TRIBO-ELECTRIC SERIES

 TRIBO-ELECTRIC SERIES 

The series in which the substances are arranged according to their tendency to lose or gain electrons on rubbing each other. The higher the position of the substance, it has the tendency to lose electrons and to gain positive charge on rubbing.

The series of common substances  follows,

Air

Human hair

Rabbit's fur

Glass

Hair

Silk

Paper

Cotton

Steel

Wood

Amber

Rubber balloon

Plastic wrap

Ebonite

Example:

When glass rod is rubbed with silk, the rod loses electrons and get positively charged whereas silk gains electrons from the rod and gets negatively charged. Please check the position of glass in relation with silk. Glass is placed above silk.

Longitudinal and Transverse Waves

 LONGITUDINAL WAVES

The waves in which the particles of a medium vibrate in the  direction of propagation of wave are called a longitudinal wave.

These waves propagate in the form of compressions and rarefactions.

Example: Sound waves in air.

TRANSVERSE WAVES

The waves in which the particles particles of a medium vibrate in the direction perpendicular to the direction of propagation of waves are called transverse waves.

These waves propagate in the form of Crests and troughs 

Example: water ripples

PRESSURE COOKING

 QUESTION 

Why is food cooked quickly in a pressure cooker ?

Boiling:

 The process in which a liquid changes into gaseous state at a constant temperature and pressure is called boiling. 

Example: water boils at 100°C to change into steam.

Boiling  point:

The temperature at which boiling takes place at a given pressure is called boiling point. 

Example: The boiling point of water is 100°C. 

Boiling of  a liquid starts when the vapour pressure of the liquid heated is equal to the air pressure above the liquid surface. If we can increase the pressure above the liquid surface then boiling is  delayed and hence the boiling point is increased. 

So, when steam is retained inside the cooker, boiling point of the liquid increases to 120-125°C.

As the food is cooked at higher temperature in the pressure vessel, it is cooked easily and quickly.

PRISM and ANGLE OF PRISM

 PRISM

Prism is a transparent retracting medium bounded by at least two plane surfaces inclined at a certain angle.

ANGLE OF PRISM

The angle between two retracting surfaces of a prism is called angle of prism.

ANGLE OF DEVIATION

The angle between the direction of incident ray and the emergent ray is called angle of deviation. 

The angle of deviation depends on

1. Angle of incidence. 

With the increase in the angle of incidence , the angle of deviation first decreases ,then reach a minimum value then it also increases.

2.Refractive index of the material of the prism.

With the increase in the refractive index angle of deviation increases.

3. Colour o wavelength  of light 

With the increase in wavelength of light used, the angle of deviation decreases . Red is deviated the least and violet the most. 

4. Angle of prism or thickness of the prism 

The angle of deviation increases with increase in the  the angle of  de prism.

PASCAL'S LAW

 PASCAL'S LAW:

The pressure exerted at any point in a confined liquid is transmitted equally and undiminished in all direction throughout the liquid.

Pressure at a point in a liquid =F/A, Pressure at any other point in the liquid=F'/A'

As pressure is transmitted equally,   F/A=F'/A'

Applications

1.Hydraulic press.

2. Hydraulic lift.

3.  Hydraulic brakes.

CRITICAL ANGLE FOR DIFFERENT COLOURS OF LIGHT

 CRITICAL ANGLE:

The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.

FACTORS:

The critical angle of a medium depends on

1. The relative refractive index of the denser medium with respect to the rarer medium. Greater the refractive index, smaller the critical angle. 

For example: refractive index of glass is 1.5, critical angle is 42° and for diamond, refractive index is 2.4, the critical angle is 24°

2. The colour(wavelength) of the light. Longer the wavelength, greater  is the the value of the critical angle.

For example:  Red has longest wavelength among seven colours, so it has maximum value of the critical angle for a medium.

Atmospheric pressure

 What is atmospheric pressure?

Answer: The force exerted per unit area on the surface of the earth due to the weight of air  column above it is called atmoshpheric pressure.

Its SI unit is N/m² or pascal. Other units are dyne/cm², mm of mercury, bar, torr,

Unstable Hydrogen isotope can not emit alpha rays. Why?

 QUESTION: Unstable hydrogen can not emit alpha particles Why ?

Unstable hydrogen, though radioactive, can not emit alpha rays because during alpha decay two protons and two neutrons are emitted which are not present in the hydrogen atom. Hydrogen  has only one proton in its nucleus.

EARTHEN POT: An earthen pot keeps water cool..

 QUESTION: An earthen pot keeps water cool. How ?

Answer: The earthen pot has fine pores through which water comes out to the outer surface where evaporation takes place. During evaporation, the liquid absorbs heat from the surrounding keeping it cool.

g and G, The differences. Class 9

 

The acceleration due to gravity (g) 

1. It is the acceleration caused by the gravitational force.

2. Its value depends on the mass and radius of a planet or heavenly body.

3. Its value changes from place to place.

4.Its a vector quantity.

The universal gravitational constant(G)

1. It is numerically equal to the force of attraction between two bodies each of mass 1kg separated by distace of 1m.

2. Its value does not depend on the mass and size of bodies. 

3. Its value is constant everywhere in the universe. 

4. It's a scalar constan.

3. 

NEW SIGN CONVENTION OF CONVEX AND CONCAVE LENS.

 New sign convention for convex and concave lenses. 

1. The optical centre of the lens matches with the origin of  two dimensional cartesian system ( the point of intersection of x-axis and y-axis).

2. The object should be placed at the left side of the lens( towards negative x-axis).

3. The distances are measured from the optical centre of the lens( the origin). Assign the signs according to the signs of the segments of the axes.(x or y axis).

For CONVEX LENS.

4. The  object distance (u) is always negative.

5. The image distance (v) is positive for real images.

6. The image distance (v) is negative for virtual image.

7. The focal length (f) is positive. (Second local length)

For CONCAVE LENS.

8. The object distance (u) is always negative.

9. The image distance (v) is always negative as it forms virtual image.

10. The focal length (f) negative. (Second local length)

For OBJECT and IMAGE  lengths.

11. The length of the image is positive as it is always kept erect.

12. The length of the real image is negative  as it is inverted.

13. The length of the virtual image is positive. 

For MAGNIFICATION, (m=Image  length÷ object  length)

14. m is negative for real images.

15. m is positive for virtual image.

For NUMERICALS,

16.  Do not assign  any sign for unknown quantities. 

LENS FORMULA 

17.  Magnification (m)= Size of the image÷size of the object.

18. m= image distance  ÷object distance.

19.  Lens formula: 1/f= 1/v-1/u.

DOES CONVEX LENS FORM VIRTUAL IMAGES?

Question: Does convex lens form virtual images?

Answers: Yes, convex lens can form virtual image when the object is placed at the position between the first focus and the optical centre of the lens. The image is upright and magnified. Its formed on the same side of the lens where object is placed.

REVISION QUESTIONS ON KINETIC THEORY OF MATTER CLASS 8

 PHYSICS, CLASS 8

ALL QUESTIONS ARE COMPULSORY.

Q1. WHY DO WE SMELL HOT FOOD FROM A DISTANCE? 2

Q2. WHY IS THERE NO CHANGE IN TEMPERATURE DURING THE MELTING  PROCESS? 2

Q3. DIFFERENTIATE BETWEEN BOILING AND EVAPORATION WITH 3POINTS. 3

Q4. STATE AND EXPLAIN ANY THREE POSTULATES OF KINETIC THEORY. 3

Q5. DEFINE TWO TYPES OF INTERMOLECULAR FORCE WITH ONE EXAMPLE OF EACH. 2

Q6 EXPLAIN THE PROCESS OF MELTING ON THE BASIS OF KINETIC THEORY OF MATTER. 2

Q7. EVAPORATION CAUSES COOLING . GIVE REASON WITH ONE EXAMPLE. 2

Q8. WHY DOES A CAMPHOR GET CONVERTED TO FUMES ON HEATING? 2

Q9. WATER ______________HEAT DURING FREEZING AND _________ HEAT DURING BOILING. FILL IN THE BLANK WITH “ GAINS OR LOSES" 2

......*************......

Answers: 

A1. The molecules of food in gaseous form are free to move in  large intermolecular space available between them and the air and these molecules show zigzag motion  to different parts of a house. So, we can smell hot food from a distance.

A2. During melting process, the heat given to a solid is utilized to do work against intermolecular force of attraction. The heat energy changes completely into potential energy. Hence temperature does not change.

A3.                   Boiling                                                        I                   Evaporation

a) Boiling is a fast process.                                    I        a) Evaporation is a slow process.

b) Boiling does not  cause cooling                        l       b) Evaporation causes cooling.

c) Boiling takes place at a particular                    I      c) Evaporation takes place at any temperature. 

Temperature 

A4. I) Matter is made of atom or molecule..

Matter is made of small particles called atoms or molecules which cant be seen by naked eyes

ii) The molecules are always inconstant, random motion.

The particles continuously move in a zigzag path .

iii) The molecules possess kintic and potential energy.

Due to their motion the molecules possess kinetic energy. As they interact with other molecules, they change their position and possess potential energy.

A5. I) Force of cohesion : The force of attraction that exists between similar kinds of molecules.  Example: The force of attraction between molecules of water. 

ii) Force of adhesion: The force of attraction that exists between molecules of different substances. Example: The wetting of glass due to the force of adhesion between the molecules of water and glass. 

A6.  When a solid is heated,  the molecules gain kinetic energy and start overcoming the strong intermolecular force of attraction. At a particular temperature, the molecules completely overcome the force of attraction and the molecules becomes free to flow and the solid changes  into liquid . This process is called melting. 

A7. Evaporation causes cooling because the liquid absorbs heat from the body and the surrounding to evaporate. Earthen pot keeps water cool by evaporation of water from the surface of the pot.

A8. Camphor molecules have very weak intermolecular force of attraction. By absorbing  heat from surrounding these molecules overcome the force easily and molecules becomes free like gaseous molecules. Hence it is directly converted to fumes.

A9. Loses, gains.

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