CONCEPT:

Light year:

• It is the distance traveled by light in one year. 
  • A light-year is a convenient unit of measurement of large distances.
  • Generally, a light-year is used to represent astronomical distances. 
  • One light-year is approximately 9 trillion (9 × 1012) km.

EXPLANATION:

• A light-year is a unit of length. Therefore option 3 is correct.​​
• It is the distance traveled by light in a vacuum in one Julian year.
• A light-year is used in astronomy to measure the distances of nearby stars.

⇒ 1 light-year = 1ly = 9.467 × 10¹⁵ m.

Additional Information

Astronomical unit:

• It is defined as the mean distance of the earth from the sun. It is used in astronomy to measure the distance of planets.

⇒ 1 astronomical unit = 1 AU = 1.496 × 10¹¹ m

Parsec (parallactic second):

• It is the largest practical unit of distance used in astronomy.
• It is defined as the distance at which an arc of the length of 1 astronomical unit subtends an angle of 1 second of arc.

⇒ 1 parsec = 3.08 × 10¹⁶ m

CONCEPT:

• The Gold Foil Experiment: A very thin sheet of gold foil was bombarded with alpha particles.
  • The experiment result of the Bombardment of gold foil with alpha particles showed that a very small part of alpha particles was deflected.
  • This experiment showed that the atom consists of a small and dense positively charged interior surrounded by a cloud of negative charge electrons.

Figure (A) The experimental setup for Rutherford's gold foil experiment

Figure (B) Rutherford found that a small percentage of alpha particles were deflected at large angles

EXPLANATION:

• In Rutherford's Gold foil, alpha-particle scattering experiment, Very thin sheets of gold foil were bombarded with fast-moving alpha particles.
• Since alpha particles are positive charge ions, He used them to know the exact position of positive charge inside the atom.
• He found a very small, dense, positively-charged nucleus at its center (bottom).
• So the correct answer is option 1.

CONCEPT:

• Electric current: The rate of flow of electric charge is called electric current. It is denoted by I.

Electric current (I) = Charge (Q)/time (t)

Charge flown (Q) = I × t

CALCULATION:

Given that:

Current (I) = 0.6 A

Time (t) = 6 mins = 6 × 60 sec

Charge (Q) =?

Electric charge = Current × Time

Q = I × t

Q = 0.6 × 6 × 60 C

Q = 216 C

CONCEPT:

• Electric Power: The rate at which electrical energy is dissipated into other forms of energy is called electrical power i.e.,

Where V = Potential difference, R = Resistance and I = current.

EXPLANATION:

• From the above equation, it is clear that the power loss in a conductor of resistance ‘R’ across which a potential difference of ‘V’ is equal to V²/R.

Mistake Point

• The expression V²/R and I²R only used for ohmic materials.

CONCEPT:

• Electric potential is equal to the amount of work done per unit charge by an external force to move the charge q from infinity to a specific point in an electric field.

• Potential due to a single charged particle Q at a distance r from it is given by:

Where, ϵ0 is the permittivity of free space and has a value of 8.85 × 10^-12 F/m in SI units

• Potential at a point P due to a system of charged particles Q₁, Q₂, Q₃, ... Qn having distances r₁, r₂, r₃, ... rn respectively from point P is given by:

• An Equipotential surface is a surface with a constant value of the potential at all points on the surface. 
• The following figures show equipotential surfaces due to a point charge and an electric dipole.

• The relation between electric potential and the electric field is,

Where E is the electric field, V is the electric potential, and x is the direction of the electric field in space

EXPLANATION:

• An equipotential surface is a surface with a constant value of the potential at all points on the surface.
• The electric field is always perpendicular to an equipotential surface.
• The force on a positive charge is always in the same direction as that of the electric field.

⇒ F = qE

• Therefore statement B is correct.
• The relation between electric potential and the electric field is,

• Thus if E is positive along with dx, dV decreases when moving along dx. 
• The direction of the electric field is in the direction of decreasing potential. Therefore statement D is correct. Hence, option 4 is correct​

CONCEPT:

Atmospheric Pressure:

• The pressure of the atmosphere at any point is equal to the weight of a column of air of unit cross-sectional area extending from that point to the top of the atmosphere.
• At sea level the atmospheric pressure is given as,

⇒ P_a = 1.013 × 10⁵ Pa = 1.013 bar

⇒ 1 bar = 10⁵ Pa

EXPLANATION:

• At sea level, the atmospheric pressure is given as,

⇒ Pa = 1.013 × 10⁵ Pa = 1.013 bar

• Italian scientist Evangelista Torricelli (1608-1647) devised for the first time a method for measuring atmospheric pressure.
• A long glass tube closed at one end and filled with mercury is inverted into a trough of mercury as shown in the figure.
• This device is known as a ‘mercury barometer’.
• In the experiment, it is found that the mercury column in the barometer has a height of about 76 cm or 760 mm at sea level equivalent to one atmosphere (1 atm). Hence, option 3 is correct.

CONCEPT:

• Friction: The property of a surface that opposes the relative motion between two surfaces is called friction.

Friction (f) = μ N

where μ is the coefficient of friction and N is the normal force

There are three types of friction:

• Static friction: The friction that exists between an object at rest and a rough surface on which it is placed, such friction is termed as static friction.
  • The coefficient of friction due to this friction is called a static friction coefficient.
• Kinetic friction/sliding friction: The friction between the two surfaces when there is a relative motion between the surfaces then it is called kinetic friction.
• Rolling friction: When a block is rolling (usually a circular body like a ball, tire) on a surface then the friction acting is called rolling friction.

EXPLANATION:

1. Friction opposes the relative motion between two surfaces. So the statement A is true.
2. When the body is moving then the friction acting is called sliding friction. So statement B is true.
3. Friction in machines wastes energy and also causes wear and tear. Statement C is also correct.
4. In the case of rolling motion, there is no relative motion between the two contact surfaces. So there is no work done by the friction in a rolling motion. That's why the rolling friction is less than the sliding/kinetic friction. So statement D is not true.

CONCEPT:

Bio-mass energy:

• Bio-mass is the energy source that is received from animal wastes and plant materials.
• For example, cow-dung, various plant materials like the residue after harvesting the crops, vegetable waste, and sewage are decomposed in the absence of oxygen to give biogas.

EXPLANATION:

• Atomic energy is not a form of biomass energy. 
• Biomass energy is the energy obtained from organic fuels. 
• Coal is a fossil that is formed by dead plants and its formation takes millions of years.
• Wood is obtained from trees of forest which are organic. 
• From the above, it is clear that wood, gobar-gas, and cow-dung are an example of biomass energy sources. Therefore option 1, 2, and 4 is correct.
• On the other hand, nuclear energy is obtained from nuclear fusion and fission. Therefore option 3 is incorrect.
• So, the correct option is atomic energy.

• The atomic structure consists of protons, electrons, and neutrons. The nucleus of an atom consists of Protons and Neutrons.
• All the three associated with the atomic model.
• J Chadwick:
  • He was a British physicist. 
  • He was associated with the discovery of Neutron and also awarded the noble prize for the discovery of neutrons.
• JJ Thomson:
  • He was a British Physicist.
  • He was credited with the discovery of the electron.
• Rutherford:
  • Rutherford was awarded the 1908 Nobel Prize in Chemistry for his theory of atomic structure.
  • He discovered the nucleus of the atom in 1911.
  • He is known as the father of nuclear physics.

CONCEPT:

• Refraction of Light: The bending of the ray of light passing from one medium to the other medium is called refraction.

• The refraction of light takes place on going from one medium to another because the speed of light is different in the two media.
• Greater the difference in the speeds of light in the two media, greater will be the amount of refraction.
• A medium in which the speed of light is more is known as optically rarer medium and a medium in which the speed of light is less is known as an optically denser medium.

EXPLANATION:

• When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal. Therefore option 2 is correct.

Important Point

• When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal.