Skip to main content

Pressure of Atmosphere (Air) & Liquid – Physics GK Notes [PDF]

Pressure Important Point (General Knowledge)

Complete Notes and important points on the topics Pressure in Physics for your upcoming exams such as UPSC, IAS, Banking, SSC, Railway, etc.

Contents:

What is Pressure
Atmospheric Pressure
Pressure in liquid

Formula:Pressure = Force/Area
Unit:CGS: dyne/cm2 SI: Pascal or N/m2
Dimension:[ML-1T-2]
Quantity:Scalar

What is Pressure?

Before knowing the pressure I hope you have some kind of knowledge about Force and Area. Both Force and area are a vector quantity. We realize that force can act on a point or on an area. But have you ever wonder why a sharp knife is easier to cut a piece of onion rather than a blunt one? In this case, you probably apply the same amount of force on the onion. The answer is how the force act on the onion. If you apply some amount of force on a larger area the effect will be smaller. Form this concept a new physical quantity is required to describe this effect of force on an area.

Definition of Pressure: Force that applied normally on unit area is known as Pressure.

Formula: Pressure (P) = Force (F)/Area(A) or P = F/A

What is the unit of Pressure?

The unit of Force and area is Newton (N) and square meter (m2 ) respectively. Thus from the formula of pressure, we get the unit of pressure is N/m2 in the SI system and dyne/cm2 in the cgs unit. In SI system N/m2 is also known as Pascal (Pa).

1 Pa = 10 dyne/cm2.

Thus we get that pressure is force/area. Both force and area are vectors. In this case, a vector quantity (force) is divided by another vector quantity (area). So here a question has arisen. Is pressure a scalar or a vector quantity? The answer is that Pressure is a scalar quantity. Remember: We can not simply divide a vector by a vector.

Atmospheric Pressure:

The air is surrounding the earth is called the atmosphere. It is extended about 100 km high from the earth’s surface. The air contains various types of gases such as Nitrogen, Oxygen, Carbon dioxide, etc. So air has mass. And the earth attracts masses around it. The force by which gravity attracts masses is called weight. So the air has weight. The weight of air per unit area of the earth’s surface is called atmospheric pressure.

How to measure atmospheric pressure?

Look at the picture. The shaded portion is filled with mercury. The upper side of the tube is airtight so that no air can fill the tube from the upper side. It is seen that at STP (Standard Temperature and Pressure) the mercury column gets up to a height of about 76 cm. So the pressure of air is equal to the weight of mercury of 76 cm height per unit area (1 cm2). That is 76 × 13.6 × Gravitational acceleration (980) = 1.013×106 dyne cm-2. This is called 1 atmospheric pressure or 1 atm. Another unit of atmospheric pressure is torr and bar. 1 torr = height of 1 mm mercury = 133.18 Pa. 1 bar = 105 Pa.  And 1 atm = 1.01 bar.

The atmospheric pressure decrease as it increases the height from the earth’s surface. So at the high altitude (on the mountain), at low pressure, it is difficult to cook.

Atmospheric pressure is measured by Barometer. If the reading of the barometer falls suddenly means the pressure of that region is low and there is a possibility of a storm.

The slow fall of barometric reading is an indication of rainfall. Again if the reading of the barometer rises slowly means the weather is clean and dry.

Pressure in liquid :

Pressure also exerted by liquid to its all directions. Look out the picture. The height of the liquid level is h. and the area considered is A on which the liquid exerts pressure.

So the volume of liquid is = Ah. let the density of the liquid is = ρ. The mass of the liquid is = Ahρ. And the weight of the liquid is = Ahρg, where g = Gravitational acceleration. Again we know the formula of Pressure which is

Pressure P = Weight / Area
or, Pressure P = Ahρg / A

or P = hρg

This is how much pressure exerts at depth h by a liquid. From the above formula, we can see that the pressure is proportional to the depth of liquid. It is also proportional to the density. Thus the density of mercury is more than water. Therefore at the same depth, mercury exerts more pressure than water. The pressure also proportionally depends on the gravitational acceleration.

Read More: Work Power Energy > Important Physics GK [PDF]

In the case of static liquid, the pressure is the same for all points on the same horizontal plane. That’s why the surface of the water in a pot is flat. At a particular point, the liquid exerts the same pressure in all directions.

Read Also: Motion – Its Equations > Important Physics GK [PDF]

How do pressure effect on melting point and boiling point? By the effect of pressure melting point may increases or decrease depending on the substance. If the substance expands on fusion, the melting point increases with an increase in pressure. Example: Wax. Again if the substance contract on fusion, then the melting point decrease on an increase in pressure.

Download As PDF

What is the unit of Work or Energy?

Units of Energy, Work [PDF]

What is the unit of Work or Energy?

There is various type of units of energy as we used in a various case. In this topic, I am going to discuss the various units of energy. So at first, I recommend you to have a quick look at Work Power Energy topics so that if you do not know what is work and power, you can understand those.

The S.I unit of energy is Joule (J) and the C.G.S unit of energy is erg.

1 J = 107 erg.

The higher units of energy are :

Watt-hour (Wh) and Kilowatt hour (KWh) :

Watt-hour (Wh): 1 watt-hour is the energy spent (or work done) by a source of power 1 Watt in 1 hour. I,e.,

1 watt-hour = 1 watt × 1 hour

= 1  J / Sec × 3600 sec

= 3600 Joul = 3.6 kJ

Kilowatt-hour: One kilowatt-hour (1 kWh) is the energy spent (or work done) by a source of power 1kW in 1 hour. i.e,

1 kilowatt-hour (kWh) = 1 kilowatt × 1 hour

= 1000 Joul/sec × 3600 sec

= 3.6 × 106 Joul = 3.6 Mega Joule.

Note: Watt (W) and kilowatt (kW) are the units of power, while Watt-hour (Wh) and kilowatt-hour (kWh) are the units of work or energy since power × time = work or energy.

Calorie: heat energy is usually measured in calories. 1 calorie is the heat energy required in raising the temperature of 1 g of water from 14.5°C to 15.5°C (or through 1°C). It is related to joule as

1 J = 0.24 calorie or 1 calorie = 4.2 Joule

The kilocalorie is the bigger unit of heat energy.

1 Kilocalorie = 100 calorie = 4200 Joule.

Electron volt: The energy of the atomic particles is very small, so it is measured in electron volt (eV).

1 eV is the energy gained by an electron when it is accelerated through a potential difference of 1 volt. i.e.,

1 eV = Charge on an electron × 1 volt

= 1.6 × 10-19 coulomb  × 1 volt

= 1.6 × 10-19 Joule.

1 eV = 1.6 × 10-19 Joule.

I hope this will help you.

Download Units of Energy, Work [PDF]

Save as PDF


More Physics General Knowledge (GK)

SATELLITE related General knowledge

Satellite – GK [PDF]

SATELLITE related General knowledge

What is satellite?

A satellite is an object which revolves around a planet. The revolution occurs due to the effect of the Gravitational Force that acts between any two masses. There are two types of satellite one is natural satellite and the other is an artificial satellite. Natural satellites are created by nature and they are much bigger than the artificial ones. On the other hand, artificial is man-made.

In our solar system, Mercury and Venus have no natural satellite. Our earth has only one satellite which is Moon. Mars has two moons, Jupiter has 67 moons, Saturn has 62, Uranus has 27 and Neptune has 14.

The orbital speed of a satellite does not depend on its mass. This means if two or more satellites having different masses revolving in the same orbit will have same orbital speed. This speed depends only on the distance between satellite and planet. If the distance increases the speed will decrease.

Read : Kepler Laws of Planetary Motion > Physics GK [PDF]

If we place a satellite near the earth’s surface its speed should be 7.9 km per second.

Russia was the first to send a satellite into space on 4 October 1957, Name of the satellite was Sputnik 1.

Geostationary Satellite :

Geostationary satellites are kept at high altitudes, about 36,000 km above the earth’s surface. The main characteristic of this satellite is that it will remain at a fixed position with respect to earth. This means it has the same angular velocity and direction with the earth. It’s time period or time of revolution is 24 hour. Therefore if it is possible to watch this satellite from our earth, it would be always visible.  The orbit in which this satellite revolves is called Geosynchronous orbits.

Polar orbits Satellite :

These satellites revolve around north-south poles and pass over equator once in a single day. They are kept about 700 km above the earth’s surface. They are used mainly for mapping purposes.

Escape Velocity :

Escape velocity is a certain velocity by which an object can escape from a gravitational field. Therefore if we project a body from earth surface with escape velocity it will never return to earth again. For earth, the value of this escape velocity is 11.2 km per second.

Escape velocity is totally independent of its mass, size density, and direction of projection. For a particular planet, the escape velocity is fixed for all objects. For the moon, its value is 2.4 km/s. If Ve is the escape velocity, g is gravitational acceleration and R is the radius of the planet then, –

Ve = √(2gR)

Download As PDF

Kepler Laws of Planetary Motion

Kepler Laws of Planetary Motion > Physics GK [PDF]

General Knowledge on Kepler Laws of Planetary Motion

Kepler was a great astronomer and physicist from Germany. His theory about planetary motion open a new revolutionary direction in the field of physics.

He formulated three famous laws on the motion of any planetary object.

The laws are –

1.>  All planets in our solar system revolve around the sun in elliptical orbits. The sun will remain rest at one of the foci of the elliptical orbit.

2.>  The line joining a planet to the sun i.e., the position vector or radius vector traces out equal areas in equal times. In other words the areal velocity of the radial vector is constant.

3.>  The square of the period of revolution of the planet round the sun is directly proportional to the cube of the major axis of the ellipse which the planet describes.

Explanation of Three Kepler’s Laws :

The revolution of planets occurs due to the gravitational force between the sun and planets. Though we assume that all the planets move around the sun along a circular path for simple calculation, it is not practically correct. Actually all the paths are elliptical. For ellipse there are two axes, one is semi-major axis and the other is the semi-minor axis.

ellipse-Kepler laws
ellipse-Kepler laws

 

From the above figure, we can see that the red line (a) is semi-major axis and the yellow (b) line is the semi-minor axis. F1 and F2 are two foci. The sun is situated on one of its focus (F1).

Keplar's laws of motion

The radius vector (the blue line from the figure) swipe out equal area in equal time. Let the planet moves from point P to point Q in a fixed interval of time and The amount of area is swiped out. Again it moves from point R to point S in that fixed interval of time, and B amount of area is swiped out by it. Then from Kepler’s law of motion the area A will be equal to area B.

If T is the time period of the planet i.e., time is taken to complete one revolution, and R is the radius vector (the line joining sun and planet) then, according to Kepler’s third law of motion, we can express as –

T2 ∞ R3

From the above expression it is clear that, when the planet is located far from the sun, the time period is smaller. This means that the planet will move with less velocity. And when the planet is near to the sun, the velocity is greater.

Read: Work Power Energy > Important Physics GK [PDF]

Download Kepler Laws of Planetary Motion PDF

Download as PDF

 

Gravitation, Newton's Law, Potential energy.

Gravitation – Physics > GK

Gravitational Force and Gravity

Contents

    1. Newton’s Law of Gravitation;
    2. Gravity
    3. Gravitational Potential

Gravitation related General Knowledge is one of the most important topics for various types of competitive examinations such as IAS, UPSC, Railway, Banking, SSC, CGL, CHSL, MTS, etc. Therefore we have brought to you the proper and appropriate knowledge and concept related Physics by chapter wise.

In this topic we are going to discuss Gravitation force, Newton’s Law of Gravitation, Center of mass concept, gravity, and Gravitational Potential energy.

Newton’s Law of Gravitation, Gravitational Force:

Definition of Gravitational Force: Every objects in this universe attract each other by a force called gravitation force.

Any two objects in this universe attract each other along a line joining between them. The magnitude of this force of attraction depends on the product of their masses and inversely proportional to the square of the distance between them.

Let us consider two masses, m1 and m2 are separated at a distance r. According to Newton’s law of Gravitation if F is the force between these two objects, then, –

Fm1×m2 Where r is constant;

F 1/r2 where m1, m2 are constant;

Combining these two rule we get

Fm1×m2/r2

or, F = G×m1m2/r2
Gravitation force
Gravitation force

Where G is known as Universal Gravitational Constant. The above equation is the mathematical form of Newton’s Law of Gravitation.

Value of Universal Gravitational Constant:

From Newton’s Law of Gravitation, we get

G = F × r2 / (m1 × m2);

From the above equation, we see that if each masses of two bodies m1 and m2 is 1 kg and they are kept at a distance of 1 meter, then the amount of force by which they attract each other is the Universal Gravitational Constant. It is denoted by ‘G‘. Its unit is Nm2/kg2

The absolute value of Universal Gravitational Constant is 6.67×10 -10 Nm2/kg2. This amount of force is too small that we can not feel. Gravitational force is always directed towards the center of masses of the bodies. That’s why it is a vector quantity.

The dimension of G :

The dimension of Universal Gravitational Constant is [M-1L3T-2].

Gravitational Attraction for Extended Body:

How to calculate the force of attraction for extended bodies or irregular-shaped bodies? Now, look out the following problem. Object-A and object-B are irregular shaped. While calculating the gravitation force we need to have the distance between them. But how can we obtain the distance? Since they are not point masses.

understanding centre of mass
Center of mass of an irregular shaped object.

The problem is solved by considering their center of masses or center of gravity. Every object in the universe has a particular point where all the mass is considered to be concentrated. This point is called the ‘Centre of Mass‘. For regular shaped and homogeneous objects such as a sphere, the center of mass lies in its center.

Gravity :

The gravitational force by which our Earth attracts an object or body towards its center is known as gravity. Due to this gravity, each object on the earth’s surface falls on earth.

Gravitational Acceleration :

We know that when a body moves due to the application of force, acceleration is always created on it. How much acceleration will be created? The answer is obtained from the equation –

Force = Mass * Acceleration;

This concept is also applicable to gravity. Thus when Earth attracts an object by its gravity, an acceleration produced on the object. This acceleration is known as ‘Gravitational Acceleration‘ and it is denoted by ‘g‘. The value of ‘g‘ is 9.8 m/s2. It depends on the shape, size, and mass of the body.  Gravitational acceleration is a vector quantity.

Variation of gravitational acceleration on the earth’s surface.

Though the value of gravitational acceleration is considered as constant, it is not. It varies on the earth surface.

  • Its value is maximum at the two poles of the earth. Why? Because the linear velocity of the surface of the pole is less than any other place on the Earth.
  • The value of g is minimum at the equator. Why? Because the linear velocity of the surface of the earth is maximum.
  • The value of g decreases as the height from the earth’s surface increases.
  • The value of g also decreases as we go inside the earth.

If the angular velocity of the earth becomes seventeen times its present value, every object on the equator well become weightless. Why? Because when angular velocity increases its centrifugal force also increases. This centrifugal force is always opposite to gravity.

Gravitational Potential:

The gravitational potential at a point in a gravitational field is measured by the amount of work done in bringing a unit mass form infinity to that point.

It is a scalar quantity and generally, it is denoted by V. In the gravitational field if you want to move an object from point A to point B, then it required some energy. If you move the object along the direction of the field, the energy is negative. And the energy is positive if you move an object opposite to the direction of gravitational force.

If you move an object from infinite to some point in the field the potential is given by the following formula.-

V = -Gm/r, where m is the mass of the object.
gravitational potential
potential varies as 1/r

Potential energy of Earth:

The gravitational potential energy or P.E of Earth is measured by the height you lift an object multiplied by its mass and gravitational acceleration i.e. –

Gravitational Potential energy of Earth W -= mgh;

The unit of P.E. is Joul in SI system and erg in c.g.s system.

Download Gravitation – Physics > GK as PDF

Download As PDF