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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

(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?

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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