Rating of Electric Appliances


 
 
Concept Explanation
 

Rating of Electric Appliances

All the electrical gadgets are provided with a rating, for example on  an electric bulb you see that apart from the name and the symbol of the company, you will find values of power and potential difference. For example, it could be 60 W, 220 V. It means that 220V should be applied across this bulb, and when 220V is applied, the power consumed will be 60 W. You will find similar markings on all electric appliances. For an electric appliance, the values of power and voltage taken together form what is called the rating of the appliance.

From the rating of an appliance, you can easily calculate its resistance by using the equation

P=frac{V^{2}}{R}

Note that higher the power rating, smaller is the resistance because from equation it is clear that power is inversely proportional to resistance. So, a 1000-W heater has less resistance than a 100-W bulb.

The current drawn by an appliance can be calculated by using the relation of Ohms Law

V= IR

Rightarrow R= frac{V}{I}

and using the relation for the power

P=frac{V^{2}}{R}

Rightarrow P=frac{V^{2}}{frac{V}{I}}= frac{V^2}{1}timesfrac{I}{V}= IV

Rightarrow I=frac{P}{V}

Illustration:  A bulb is rated 40 W, 220 V. Find the current drawn by it when it is connected to a 220-V supply.

Solution. Since the bulb is rated at 220 V and it is connected to a 220-V supply, the power consumed will be 40W. The current drawn by it is

                                I=frac {P}{V}=frac {40W}{220V}=frac {2}{11}A.

Sample Questions
(More Questions for each concept available in Login)
Question : 1

Marking on a bulb is 60 W,220V. What does it signify?

Right Option : A
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