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## Why don’t the birds perching on high power electric lines do not get an electric shock? What is an electric shock? It’s a a sudden discharge of electricity through a part of the body.

For a sudden discharge of electricity, the prime need is a potential difference. When the bird perches on an electric line, it is at a high potential, but it is not subjected to a high potential difference between two parts of its body and hence it does not get a shock. The entire body is raised to the same potential as that of the electric line.

So, the electric potential difference is the VILLAIN. ## Quick Revision Study Materials for Class XII ## Can the frictional electricity produced during landing of a plane be used for constructive purposes?

As we know that in aeroplane during landing the charged produced in it due to the friction with ground are earthed through its wheels; can this charge be stored with the help of a capacitor and used for useful purpose?

I feel that this is a metter for research.

If you feel otherwise and know some method to tap this potential, please post. ## Random collection of Physics Viva Questions

1. State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
2. How can we increase the sensitivity of a potentiometer?
3. Define figure of merit of a galvanometer.
4. Which has more resistance – a galvanometer or a milliammeter?
5. How does an LED emit light?
6. What is the difference between an ordinary diode and an LED?
7. Define principal axis of a convex lens?
8. What happens to the focal length of a concave mirror if it is immersed in water?
9. What are the factors affecting the internal resistance of a cell?
10. What are the difference between primary and secondary cell?
11. Why can’t we use a dry cell for starting a car?
12. What happens to the resistivity of a wire when it is doublefolded?
13. How does the resistance of a wire depend on its dimensions?
14. Why are the resistances used in a resistance box is like 1,2,2,5,10,20,20,50,100,200,200,500,1000,2000,2000,5000 etc?
15. Why constantan or manganin wires are used for making the resistance coils in resistance box?
16. What is a standard resistance?
17. What are the characteristics of a standard resistance?
18. What are the precautions to be observed while doing electricity experiments in general?
19. What is a galvanometer?
20. What is the resistance of an ideal ammeter?
21. Why is ammeter always connected in series and voltmeter always connected in parallel?
22. How can we convert a galvanometer into an ammeter or a voltmeter?
23. What is shunt?
24. What is AVO meter?
25. What is the effect of temperature on the resistance of a conductor?
26. Why does the resistance of a conductor increases with temperature, whereas that of a semiconductor decreases with temperature.
27. What is conductance?
28. What are non ohmic devices? give an example.
29. What are superconductors?
30. Define emf
31. Why emf is said to be a misnomer?
32. What happens if the battery used in the primary circuit of a potentiometer has less emf compared to the emf of the cell used in the secondary circuit?
33. If you find that the galvanometer reading is shaky, what error can you expect?
34. What do you mean by figure of merit of a galvanometer?
35. Why a moving coil galvanometer is called so?
36. What is the principle of a galvanometer?

## Electric Current and flow of electrons

I have read that the FLOW OF ELECTRONS is called electricity. I wanna know that what is meant by flow of electrons? Pleaz answer me….

Electric current is the flow of charges. (Measured as the rate of flow of charges).

In conductors electric current is caused by the flow of free electrons. When a potential difference is applied across a conductor, the electrons are accelerated towards the positive potential. (But, due to the presence of plenty of electrons, they cannot move with acceleration as they collide with each other during their motion.) There is a net drift of electrons across any cross section and a current is developed.

## 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

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