Concept:

Magnifying Glass

• A magnifying glass is a convex lens used to make an object appear much larger than it actually is. This works when the object is placed at a distance less than the focal length from the lens.
• A convex lens is used as a magnifying glass as it converges parallel beams of light into one concentrated beam of light. The convex lens is used in the magnifying glass.
• It helps in magnifying an object that is placed. In order to see an image, the frame is placed right above the object.

Explanation:

• So, in order to use the convex lens as the magnifying glass, the object should be kept on the focal length. 
• The focal length is the distance between from center of the mirror to the focus point, so the image should be kept at a distance of 10 cm from the mirror.
• So, the correct option is from the center of the mirror to 10 cm in front of the lens.

Additional Information

• Spherical mirrors are of two types - Convex mirror, and Concave mirror.
  • The image formed by the spherical mirrors depends on the position of the object.

• Convex mirror: If the outer surface of the spherical mirror is the reflecting surface then it is called a convex mirror. It is also called the diverging mirror.
  • The nature of the image formed by a convex mirror does not depend on the distance of the object from the mirror. These images are always virtual, smaller than the object, and situated behind the mirror.
• Concave mirror: If the inner surface of the spherical mirror is the reflecting surface, then it is called a concave mirror. It is also called a focusing mirror/converging mirror.
  • The size of the image produced by these mirrors can be large

CONCEPT:

• The Law of Conservation of Energy states that the total energy of an isolated system remains conserved (constant), that is, it can neither be created nor be destroyed but it can be transformed from one form to another.
• One of the most common examples of the Law of Conservation of Energy is: water is stored at a height with some potential energy, when the water starts flowing downward then its potential energy is converted into kinetic energy.

• Whereas different forms of energy can be given as

• Whereas mathematically it can be represented as,
• Total energy initial = total energy final

Calculation:

Given that,

The electrical energy consumed by the water heater, E_elec = 500 J

And the efficiency of water heater = 80%

This means that the water heater can convert a maximum of 80 % of electrical energy into thermal energy whereas the rest will be lost due to heat exchange by the environment, resistance, etc.

Thus, maximum thermal energy can be produced by the water heater will be

CONCEPT:

Thomson atomic model:

• Thomson's atomic model was proposed by William Thomson in the year 1900.
• According to this model an atom consists of a positively charged sphere with electrons embedded in it.
• An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.
• Thomson’s model of an atom is similar to a plum pudding.

​EXPLANATION:

• According to this model an atom consists of a positively charged sphere with electrons embedded in it.
• An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.
• Thomson’s model of an atom is similar to a plum pudding. Hence, option 1 is correct.

CONCEPT:

Refraction:

• When a ray of light passes from one medium to another it suffers a change in direction at the boundary of two media is called refraction.
• The change in direction because the speed of light travels at different speeds in different mediums.
• Refractive Index of a medium defines that it is denser or rarer
• Refractive Index = Speed of light in Vacuum /speed of light in that medium.
• Refractive Index = η =c/v where, c = Speed of light in Vacuum, v= speed of light in that medium.

EXPLANATION:

• When a light ray is passed from rarer medium to denser medium then it bends toward the normal and when a light rays passed from denser to a rarer medium, then it bends away from the normal.
• When light enters through the prism it undergoes refraction twice, as shown in the figure.
• The angle of minimum deviation 'D' is the difference between the angle of incidence and refraction of a ray of light passing through the surface between one medium and another of different refractive indexes.
• The Prism formula is given by

Where A is the angle of the prism and D is the angle of minimum deviation.​

• Therefore option 1 is correct.

CONCEPT:

Acceleration Due to Gravity:

• The force of attraction exerted by the earth on a body is called gravitational pull or gravity.
• We know that when a force acts on a body, it produces acceleration. Therefore, a body under the effect of gravitational pull must accelerate.
• The acceleration produced in the motion of a body under the effect of gravity is called acceleration due to gravity, it is denoted by g.

• The acceleration due to gravity on the surface of the earth is given by,

EXPLANATION:

• We know that the gravitational acceleration on the surface of the earth is given as,

Where R = radius of the earth

• By equation 1 it is clear that the gravitational acceleration will be more where the radius of the earth is less.
• We know that the earth is not a perfect sphere, i.e., the Earth is flattened at the poles and bulges out at the equator.
• The equatorial radius of the earth is nearly 21 km larger than the polar radius.
• So the gravitational acceleration at the poles will be more than the equator.
• Due to the rotational effect of the earth, the gravitational acceleration of the earth gets reduced at the equator because a centrifugal force acts on the object.
• The relation between the gravitational acceleration at the equator and the poles is given as,

⇒ g_e = g_p - ω²R     -----(2)

Where ω = angular velocity of the earth

• By the above explanation, it is clear that the gravitational acceleration at the poles is more than the equator because,
  1. ​The radius of the earth at the equator is more than the radius of the earth at the poles.
  2. Due to the rotation of the earth.
• Hence, option 3 is correct.

The correct answer is In the affected eye, the image of a distant object is formed beyond the retina

• Myopia is a defect of an eye in which it cannot see distant objects clearly.
• A person with Myopia can see nearby objects clearly.
• Myopia is caused by due to - High converging power of the lens, and Eye-ball being too long.
• Due to the high converging power of the lens, the image is formed in front of the retina and a person cannot see clearly the distant objects.
• Myopia or short-sightedness can be corrected by wearing spectacles containing a concave lens.
• There are three common defects of vision :
  • ​Myopia or short-sightedness
  • Hypermetropia or long-sightedness
  • Presbyopia.

CONCEPT:

• Electromotive force (emf) - It is the potential difference between the terminals of the cell when no current is drawn from a cell.
• Inductance - It is the property of a coil to induce emf in the same coil or another when placed near it due to the changes in the current flowing through the coil.

EXPLANATION:

Self-Induction

• Every current-carrying coil produces a magnetic field when it is linked to the same coil.
• When there is a change in the current flowing through the coil, the magnetic flux also changes and it is linked to the coil itself.
• According to Faraday’s law of induction, the change in a magnetic field produces an induced emf which opposes the change.
• Hence, the phenomenon in which an induced emf is produced in a coil due to a change in the current flowing in the coil is known as self-induction.
• The induced emf is referred to as self-induced emf, which is equal to the rate of change of flux.
• Mathematically, it is written as

• The negative sign denotes the induced emf that opposes the applied emf.

Where 'L' is the constant of proportionality and it is called self-inductance of the coil.

Thus, the self inductance of a coil is numerically equal to the ratio of the induced emf to the rate of change of current in the coil.

Additional Information

CONCEPT:

Speed of a longitudinal wave in an ideal gas:

• In a longitudinal wave, the particles of the medium oscillate forward and backward in the direction of propagation of the wave.
• They cause compressions and rarefactions of small volume elements of the medium.
• The speed of a sound wave in an ideal gas is given by:

where P = Pressure of the gas and ρ = density.

EXPLANATION:

Factors affecting the speed of sound are:

a) Elasticity:

Elastic properties relate to the tendency of a material to maintain its shape and not deform when a force is applied to it. Elastic materials have closely bounded atoms and hence the speed of sound is more inelastic materials

b) Density:

Density describes the mass of the substance per unit volume. If the material is denser because its molecules are larger, it will transmit sound slower

c) Temperature:

Sound travels faster in a warm environment than in a cold environment

Key Points

• Nuclear Fuel Cycle:
  • ​The nuclear fuel cycle consists of front-end steps that prepare uranium for use in nuclear reactors and back-end steps to safely manage, prepare, and dispose of used or spent but still highly radioactive spent nuclear fuel.
  • Uranium is the most widely used fuel for nuclear fission in nuclear power plants.
  • Nuclear power plants use a certain type of uranium U235 as fuel because its atoms can be easily divided.
  • Although uranium is generally about 100 times higher than silver, U235 is relatively rare and only accounts for more than 0.7% of natural uranium.
  • Uranium concentrate is separated from uranium ore in a uranium plant or separated from sludge in an on-site leaching facility. It is then processed in a conversion and enrichment facility to increase the U235 level of commercial nuclear reactors to 3% -5% and made into reactor fuel pellets and fuel rods at the reactor fuel fabrication plant.
  • They use nuclear reactors in combination with the Rankine cycle, where the heat generated by the reactor converts water into steam, which spins a turbine and a generator. The nuclear reactor is a key component of a nuclear power plant, as it contains the fuel and its nuclear fission chain reaction, along with all of the nuclear waste products.
  • Uranium is the dominant nuclear fuel used in nuclear reactors, and its fission reactions produce heat within a reactor. This heat is then transferred to the reactor's coolant, which provides steam to the turbine of the nuclear power plant.

CONCEPT:

Eddy current:

• When a changing magnetic flux is applied to a bulk piece of conducting material then circulating currents called eddy currents are induced in the material.
• Because the resistance of the bulk conductor is usually low, eddy currents often have large magnitudes and heat up the conductor.
• These are circulating currents like eddies in the water.
• The experimental concept was given by Focault hence also named “Focault current”.
• The production of eddy currents in a metallic block leads to the loss of electric energy in the form of heat.
• Eddy currents are minimized by using laminations of metal to make a metal core. The laminations are separated by an insulating material like lacquer.
• Eddy currents are undesirable in the metallic cores of transformers, electric motors, and other such devices in which a coil is to be wound over a metallic core since they heat up the core and dissipate electrical energy in the form of heat.
• Eddy currents are used to advantage in certain applications like:
  1. Magnetic braking in trains
  2. Electromagnetic damping
  3. Induction furnace
  4. Electric power meters

EXPLANATION:

Electric power meters:

• The shiny metal disc in the electric power meter (analogue type) rotates due to the eddy currents.
• Electric currents are induced in the disc by magnetic fields produced by sinusoidally varying currents in a coil. Hence, option 2 is correct.