"Questions and Answers on Physics Concepts

[ritwik06]

1. Water in an electric kettle connected to a 220V supply took 5 minutes to reach its boiling point. How long would it had taken if the supply had been of 200v?
My ans: 6 minutes

2. Does the e.m.f. of the cell depends on the size of the plates?

3. Tail of a comet always points away from the sun due to centrifugal force. True or false?
Ans: True.

4. If we need a magnification of 375 from a microscope of tube length 15 cm and an objective of focal length 0.5 cm then what focal length of eyepice should we use?
Guess: 6cm

5. The door of running refrigerator inside a closed room was left open then the room will be warmed up gradually. true or false? Ans: True.

6. The blue colour of water in sea is due to reflection of the blue sky in water. True or false?
Again my ans: True.

7. For the same kinetic energy , the momentum shall be maximum for -
i) electron ii) proton iii) deutron iv) alpha particle

My ans: electron

8. The penetrating power of x-rays increases with the increase in its frequency. True or false?

9. The conductivity of a magnetic substance for the lines of force with respect to air is called magnetic flux density. True or false?
Ans: True

10. Boiling water is changing into steam. Under this condition the specific heat of water is infinite?

11. If the distance between the two molecules is less than the equilibrium distance then the force between them will be repulsion. True or false?
Ans: true.

12. The places where angle of dip is 90 degree are known as magnetic poles of earth. True or false?
Ans: True.

13. On heating a liquid of co-efficient of cubical expansion a in a container of co-efficient of linear expansion a/3, the liquid will-
1) rise ii) fall iii)stationary

Ans: rise

Please tell me if I am wrong. You can answer only those questions which you perfectly know. Thanks.

 

[Astronuc]

How about showing one's work, e.g. the reasoning behind the answer or the equations involved. Also, there are several questions not answered.

Hint on 2, the size of the electrode affects current density, but not electric potential, which is dependent on the compositions of anode and cathode.

3. No. Something from the sun interacts with the comets tail.

 

[SGT]

The tail of a comet points away from the Sun because it is blown by the solar wind.

 

[ritwik06]

I have tried to work them out. But those are olympiad questions: just out of course. Please help me. Do you mean to say that the tail always points away due to electrical repulsion or radiation pressure or something like that?

 

[tim_lou]

1. $$P=\frac{V^2}{R}$$

2. my guess is true, if the plate is significantly smaller than the wavelength of cell phone signals.

7. $$KE=\frac{p^2}{2m}$$
the answer can't be the electron

8. It should, since for photon, $$p=\frac{hf}{c}$$

10. I think it should be zero. Since any additional heat to the water will not increase it's temperature (still boiling) (at least for an infinitesimally small time interval).

11. not true if it is in a unstable equilibrium.