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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)

Resistors Combination – Numerical Problem

What is the effective resistance between P and Q in the diagram?

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Asked Malik Ahmed

Answer : 8 ohms

The Two arms have resistances 32 ohm and are in parallel, Their effective resistance is 1 ohms. This is in parallel with the middle arm. So the effective resistance is 8 ohms.

 

OR

The three arms have resistances 32, 1 and 32 respectively and are in parallel. Therefore, taking their reciprocals and adding will give the answer.

Electrons in metals at 0K

We know that at 0K all the electrons will be in valence shell of semiconductors………..is it the case with even metals?

-Velpuri asked.

Please refer to the following links for details

1. http://hyperphysics.phy-astr.gsu.edu/hbase/solids/fermi2.html

2. http://www.physics.umd.edu/courses/Phys798S/anlage/Phys798SAnlageSpring06/Free_Electron_Model.pdf

3. http://www.cmmp.ucl.ac.uk/~ikr/3225/Section%206.pdf

4. http://www2.phy.ilstu.edu/~marx/ph355/Kittel_Solid%20State%20Physics/c06.pdf

Current Electricity – Drift Velocity based numerical

Two different wires X and Y of same diameter but different materials are joined together in series     across a battery . If the number density of electrons in X is  twice that in Y . Find ratio of drift velocity of electrons in two wires .

Asked Kavita

Answer:

In series combination, the current is the same.

I = nAev

=> n1v1 = n2 v2

=> v1:v2 =n2:n1

 

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