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Theory:

Electricity is used in both households and industries. The amount of electricity consumed is determined by two factors. They are
  1. The amount of electricity consumed
  2. The duration of usage
The product of electric power and its usage time is used to calculate the electrical energy consumed.
 
Electrical energy =Electricpower×Time
 
If an electric appliance of power \(200\ watt\) is utilised for \(3\ hours\), then the total electric power consumption is
 
200×3 =600watthour
 
Electrical energy consumption is quantified and stated in watt-hours, even though the SI unit is watt-second. But in reality, a larger electrical energy unit is required. The higher units of electrical energy are watt-hour (\(W\ h\)) or kilowatt-hour (\(kW\ h\)).
 
1 watt hour =1 watt×1hour=1W×60×60s=3600 W s
 
In terms of joules,
 
1watthour=3600J
 
Kilowatt is the larger unit of power that is most commonly used in the devices.
 
1kilowatthour=1kilowatt×1hour=1kW×60×60s=1000W×3600s=3.6×106J
 
In terms of megajoules,
 
1kWh=3.6MJ
 
The energy consumed for domestic purposes is measured using the electric meter in the units of a kilowatt-hour.
\(\)
Horse power:
Another unit called horse power (\(hp\)) is also used to express electric power. It is one of the units in the foot-pound second (FPS) system or English system.
 
1 horse power (hp= 746 watt
 
Example:
Two bulbs are have the ratings as \(60\ W\), \(220\ V\) and \(40\ W\), \(220\ V\) respectively. Which one has a greater resistance?
 
Electric power, P=V2R
 
For the same value of \(V\), \(R\) is inversely proportional to \(P\). Therefore, the lesser the power, the greater the resistance. Hence, the bulb with \(40\ W\), \(220\ V\) rating has greater resistance.