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Electrical energy : Electrical energy Next Map
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W =VIt : W =VIt Previous Next Map
Slide 5 : SI unit of electrical energy is joule 1J is defined as the energy supplied by the external source in providing a current 1 ampere for 1 second in the conductor under a potential difference of 1V Previous Next Map
Slide 6 : Watt-hour (Wh) and kiloWatthour(kWh) are the commercial units of electrical energy. 1Wh =1W x 60 x 60 s = 3600J
1Wh = 3600J 1kWh =1kWx1hour=
1000 W x 60x60s =3.6 x 106J
1kWh = 3.6 x 106J = 1 unit of electrical energy Previous Next Map
Numericals : Numericals The current through a 12V tungsten filament lamp connected to a 12 V battery is 3 A. Calculate the resistance of the filament and the electrical energy consumed in 5 hours?
V =12V, I=3A , t = 5 hours
5 hours = 5 x 60 x60 s
R = V/I = 12/3 = 4W
W = VIt = 12 x 3 x 5 x 60 x60 = 648000 J Identify data Steps to solve numerical problems Convert data into (SI) proper units Choose equation Substitute and solve equation Check the answer Previous Next Map
Slide 8 : A current of 0.2 A flows through a wire whose ends are at a potential difference of 12 V. Calculate the energy produced in 1 minute.
1 minute = 60 seconds
W = VIt = 12 x 0.2 x 60 = 144 J Previous Map
Electrical Power : Electrical Power Next Map
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Slide 11 : SI unit of electrical power is watt(W).
Electrical power consumed is 1 W when a potential difference of 1V causes a current of 1 A to flow through a conductor Bigger units of power and their relation to W
1kilowatt(1kW) = 1000W
1Horse-power (1HP) =746W Previous Next Map
Numericals : Numericals Two devices A and B are rated 100W, 120V and 10W,120V respectively. They are connected across a 120 V in series. Calculate the power consumed by each bulb.
Resistance of A = V2/P = 1202/100 =144W
Resistance of B = V2/P = 1202/10 = 1440 W
Current through A and B = V/RA+RB = 120/1584 = 0.08A
Hence, Power consumed by A = I2RA = 0.082 x 144 =0.92W
Power consumed by B = I2RB = 0.082 x 1440 = 9.216W (In series circuit same current flows through both A and B. Hence power has to be calculated using the formula P=I2R. Previous Next Map
Slide 13 : Calculate the electrical energy in kWh consumed in a month, in a house using 2 bulbs of 100W each and 2 fans of 50W each, if the bulbs and fans are used for an average of 6 hours in a day.
If the cost per unit is Rs. 3, calculate the cost per month.
Total power consumed = 2x100 + 2x50 = 300W.
Electrical energy consumed per day = P X t = 300x6 = 1800Wh
Electrical energy consumed per month = 1800 x 30 = 54000 Wh =54kWh.
Cost per month = 54 x 3 = Rs. 162 Previous Map
Key points : Key points Electrical energy W = QV =VIt = I2Rt = V2t/R
W = Power x time
Electrical power P = VI = I2R = V2/R