Why is food cooked faster in pressure cooker?

Water boils at 100°C at normal pressure of 1 arm or 100000 Pascal. Boiling temperature increases  with the increase in pressure above the liquid surface. When we cook food in pressure cooker, the pressure increases due the retention of vapour and hence the boiling temperature increases in the pressure cooker upto 125°C As food is cooked at higher temperature than 100°C , it is cooked faster in the pressure cooker.

SI UNIT OF TEMPERATURE

Why is Kelvin considered as SI unit of temperature?

Answer: From kinetic theory of matter , it is understood that the average kinetic energy of molecule is directly proportional to the  temperature in Kelvin. When the temperature in this scale is zero then the molecular motion ceases irrespective of nature of any substance. This can be taken as the starting temperature . Hence Kelvin scale is taken as an absolute scale and hence SI unit.

What is loudness of a sound?

Loudness of a sound is the characteristic of a sound which depend on the intensity of  energy it carries.
Intensity of a sound is the amount of energy passing through a surface area of one square metre held perpendicular per sec.
Intensity depends on square of amplitude of the wave passing. Hence we can say Loudness is directly proportional to the square of Amplitude. If amplitude is doubled, loudness becomes four times.
Loudness depends on distance of source from the listener.
Loudness depends on resonance also.

ARCHIMEDES' PRINCIPLE

"Any body immersed partially or completely in a fluid experiences an upthrust which is equal to the weight of the fluid displaced by the immersed part of the body"

Ideal fuel

What  are the characteristics of an ideal fuel?

Answer: The characteristics are:
 1. It has high calorific value. It means the fuel produces large amount of energy per unit mass  of fuel on burning .
 2. It should not emit harmful gases or products on burning.
 3.It should  easily be stored and transported .

Different persons have different voice. Why?

Each vibrating body has it's own set of frequencies. The minimum frequency of any vibrating body is called its fundamental note. Other frequencies are called harmonics. Combining these two types of frequencies , we get its waveform. The waveform decides the types ( Quality) of sound produced.

      As each source has it's own quality of sound, different sources or different persons have different type of sound.

Why is hollow box provided in a guitar?

The vibrating srings force air molecules in the box to vibrate with greater amplitude to produce loud sound.

Anamolous expansion of water

It's a general property of any substance that it expands on heating and contract on cooling but certain substances behave just different way at certain range of temperature. Water  is an example.
Water expands on cooling from 4°C to 0°C or you can say water contracts on heating from 0°C to 4°C.
This unusual behaviour of water is called Anomalous expansion  of water

Why does bar magnet align itself along North and South direction?

Due to the continuous flow of molten magma and lava inside the earth, there is the magnetic field of the earth as if there is a huge bar magnet inside the earth along its axis little different from geographic axis.

    The earth's magnet has it north pole towards the geographic south and vice versa.The magnet has the bar magnet suspended is affected by this field and hence shows the N-S direction.

GREEN HOUSE EFFECT

Infrared radiations from the Sun have short wavelength (high frequency) s they are emitted from the surface of the sun at high temperature.These radiations have more energy due to high frequency and hence they can penetrate through a glass or plastic house. But the radiations reflected or emitted from the plants in the house are of longer wavelength so they cannot penetrate through the house. So heat is retained inside the house and temperature increases.
This rise in temperate is called green house effect

What is relative density?

We compare the density of different substances with the density of water at 4°C.

Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance".

4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only.

There is no unit of relative density as it is  the ratio of two similar quantities.

The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7.
It means that the substance is 7.7 times heavier than water.

LAW OF FLOATATION

"For a floating body, the upthrust experienced by it is equal to its weight."
Net force  on the floating  body is zero

What is Regelation?

The melting point of ice is 0°C at normal pressure but when you increase the pressure on ice it melts at the temperature below 0°C( i.e. melting point of ice decreases with pressure).
When the pressure applied is released then water formed from ice starts freezing. This phenomenon is called REGELATION.