## A Problem from Optics – Reflection of light

Convex and concave mirrors have same radii of curvature R. The distance between the mirrors is 2R. At what point on the common optical axis of the mirrors should a point source of light A be placed for the rays to converge at the point A after being reflected first on the convex and then on the concave mirror?

## Calculating charge on a body with excess of electrons

How would I calculate the charge on a small sphere with an excess of 5.0 x 10 14 electrons?

We know that charge on an electron is (-) 1.6 x 10-19 C

If the sphere has 5.0 x 10 14 electrons in excess then the body has a net negative charge and is given by q = ne

Charge on the sphere = 5.0 x 10 14 x (-) 1.6 x 10 -19 = 8 x 10 -5 C

## How was your Physics Practical Examination?

Hello students!

It’s the time of AISSCE Practical Examination.

How was your Physics Practical Examination and Viva?

Which questions were asked to you?

Which were the experiments and activities you were asked to perform?

How farely did you do it?

Share your experience here as comments to this post. Your responses will be a boon for the others.

You can post the questions you were asked during Viva so that others can find the answers to them and learn for the Viva during their turn.

## 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?

## Which way is better for giving AIEEE exam? ONLINE or OFFLINE.

Which way is better for giving AIEEE exam? ONLINE or OFFLINE.

I would personally recommend online. But the choice is up to you.

Please follow the following check to decide. Try to identify for yourself the plus points and negative points of writing AIEEE online and OFFLINE.

The most important disadvantage of choosing the OFFLINE version is the rarity of choices. You may have to travel long if you are not nearby the centre.

The decision is yours. But, think well before you decide.

## Motion in a vertical circle and conservation of energy

A stone  tied to a string of length l is whirled around a vertical circle with the other end of the string at the centre. At a certain instant of time the stone is at the lowest position and has a speed u. What is the magnitude of change in its velocity as it reaches a position where the string is horizontal?

Let’s assume that the potential energy at the lowest position be zero. So, when the string is horizontal, the stone has risen by a vertical height l, the length of the string which is also the radius of the vertical circle.

If v is the magnitude of velocity at the horizontal position, then according to the law of conservation of energy,

KE+PE at the lowest position = KE+PE at the horizontal position

$\frac{1}{2}mu^{2}=\frac{1}{2}mv^{2}+mgl$

From the equation above, v-u can be calculated.

$v=\sqrt{u^{2}-2gl}$

The following links will help you for deeper understanding and you can browse through some solved problems from the topic too.

## Why protons attract inside a nucleus?

What is the reason for proton- proton attraction inside the nucleus according to nuclear physics? (Sanjeev Asked)

Answer: Inside the nucleus where the nucleons are very close to each other, the force which holds them together is the nuclear force, which is the strongest force in nature. The electrostatic force between protons is negligibly small compared to the nuclear force. But the nuclear force is of a very short range, 10^-15 m.

The nuclear force is charge independent, i.e; the nuclear force between proton and proton, proton and neutron as well as neutron and neutron are almost the same.

Yukawa’s meson theory suggest that the nuclear force is an exchange force. The nucleons are bound because of constant exchange of the mesons.

The nuclear force is only felt among hadrons. At small separations between nucleons (less than ~ 0.7 fm between their centers) the force becomes repulsive, which keeps the nucleons at a certain average separation, even if they are of different types. At distances larger than 0.7 femtometer (fm) the force becomes attractive between spin-aligned nucleons, becoming maximal at a center-center distance of about 0.9 fm. Beyond this distance the force drops essentially exponentially, until beyond about 2.0 fm separation, the force drops to negligibly small values.

At short distances (less than 1.7 fm or so), the nuclear force is stronger than the Coulomb force between protons; it thus overcomes the repulsion of protons inside the nucleus.

However, the Coulomb force between protons has a much larger range due to its decay as the inverse square of charge separation, and Coulomb repulsion thus becomes the only significant force between protons when their separation exceeds about 2 to 2.5 fm.

(There are many terms introduced in the explanation. You can discuss them as comments for obtaining further details, if required)

Reference:

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