Notes on Electric Power and Electric Energy
- The quantity work has to do with a force causing a displacement. Work (force × displacement) has nothing to do with the amount of time that this force acts to cause the displacement.
- Sometimes, work is done very quickly and times the work is done rather slowly. The two people might do the same amount of work.
- Two boys are lifting a load of 20kg to a height 1m. One may lift he load in less time then other. The quantity that has to do with the rate at which a certain amount of work is done is known as the power. Power is the rate at which work is done. It is the work/time ratio. Mathematically, it is computed using the following equation.
Electric Power
- The electrical energy is supplied to the load by an energy source such as an electrochemical cell.
- A cell does work upon a charge to move it from the low energy to the high energy terminal. The work done on the charge is equivalent to the electrical potential energy change of the charge.
- Thus, electrical power, like mechanical power, is the rate at which work is done. Like current, power is a rate quantity.
- It is defined as the amount of electric energy supplied to a circuit or consumed by a load in unit time. Its mathematical formula is:
Electric Power (P) =
But we know that the work done ‘W’ by current ‘I’ when it flows for time ‘t’ under a potential difference ‘V’ is given by W = V x I x t [W = V Q]
Electric power = Potential difference × current
Now if an electrical appliance is operated at a potential difference of 1 volt and the device carries a current of 1 ampere, then power becomes 1 watt.
Therefore, 1 watt = 1 volt × 1 ampere
1 kW=1000 watt
1 hp=746 watt= 0.746 kwatt
Definition of 1 Watt
Electric power is one watt if one ampere electric current flows through a circuit having potential difference of one volt.
We have other formulae of electric power which are following:
(a) Power in term of I and R :
Since P = v × I…………(I)
Now from ohm’s law we have v= I r
(b) Power in the terms V and R :
P = V × I ——————– (i)
from ohm’s law we have V/I = R or I = V/R
Putting this value of I in equation (i) we get
Or
NOTE: When two or more bulbs connected in series.
If ‘n’ numbers of appliances have the same power ‘P’ then Ps = P/ n
NOTE: When two or more bulbs connected parallel. P = [P1+P2+P3] Net power of series combination of appliances:
If three appliances of power rating P1 , P2 and P3 at the same voltage connected in series, then net
power of combination (Ps ) given by :
If ‘n’ numbers of appliances have the same power ‘P’ connected in series, then
Net power of parallel combination of appliances:
If three appliances of power rating P1 ,P2 and P3 at the same voltage connected in parallel, then net
power of combination(Pp ) given by :
= P1 +P2 + P3
If ‘n’ numbers of appliances have the same power ‘P’ connected in parallel,
then
Pp = n P
Electric Energy:
Work done by a source of electric current to maintain electric current in electric circuit in a given time.
OR
Total work done by an electric appliance in a given time .
Electric energy (E) = Power (P) x time (t)
= V I t
S.I unit of electric energy= Joule
E= P × t
1 joule= 1 watt × 1 sec
Definition of one joule :
One joule of electric energy consumed when an appliance of one watt consumes electrical energy for one second.
The commercial unit of electric energy is Kilowatt-hour (k Wh) which also known as Board of Trade Unit ( B.O.T.U).
One kWh= 1 kilowatt × 1 hour
The kilowatt hour is the amount of electric energy consumed when an electrical appliance of 1 kilowatt rating is used for 1 hour
One kilowatt hour (kWh) = one kW × 1 hour =
How to calculate the electricity bill of electricity consumed : electricity bill delivered gives the number of units consumed.
One unit = One kWh = 3.6 × 106 joule
Relation between kWh and joule:
One kWh = 1 kilowatt × 1 hour
= 100 watt × 3600 sec
Applications of Heating Effect of Electric Current
Physics Electricity Formulae : Electricity