PHYSICS

 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,

 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.

 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.

  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 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...

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.

 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....