18 April 2012

Heating Effect of Electric Current and Electrical Power

Heating Effect of Electric Current
When charge (electron) move through conductor ,collide with atoms of the conductors due to resistance  and transfer energy to the conductor which appear  as  heat .
Hence work done by electron to overcome resistance is equal to heat generated in conductors.
Let W work done to move Q charge through a conductor of resistance R that appear as Heat
W = H
But , V = W/Q Þ  W = Q V
Þ H = QV
But, I = Q /t  Þ Q = It
\ H = V I t
 Also, V = IR
Þ H = It x IR
Þ H =I2Rt

Electrical Power:
The rate of doing work in moving charge through conductors
OR, The rate of producing heat in conductor
Or, Rate of producing electrical Energy
P = W/ t
OR, P = H/t         
But, H =I2Rt
P = I2Rt /t      Þ  P = I2R
Also we may express Power as
P = I x I x R
Using V =IR
P = VI
SI unit of POWER
P = H/t = 1J/1Sec = Watt
If 1oule energy consumed by device in 1 sec Its power said to be 1 watt
Commercial electrical Energy unit is kWh
We know that P = H/t Þ  H = Pt
If 1 kW (1000watt) energy consumed for 1 hour It is called 1unit(BOT unit)
Energy in term of joule:
H = 1kWh  = 1000W x 3600sec =1000J/sec x 3600sec = 3.6 x 106 j
Numerical X Current Electricity Numerical Practice worksheet
Q. 1. Find the work done by an electron to maintain the potential difference of 80V?
Q. 2. What is the potential difference between the ends of 16Ω resistance, when a current of 1.5A flows through it?
Q. 3. The potential difference across the the terminals of an electric iron is 240V and the current is 6 A what is the resistance of electric iron?
Q. 4. If there are 108 electrons flowing across any cross section of a wire in 4 minutes, what is the current in the wire?
Q. 5. .A copper wire has diameter o.5mm and resistivity 1.6 x 10-8 ohm m what will be the length of this wire to make the resistance of 10 ohms?
Q. 6. Find the effective resistance of resistors 0.01 ohms and 107 ohms.in series and parallel
Q. 7. Two resistors of same materials has been connected in series first and then in parallel. Draw a V – I graph to distinguish these connection.
Q. 8. Three resistors 3,4,5 ohms are joined in parallel in a circuit. If a current of 150 mA=150×10-3A flows through the resistor of 4 ohms, then find the values of the current in mA which will be flowing in other two resistors?
Q. 9. A wire of length 2cm having resistance R is stretched to have an increase of 100% of original length . Find its new resistance with respect to its original resistance.
Q. 10. An electric lamp has resistance of 400 ohms. It is connected to a supply of 250V. If the price of electric energy is Rs.1.20 per unit, calculate the cost of lighting the lamp for 20 hours.

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