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Class 10 electricity Notes : Electric Power





12. Electric Power

  • Rate of doing work or the rate of consumption of energy is known as POWER
    Mathematically,

  • SI unit of power is Watt which is denoted by letter W. The power of 1 Watt is a rate of working of 1 Joule per second. Actually Watt is a small unit, therefore , a bigger unit of electric power called Kilowatt is used for commercial purposes. Also ,
    1 kilowatt = 1000 Watts
    So,
    " the rate at which electric work is done or the rate at which electric energy is consumed is called electric power "
  • We will now derive formula for the calculation of electric power.
    From equation 14 we know that

    Now we know that work done W by current I when it flows for time t under a potential difference V is given by

    Putting this value of work done in equation 16 we get

    Hence,
    Electric Power = voltage x current



12 (a) Power in terms of I and R

  • From equation 17 we know that
    $P=VI$
    Now from Ohm's law

    Putting above equation in equation 15 we get
    $P=I \times R \times I$
    Power , $P=I^2 \times R$
  • Above formula is used to calculate power when only current and resistance are known to us.

12 (b) Power in terms of V and R

  • From equation 17 we know that P=VI Now from Ohm's law

    Or we have

  • Putting this value of I in equation 15 we get
    $P = V \times \frac{V}{R}$
    $P = \frac{V^2}{R}$
    This formula is used for calculating power when voltage V and resistance R is known to us.
  • It is clear from the above equation that power is inversely proportional to the the resistance.
  • Thus the resistance of high power devices is smaller then the low power ones. For example , the resistance of a 100 Watt bulb (220 V) is smaller then that of 60 Watt (220 V) bulb.
  • We have three formulas for calculating electric power. These are
    (1) $P = V \times I$
    (2) $P= I^2 \times R$
    (3)$P = \frac{V^2}{R}$
    You must memorize these formulas as they would be used to solve numerical problems.
  • When electrical appliance consumes electrical energy at the rate of 1 Joule per second , its power is said to be 1 Watt.
  • Rate at which electric work is done or the rate at which electric energy is consumed , is called electrical power.






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