rate at which electrical energy is consumed or dissipated.
- If W is electric energy consumed in a circuit for a given time, the power
P = W / t
- Let, the electric energy supplied by a current source for a given time t is
W = VQ
From equation ( 1
) and ( 2 )
- P = VQ / t …… ( 3 )
- But I = Q / t …… ( 4 )
From equation ( 3
) and ( 4 )
- P = VI
According to ohm’s
law V = IR
- P = ( IR )R = I^{2}R ( As V = IR )
According to ohm’s
law I = V / R
- P = VI = V ( V / R ) = V^{2} / R
Equation of Power |
VI |
I^{2}R |
V^{2} / R |
The SI unit of
power is watt.
Watt = Volt ×
Ampere
Electrical Energy
- It is defined as the amount of energy consumed in an electrical circuit for a given time.
- The electrical energy not only depends upon electrical power but also on the time for which electrical power is supplied.
- Electrical energy = Power × Time
- Unit of electrical energy = Watt – Second
Kilo watt hour
- The commercial unit of electrical energy is kilo watt hour ( kWh )/
- One Unit = One kWh
= 10^{3} × Watt ×
3600 second
= 3.6 × 10^{6 }Watt
second
= 3.6 × 10^{6 }Joule
Example
How many units are
consumed by a lamp of 100 W if it will turn on for twenty hour.
Solution
Rating of lamp =
100 W
Twenty hour
1 kWh = One unit
Number of units
consumed by lamp = 100 × 20
= 2000 Wh
= 2 kWh
= 2 Unit
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