Principle of moments

Principle of MOMENTS states that in EQUILIBRIUM the sum of clockwise MOMENT is equal to the sum of anti clockwise MOMENT.

Why do we need moisturizers in winters?

Winters in most places have very low humidity. For that,  the moisture of our body is lost easily and skin becomes dry quickly.

Temperature of surrounding falls during or after the heavy shower. Why?

Temperature of any medium or object falls when it loses the heat. After the heavy shower, water absorbs heat from the surrounding air. Water has high specific heat capacity (i.e. the quantity of heat required by unit mass for the unit rise of temperature)  so water absorbs large amount heat from the surrounding without much rise in it's own temperature. Hence, temperature falls.

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.

Water seeks it's own level.( Hydrostatic paradox)

Irrespective of shape of vessels or containers, a connected liquid maintains same level in all containers.
This happens to maintain same pressure at a given depth. As the pressure depends on depth of a point from the liquid surface.

MIRAGE. HOW IS IT FORMED ?

MIRAGE is formed by the phenomenon of total INTERNAL reflection of light. Mirage is formed in extremely high or low temperature conditions.
For example: When a sand surface (desert) or road surface is heated, the air above this surface is also heated and rises up.
As a consequence, the density of air in the lower level is less than that of upper level.
  Now , the light from any object starting from the upper layers (denser medium) travels to lower layers ( rarer medium), total internal reflection takes place i.e, the light is reflected back towards the denser medium. When these light rays reflected back, it seems that the light rays are refected from some bright reflecting surfaces on the ground.  Hence the  image is seen below the object.

What is SMOG?

Literally,  smoke + fog= smog.
Smoke is the collection of unburnt fuel particles suspended in the air.
Fog is the suspended water particles in the air.
The combination of these suspended particles gives the smog.

A flute has a number of holes. Why?

The sound produced in a flute is due to the vibration of air column in the flute. The frequency of sound produced depends on length. The frequency is inversely proportional to the length of the air column. By closing or opening the holes , the length of the air column is changed and hence the frequency can be changed.

Plant leaves appear green. Why?

Sunlight contains seven colours, viz violet, indigo ,blue, green, yellow, orange and red. The leaves appear green because they absorb all the colours and reflect green as green colour is least used in photosynthesis. This green colour reaches our eyes. Hence leaves appear green.

Can unstable hydrogen isotope release alpha particles ?

Unstable hydrogen isotope contains
1 proton
2 neutrons

Now, alpha particles are the particles with
2 protons
2 neutrons

What do you think?
As there are insufficient particles in the hydrogen isotope, no alpha particles can be released.

Shallow water runs faster than deep river water. Why?

The equation of continuity is
area of cross-section of flow of water × velocity of water = constant.
Hence  the velocity of water is inversely proportional to the the area of cross-section of water flowing.
When water runs deep then the area of cross section of the flowing water is more than when its shallow so the velocity is less for deep water.

WHAT IS SUBLIMATION ?

Sublimation is a process of transformation of state of matter where a solid changes directly into its gaseous state.
The molecules in some solids have very weak force of attraction. So the molecules by gaining very little heat from the surrounding can change into gaseous particles.

Does specific resistance/resistvity change on stretching a wire?

Specific resistance or resistivity is the resistance of a wire of unit length and unit area of cross-section.

It depends on two factors.
1. Nature of the substance.
2.  Temperature of the substance

It is not effected by
1. Length of the wire.
2. Area of cross-section of the wire.

So., when you stretch a  wire its RESISTIVITY remains constant.

Can a solenoid be used to find the direction of earth's magnetic field ( magnetic North and magnetic south)?

A solenoid is a cylindrical/helical coil whose length is more than its diameter. When we pass current through the solenoid , a magnetic field is set up in the space around the solenoid similar to that of a bar magnet.
There is ,in fact, a strong and uniform field in the core of the solenoid.At the two ends of the solenoid, poles are developed as in a bar magnet.
So the current carrying solenoid if freely can show the direction of the magnetic field of a magnet.The north end of the magnet shows the geographic north and South end shows south .

Why is high voltage preferred for long distance transmission of electric power?

The electric power is generated at 11,000V at a power station. It is then transformed to 1,32,000V using a step up transformer.
For a given electric power, when voltage increases, the electric current decreases as P=VI or I=P/V.
Hence , at high voltage low current is involved.
As current decreases, the power loss(I in the transmission lines decreases in the form of heat .
To reduce this loss, high voltage is preferred

Sun rises in the east. Why

The earth has two types of motion.
1.Revolution around the sun
2. The rotation about it's own axis.

The earth rotates about it's own axis from west to east, so sun appears to rise from east direction.

Sea Breeze...How is it formed?

theloveforphysics

Water has higher specific heat capacity* than that of land(sand) due to which water is heated at slower rate than the land. As the land is heated, the hot air above the land rises up  creating low pressure above the land. As the air above water is comparatively cooler than the air above the land , this air moves towards the land due to the pressure difference. This cool breeze blowing towards the land from sea is called sea breeze.

As heating is required for its formation,  sea breeze blows during day time.

* Specific heat capacity is the amount of heat required to raise the temperature of unit mass of a substance by 1 K or 1°C.

Why do cooking vessels have thick bases?

theloveforphysics

Cooking vessel is provided with thick base to increase the mass so that its heat capacity increases and the base can absorb more heat and impart it to the food so that food is cooked quickly and the base remains hot for longer period of time

theloveforphysics

Why do we beat carpet to remove dust particles

theloveforphysics

  Before beating the carpet , both dust particles and carpet are at rest. Hence the dust  particles have inertia of rest. When carpet is beaten, its comes to motion but due to inertia of rest, dust particles remains at rest and are separated.

Camels have broad feet. Why?

theloveforphysics

Camels have broad feet so that they can walk on sand. As the area of their feet is more, the pressure due to their weight on the sand is less. So they can walk easily without sinking their feet on the sand.

Flash of lightning is seen before the sound of thunder. Why

   Both sound and light energy are released at the same time. In air the speed of light is 300000000 m/s and speed of sound is 340 m/s. Hence the light reaches the observer instantly and sound after some time.

    For example, if an electric discharge  between the clouds takes place at a distance of 1km from the observer then lightning flash is seen instantly but sound of thunder will be heard after about 3 seconds.

Why is nuclear fusion difficult to start?

Nuclear fusion is the process of combination of lighter nuclei (eg. hydrogen)  to form a heavier nucleus. When two lighter nuclei come close to each other, there is strong electrostatic force of repulsion between the nuclei. So this process is difficult.

To carry out this process high temperature (=10000000 K ) and pressure is required. Hence it is also called thermo-nuclear reaction.

Freezer compartment of a refrigerator is placed at its top portion. Why?

Cold air from the freezing compartment settles down owing to its higher density and keeps the temperature of the contents below it low.

Infrared rays are used for thermal photography?

Infrared rays are invisible rays whose wavelength is more than that of red colour.They produce heating effect. As they have long wavelength, they are least scattered by the fog, mist and smog and dust particles. As they effect a specially treated photographic plate due to the heating effect, they are used in thermal photography.

THE DIFFERENCE BETWEEN AC AND DC.

                        ALTERNATING CURRENT
1. Its value changes continuously.
2. Its polarity changes periodically.
3. It can be transformed by a transformer.

                         DIRECT CURRENT

1. Its value doesn't change, it has  a constant magnitude.
2. Its polarity doesn't  change.
3. It can not be transformed by transformer.

The temperature is lower in shade than in direct sunlight, why ?

In the direct sunlight there are Infrared Rays. Infrared rays have strong heating effect. But in shade there is no direct sunlight with these heating Rays. Only reflected or diffused light is there. So, temperature is lower in  shade than in direct  sunlight

A spanner has long handle. Why?

A spanner is used to turn a nut to open or tighten it.
The rotational effect depends on the perpendicular distance between the force and the point of application of force. When we increase the perpendicular distance then the Moment of force increases keeping the moment same.

Efficiency of a machine is always less than 100%. Why?

Efficiency is the ratio of work output and work input. As work output is always less than work output for the following reasons, the efficiency is less than 1 or 100%.

1. The movable parts of a machines are not weightless.
2.  The movable parts are not frictionless.
3.  The parts of the machines are not rigid.
4. The strings or belts are not perfectly elastic in nature.

Why is electromagnet made of soft iron?

Electromagnet is a temporary magnet made of soft iron. Soft iron is magnetised by the magnetic field of a coil or solenoid.
Soft iron has high magnetic permeability due to which it gets magnetised quickly and it loses magnetic properties easily due to the fact that it has low magnetic retentivity .

Why are Liquids heated from below?

A liquid in a vessel is heated due to the process of convection. In convection heat is transferred by the actual movement of heated particles in upward direction. As the liquid is heated from below its density decreases and hence it rises up and cold liquid settles down and heated. This process continues till whole liquid is heated.

IF WE KEEP REFRIGERATOR DOOR OPEN, WILL IT ABLE TO KEEP THE ROOM COOL?

For a moment, it seems like that !
But the compressor does work to extract heat from the content and this heat is thrown out from the refrigerator.

If we keep the door open for long time then heat is taken from the room (from front portion)and thrown back to the room. So the temperature of the room remains the same. Hence, the refrigerator will not be able to keep the room cool.

Temperature doesn't change during phase change( melting, boiling etc..). Why?

The heat added to the solid is absorbed by the particles and it is utilised in overcoming the intermolecular force of attraction between the particles. So heat absorbed in solids is changed to potential energy without any change in kinetic energy of the molecules. As temperature in Kelvin scale is the measure of kinetic energy of particles, there is no change in temperature during melting.

CLOUDS APPEAR WHITE, WHY?

Cloud appears white due to the phenomena of scattering of white light. The water particles in the cloud are big in size as compared to air molecules and wavelength of light. Hence all the colours get scattered in equal intensity and mixture of all colours gives white light. This white light reaches our eyes giving the impression that colour of a cloud is white.

Temperature of surrounding falls after a heavy shower. Why?

Water has high specific heat capacity . It absorbs large amount of heat from the surrounding without  much rise in its temperature .Hence the temperature falls .

What is the main cause of magnetism?

The cause of magnetism is the flow of an electric charge. The rest charge is associated with an electric field only but the moving charges are associated with both electric and magnetic fields.

Our moon does not have atmosphere. Why?

The escape velocity of the moon is less than the average velocity of the atom or molecule of  gases. Hence all gas molecules escape out of moon's gravitational field .

WHY IS EARTHING OF AN APPLIANCE DONE?

Earthing is required in a circuit when
1. the appliance draws high current. High power devices draw large current.(Current =power/voltage).
2. the appliance has outer metallic case.

Why earth pin of a three pin plug is thicker and longer

It is longer so that when we put the plug into the socket, earth connection is made first for the safety.
It is thicker so that it is not inserted to the holes of live and neutral.
It has split ends so that the plug is fitted firmly into the socket to avoid sparking.

CALORIMETER

It is a device to measure the heat lost or gained by one substance mixed with another at different temperature.
It is based on the principle of mixtures or principle of calorimetry.
Calorimeter is made of thin sheet of copper kept in a wooden jacket to with other insulating materials so that the process exchange heat by conduction is checked. Its surfaces are polished to check the process of radiation. Wooden or asbestos lid is used to minimise the loss of heat by convection.

Thin sheet is used to lower the heat capacity.

Temperature falls as altitude increases, Why?

The volume of rising warm air increases with height due the availability of more space above. This expansion (Joule-Thompson effect) results in cooling effect. Hence hillstations have low temperature. This phenomena helps in the formation of clouds and various forms of precipitation.

FUSE WIRE

Fuse wire is a safety device to limit the current in an electric circuit. It gets melted when excess current flows in a circuit. It works with the principle of heating effect of electric current. It is made with an alloy of lead and tin (solder wire).
Its material has two important properties
1. It has low melting point (around 250 degree celcius).
2.  It has high resistivity.

Sky appears blue, why?

Question: Why does sky appear blue ?
Answer: Sky appears blue due to the scattering of light by air molecules. Blue components (violet, indigo, blue colours) of white light are scattered the most owing to their short wavelengths compared to other colours. This scattered blue light reaches our eyes and the sky appears blue.

It is advantageous to run before taking a long jump. Why?

When we run, our body gains inertia of motion due to which it wants to be in motion continuously. When we take long jump our body has the tendency to be thrown forward more.

NEWTON'S SECOND LAW OF MOTION

The Second law states that the rate of change of momentum is directly proportional to the applied force and the change takes place in the direction of the force applied.

If u= initial velocity,
v=final velocity,
t= time taken to change the velocity from u to v,

Then
Initial momentum = mu
Final  momentum=mv

So change in momentum = mv-mu =m(v-u)
Hence the rate of change of momentum = m(v-u)/t

Therefore F= m(v-u)/t
  F=ma.
                            

NEWTON'S THIRD LAW

The law states that "to every action there is an equal and opposite reaction and these forces act on different bodies"

Examples

1. When a rocket is launched, the burnt gases are expelled down from the rocket body. Here action is on gases by the rocket body. Hence the gases exert an equal an opposite force on the rocket body in the upward direction. This is the reaction force. With this force rocket moves in upward direction.

ACTION...on the gases by the Rocket body.
REACTION..On the rocket by the burnt gases.

2. When a gun fires bullets, action is on the bullets by the gun and reaction on the gun by the bullets. Gun recoils back due to the reaction.

ACTION: On the bullets by the gun..
REACTION...on the gun by the bullets