Why are there only two types of electric charges?

There are two types of charges because we can establish charges only by loss or gain of electrons present in the orbits of an atom. 

By losing electrons  a body gets positively charged and by gaining electrons it gets negatively charged. 

Electron is the fundamental charge. It's taken as negatively charged. 

Why do you need moisturising cream in winter season?

 Q. Why do you need moisturising cream in winter season?

Answer: As the humidity is low in winter season where there is no winter rainfall, skin loses water due to evaporation and has tendency to dry quickly. To prevent this we need moisturising cream.

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.