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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 is electricity made from water and coal?

Tazim asked: “How is electricity produced from coal and water?”


The question is whether

  • “How is electricity produced from water and how is electricity produced from water?
  • or
  • How is electricity produced from coal and water TOGETHER?

Electricity can be produced from water using hydel (hydroelectric) power stations.

Water is also used in almost all power stations like thermal and nuclear where the heat produced is used to boil water to convert it into steamwhich is then used to rotate turbines to produce electricity.

The upper reservoir (Llyn Stwlan) and dam of t...

Coal is often used as a fuel in thermoelectric power statiuons. the heat obtained by the burning of coal is used to boil water and to produce staem. the steam at high pressure is then utilized to rotate turbines and thus to produce lectricity in generators.

Flow of electrons and direction of current

We say current flow opposite to electron flow but what is current?

Sid asked


Electric current is defined as the rate of flow of charges, ie; the amount of charges flowing in 1 sec.

The direction of current was conventionally defined as the direction of flow of positive charges.
In a metallic conductor, the negatively charged particles (electrons) are moving and therefore the direction of current is taken as opposite to the direction of flow of electrons.
Hope the concept is clear.

Capacitance, voltage and Potential difference

the capacity of parallel plate condenser is 5 micro Faraday. When a glass plate is placed between the plates of the conductor  its potential become 1/8th of the original value. The value of dielectric constant will be



The charge remains same

Use Cm=KCo
V reduced to 1/8 means capacitance increased 8 times since Q=CV
So, the answer is evident

Doubts from Faraday’s Experiments on electromagnetic induction

I wanna ask that in faradays’s first experiment with coil and magnet,when the magnet is moved away from coil the deflection in galvanometer becomes opposite as compared to first case…..what leads to this and if that means direction of current becomes opposite so HOW and WHY the direction of this changes…..plz can you tell me the practical reason of this…..its really confusing me!!!

Asked Arti

Answer will be posted soon

(Visitors are welcome to post their response via comments)


Sir,im not understanding how transformer works?while current is going from one station to other we keep high voltage and very less current to decrease power loss but current is proportional to voltage how can u make low current using high voltage?

Asked  Avinah Boinepalle



A transformer works on the principle of mutual induction. [mutual induction is the phenomenon of production of emf in a coil due to the change in the strength of current through a neighbouring coil]

Transformers are of two kinds – step up and step down.

A step up transformer converts low voltage (high current) ac to high voltage (low current) ac.

In the case of an ideal transformer, there is no power loss.

In that case,

input power = out power

Vin Iin = Vout Iout

[This equation shows how an increase in output voltage results in a decrease in out put current]

Long distance transmission of ac requires low  current as the heat dissipated during transmission is proportional to the square of current.

If we use a step-up transformer , it steps up the voltage and consequently steps down the current so that VI is the same.

In the case of real transformers, there are some power losses within the transformner, however, an increase in output current through mutual induction will essentially result in a decrease in output current.

Hope that the matter is clear now. In case of doubt, please post them as comments to this post.

Resistance and Resistivity

Simran Asked

“Two wires A and B of equal lengths,different cross-sectional areas and made of same metal a)name the property which is same for both the wires. b)name the property which is different for both the wires”


Since both the wires are of same metal, the resistivity will be same for both whereas since the dimensions (Area of cross section here) are different, the resistance will be different

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