LENZ'S LAW

LENZ'S LAW is based on law of conservation of

a) mass

b) charge

c) momentum 

d) energy.

Ans: d)   energy 

Why is uniform circular motion an accelerated motion?

 Q: Why is uniform circular motion an accelerated motion?

A: The velocity of  Uniform Circular motion changes continuously due to its changing direction. As there is change in velocity, the motion is accelerated motion.

DOOR HANDLES

 Question: Why are door handles fitted at the edge of the door ?

Answer: Door handles are fitted at the edge to increase  the perpendicular distance of the force from the axis of rotation so that the minimum force can be applied to achieve the given torque.

Fog/Cloud appears white. Why?

 Q: Fog/ cloud appears white. Why ?

Ans: Light is scattered by the fog particles. As the size of the particles is bigger than the wavelength of light, all colours are scattered to equal extent giving back white light. 

The property of light responsible for white clouds: scattering of light.

Convex lens behaves as a concave lens! Surprising!!

 Convex lens can behave as a concave lens and vice versa when the lens is placed in a medium with refractive index more than that of the lens.

Convex lens diverges and concave converges the rays.

What is an image? How is an image formed?

Ans:  The replica of an object  formed by the intersection of reflected rays is called an image.

Image is formed when the reflected rays actually meet at a point        ( real image)or appear to meet( virtual image)

ABSOLUTE REFRACTIVE INDEX AND RELATIVE REFRACTIVE INDEX

 ABSOLUTE  REFRACTIVE INDEX 

The absolute refractive index of a medium is the ratio of speed of light in vacuum and the speed of light in the medium.

Absolute refractive index = Speed of light in vacuum or air

                                                    Speed of light in the medium 

For example,

A.r.i of glass = speed of light in vacuum/speed of light in glass 

3×10⁸/2×10⁸= 3/2=1.5

Its value is equal or more than 1. For vacuum or air, its value is 1. So its minimum value is 1.

RELATIVE REFRACTIVE INDEX 

The relative refractive index of a medium (a) with respect to another medium (b) is the ratio of speed of light in medium (b) and the speed of light in medium (a).

Relative refractive index of  (a) wrt (b)=Speed of light in medium (b)

                                                                        Speed of light in medium (a)

For example, 

r.r.i of glass with water=speed of light in water/speed of light in glass 

=2.25×10⁸/2×10⁸=9/8=1.1

r.r.i of water wrt to glass= speed of light in glass/speed of light in water

=2×10⁸/2.25×10⁸= 8/9=0.88

Its value can be less than 1, more than 1 or equal to 1

Why are inner pulleys in a block and tackle system smaller than others?

 Answer: 

Inner pulleys are smaller than others to eradicate the frictional force between the ropes and inner pulleys while in use.

Spanner has a long handle. Why?

Answer: When perpendicular distance between the force and the axis of rotation is large, smaller force is required to produce  the desired torque.

TERMINAL VOLTAGE

Terminal voltage of a dry cell is 1.3 V. It means that the cell can supply 1.3J of energy to move unit positive charge across its terminals in the external circuit connected to the cell.

EMF of a dry cell

Emf of a dry cell is 1.5 V. It means that the cell can supply 1.5 joule of energy to move 1 coulomb of charge all around the circuit ( outside and inside  the cell).

MCB

 MCB stands for 

a) miniature circuit board 

b) miniature circuit band

c) miniature circuit breaker

d) miniature circuit blocker

Ans: c

RADIOACTIVITY

Q1: What is radioactivity?

A:  The  phenomenon of spontaneous emission of alpha particle or beta particle and gamma rays  from an unstable nucleus is called radioactivity. 

 Q2: When does a nucleus  become unstable ?

A: When the number of neutrons become much more than that of protons.

Q3: Which part of the atom takes part in radioactivity ?

A: Nucleus

Q4: What happens to the radioactive behaviour of a radioactive sample if it is chemically oxidized ?

A: The radioactive behaviour does not change. As oxidation is the chemical change, it does not affect the nuclear activity.

Q5: How does the atomic number and mass number change if a radioactive nucleus releases one alpha particle ?

A: Atomic number decreases by 2 and mass number decreases by 4

Q6: How does the atomic number and mass number change if a radioactive nucleus releases one beta particle?

A: Atomic number increases by 1 and mass number does not change.