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Energy savings calculation!!

An energy saver light bulb of 11 W gives the same illumination as a conventional 60W light bulb. (a) If a typical light bulb is used on average for five hours a day, how much energy is saved per year by one light-bulb? (b) If each household used eight such light-bulbs, ad there are 20 million households in a country, how much energy would that country save annually?

Asked SNB

led(Answers to this question invited from all users and visitors)

How many bulbs can be connected in parallel

English: Incandescent light bulbs.

I need to know how many 15W bulbs can be connected in parallel If I use an extension cord in the 5A socket? If I use the 15 A socket then how many bulbs can I connect in parallel?

Asked Anil


Assuming the voltage to be 220 V,

The current drawn by a single bulb is 15/220=0.068 A

So the no. of bulbs which can be connected in parallel = 5/.068=73 (avoiding the fractional part)

If you use 15A, then 220 bulbs can be connected. (follow the same procedure)

Wave particle duality

Let n and m denote the number of photons emitted by a red and blue bulb respectively of equal power in a given time.Is n=m?



The energy of a photon is hν

where ν is the frequency.

Power = energy/time

==> nhν1 = mhν2

==> nν1 = mν2

clearly n not equal to m

Where is electricity stored after it is generated but before it is supplied?

This question was posted by Chithraja. This is a common doubt arising naturally in the minds of children as well as adults who think. But they never ask, or if they ask the questions remain unanswered.

I will explain the question itself first. When we switch on a bulb we get the energy. But what happens to the electricity when we are not using it? Is it just wasted if we don’t use it?

Then, won’t it be better using as much of it when available rather than saving it by not using electrical appliances?

These are all associated questions which I have also asked several times.

Before I actually answer the question I am waiting for responses from visitors, students and teachers

Low ENERGY BULB WARNING ” …from the British Ministry of Health

Warning  from the British Ministry of health about Energy  Saving bulbs   These  types of bulbs which are called Energy Saving or  low Energy bulbs, if broken, cause serious danger!   So much so, that if one breaks, everybody will  have to leave the room for at least 15 minutes,  because it contains Mercury  (poisonous) which  causes migraine, disorientation, imbalances and  different other health problems, when inhaled.   It causes many people with allergies, severe  skin conditions and other diseases just by  touching this substance or inhaling it. Also  the ministry warned to NOT clean the debris of the  broken bulb with a vacuum cleaner, because it  would spread the contamination to other rooms in  the house when using the vacuum cleaner again.   It must be cleaned up with a normal broom  or brush, be kept in  a sealed bag and disposed of right away from the  house in a bin for hazardous  materials.  
Notice:  Mercury is dangerous, more poisonous than lead or  arsenic!!!!

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