2nd march 2009

Q1. Why can Birds touching  power lines do not get a shock?

A1, It’s because the electricity is always looking for a way to get to the ground, but the birds are not touching the ground or anything in contact with the ground. A bird's body is a poor conductor as compared to copper wire, so there's no reason why electrons will take a detour through the bird. 

But if the bird touches another wire too, or somehow comes in contact with ground .... circuit gets completed.and bird gets a shock.

A human if touches a single wire does get a shock unlike a bird because he is in contact with ground...which completes the path of electrons and results in a shock.(see fig.)

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Q2. What will happen if you supply a fan with a dc?

A2.In case of 220v A.C. supply, fan (induction motor) will run since it works 

only on indution principle and to produce EMF, "rate of change of flux" should 

be occuring which is only possible if A.C. supply (A.C. voltage & current)is  

given to the fan..

In case of DC supply, rate of change of flux will be zero,so EMF produce is

zero and there will be no torque & no motion. on the other hand, 

fan will heat up & winding may burn.

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Q3. What is skin effect?

A3. The tendency of alternating current(AC) to distribute itself within the conductor

so thatthe current density near the surface of conductor is greater than that at the

core,in other words, current flows through the 'skin' of the conductor.

Skin effect causes the effective resistance of conductor to increase with frequency

of current.

Skin effect is due to the eddy currents set up by AC current.

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Done for today..

Bye for now