Physics Archives

231. In which one of the following devices, the light energy is converted i

In which one of the following devices, the light energy is converted into the electrical energy?

[amp_mcq option1=”Light-emitting diode” option2=”Laser diode” option3=”Solar cell” option4=”Transistor” correct=”option3″]

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The correct option is C, the Solar cell.

The question asks for a device where light energy is converted into electrical energy. This process is known as the photovoltaic effect.
– Light-emitting diode (LED) converts electrical energy into light energy.
– Laser diode also converts electrical energy into coherent light energy.
– Solar cell (photovoltaic cell) is specifically designed to absorb photons from sunlight and convert their energy into an electric current.
– Transistor is a semiconductor device used as an electronic switch or amplifier; it controls the flow of electrical current but does not primarily convert light energy to electrical energy (though some transistors can be light-sensitive, their main function isn’t energy conversion).
Therefore, the solar cell is the device that converts light energy into electrical energy.

Solar cells are the fundamental components of solar panels, which are widely used for generating renewable electricity. They are made of semiconductor materials, typically silicon, which release electrons when struck by photons of light, creating an electric current. This phenomenon is the basis of solar power generation.

232. A simple pendulum having bob of mass m and length of string l has time

A simple pendulum having bob of mass m and length of string l has time period of T. If the mass of the bob is doubled and the length of the string is halved, then the time period of this pendulum will be

[amp_mcq option1=”T” option2=”T /√2″ option3=”2T” option4=”√2 T” correct=”option2″]

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The correct option is B, stating that the new time period will be T/√2.

The time period (T) of a simple pendulum is given by the formula T = 2π√(l/g), where l is the length of the string and g is the acceleration due to gravity. This formula shows that the time period depends only on the length of the pendulum and the acceleration due to gravity, and it is independent of the mass of the bob.
In the given problem, the initial time period is T for a pendulum with mass m and length l. When the mass of the bob is doubled (to 2m) and the length of the string is halved (to l/2), the new time period T’ will be T’ = 2π√((l/2)/g) = 2π√(l/(2g)). We can rewrite this as T’ = 2π√(l/g) * (1/√2). Since T = 2π√(l/g), the new time period T’ is T/√2.

The independence of the time period from the mass of the bob is a key characteristic of a simple pendulum, provided the amplitude of oscillation is small (usually less than 15 degrees). This property allows pendulums to be used as reliable timekeeping devices. Real-world factors like air resistance and the rigidity of the string can affect the period slightly, but for an ideal simple pendulum, mass is irrelevant.

233. The volume of a sealed packet is 1 litre and its mass is 800 g. The pa

The volume of a sealed packet is 1 litre and its mass is 800 g. The packet is first put inside water with density 1 g cm^-3 and then in another liquid B with density 1.5 g cm^-3. Then which one of the following statements holds true?

[amp_mcq option1=”The packet will float in both water and liquid B.” option2=”The packet will sink in both water and liquid B.” option3=”The packet will sink in water but will float in liquid B.” option4=”The packet will float in water and sink in liquid B.” correct=”option1″]

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The correct option is A, stating that the packet will float in both water and liquid B.

The principle of buoyancy states that an object will float in a fluid if its density is less than the density of the fluid. If its density is greater, it will sink. The packet has a volume of 1 litre (1000 cm³) and a mass of 800 g. Its density is mass/volume = 800 g / 1000 cm³ = 0.8 g/cm³. The density of water is 1 g/cm³. Since 0.8 g/cm³ < 1 g/cm³, the packet will float in water. The density of liquid B is 1.5 g/cm³. Since 0.8 g/cm³ < 1.5 g/cm³, the packet will also float in liquid B.

When an object floats, the buoyant force exerted by the fluid is equal to the weight of the object. The buoyant force is also equal to the weight of the fluid displaced by the object. If the object sinks, the buoyant force is less than the weight of the object. The proportion of the object submerged when floating is equal to the ratio of the object’s density to the fluid’s density. In water, 0.8/1 = 80% of the packet’s volume will be submerged. In liquid B, 0.8/1.5 ≈ 53.3% of the packet’s volume will be submerged.

234. The refractive index of crown glass is close to 3/2. If the speed of l

The refractive index of crown glass is close to 3/2. If the speed of light in air is c, then the speed of light in the crown glass will be close to

[amp_mcq option1=”(3/2)c” option2=”(4/9)c” option3=”(2/3)c” option4=”(9/4)c” correct=”option3″]

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The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum (or approximately in air) to the speed of light in that medium (v). The formula is n = c / v, where c is the speed of light in vacuum/air.
Given that the refractive index of crown glass is close to 3/2, we have n = 3/2.
Using the formula n = c / v, we can rearrange it to solve for the speed of light in the glass (v): v = c / n.
Substituting the given refractive index: v = c / (3/2).
To divide by a fraction, we multiply by its reciprocal: v = c * (2/3) = (2/3)c.

The refractive index indicates how much the speed of light is reduced when it passes through a medium compared to its speed in a vacuum. A higher refractive index means a lower speed of light in the medium.

The speed of light is highest in a vacuum (approximately 3 x 10⁸ m/s) and slows down when it enters any medium, causing light to bend (refract). The refractive index is a dimensionless quantity.

235. Which one of the following terms cannot represent electrical power in

Which one of the following terms cannot represent electrical power in a circuit?

[amp_mcq option1=”VI” option2=”I^2/R” option3=”I^2R” option4=”V^2/R” correct=”option2″]

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Electrical power (P) in a circuit can be calculated using different formulas derived from the relationship between voltage (V), current (I), and resistance (R), as defined by Ohm’s Law (V = IR). The standard formulas for power are:
1. P = VI (Power = Voltage × Current)
2. P = I²R (Substituting V = IR into P = VI: P = (IR)I = I²R)
3. P = V²/R (Substituting I = V/R into P = VI: P = V(V/R) = V²/R)
Looking at the options:
A) VI – This is a correct formula for power.
B) I²/R – This is incorrect. The correct formula involving I and R is I²R.
C) I²R – This is a correct formula for power.
D) V²/R – This is a correct formula for power.

Understanding the relationships between power, voltage, current, and resistance through Ohm’s Law is fundamental to electrical circuit analysis.

The power dissipated by a resistor represents the rate at which electrical energy is converted into heat energy. These power formulas are applicable to resistive components in both DC and AC circuits (though for AC, power can also involve a power factor).

236. Two convex lenses have focal lengths of 50 cm and 25 cm, respectively.

Two convex lenses have focal lengths of 50 cm and 25 cm, respectively. If these two lenses are placed in contact, then the net power of this combination will be equal to

[amp_mcq option1=”+2 dioptre” option2=”+6 dioptre” option3=”-6 dioptre” option4=”+3 dioptre” correct=”option2″]

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The power of a lens (P) is the reciprocal of its focal length (f) in meters (P = 1/f). For a combination of thin lenses in contact, the total power (P_net) is the sum of the individual powers (P_net = P1 + P2 + …).
Given focal lengths: f1 = 50 cm = 0.5 m and f2 = 25 cm = 0.25 m. Both are convex lenses, so focal lengths are positive.
Power of the first lens: P1 = 1 / f1 = 1 / 0.5 m = +2 Dioptre (D).
Power of the second lens: P2 = 1 / f2 = 1 / 0.25 m = +4 Dioptre (D).
Net power of the combination: P_net = P1 + P2 = +2 D + +4 D = +6 D.

The power of a lens is a measure of its ability to converge or diverge light, and for lenses in contact, powers are additive.

The unit of power is the Dioptre (D), defined as the reciprocal of the focal length in meters. Convex lenses have positive power (converging), and concave lenses have negative power (diverging).

237. Dry ice is used on a performing stage to produce mist in air. The proc

Dry ice is used on a performing stage to produce mist in air. The process involved is an example of

[amp_mcq option1=”sublimation” option2=”evaporation” option3=”condensation” option4=”precipitation” correct=”option1″]

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The correct answer is sublimation.

Dry ice is solid carbon dioxide. At atmospheric pressure, solid carbon dioxide changes directly into gaseous carbon dioxide without melting into a liquid. This process is called sublimation. The visible ‘mist’ produced when dry ice is used is actually water vapour in the surrounding air condensing or freezing due to the extremely cold carbon dioxide gas. However, the primary phase transition of the dry ice itself is sublimation, which drives the cooling effect leading to the mist formation.

Sublimation is an endothermic process, meaning it absorbs heat from the surroundings, which is why dry ice is so effective as a coolant and for creating effects like mist by cooling ambient water vapour.

238. Which one of the following statements about speed and velocity is

Which one of the following statements about speed and velocity is correct?

[amp_mcq option1=”Speed and velocity both are vector quantities.” option2=”Speed and velocity both are scalar quantities.” option3=”Speed is vector quantity and velocity is scalar quantity.” option4=”Speed is scalar quantity and velocity is vector quantity.” correct=”option4″]

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Speed is a scalar quantity and velocity is a vector quantity.

– **Speed** is the magnitude of how fast an object is moving. It only has magnitude and no direction. Quantities with magnitude only are called scalar quantities.
– **Velocity** is the rate of change of displacement. It has both magnitude (speed) and direction. Quantities with both magnitude and direction are called vector quantities.

For example, saying a car is travelling at “50 km/h” describes its speed (a scalar). Saying a car is travelling at “50 km/h North” describes its velocity (a vector).

239. The presence of magnetic field can be determined using which one of th

The presence of magnetic field can be determined using which one of the following instruments?

[amp_mcq option1=”Ammeter” option2=”Voltmeter” option3=”Magnetic needle” option4=”Motor” correct=”option3″]

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The presence of a magnetic field can be determined using a magnetic needle.

A magnetic needle, like the one in a compass, is a small magnet pivoted to rotate freely. When placed in a magnetic field, it aligns itself with the direction of the field lines, indicating the presence and direction of the field.

An Ammeter measures electric current. A Voltmeter measures electric potential difference (voltage). A Motor converts electrical energy into mechanical energy using the principle of magnetic force on a current-carrying wire, but it’s not primarily an instrument for *detecting* a magnetic field’s presence in general. Other instruments for measuring magnetic fields include magnetometers, but a magnetic needle is a simple and fundamental tool for detection.

240. A DC generator works on the principle of

A DC generator works on the principle of

[amp_mcq option1=”Ohm’s law” option2=”Joule’s law of heating” option3=”Faraday’s laws of electromagnetic induction” option4=”None of the above” correct=”option3″]

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A DC generator works on the principle of Faraday’s laws of electromagnetic induction.

Electromagnetic induction is the process where a conductor moving through a magnetic field (or a changing magnetic field near a conductor) induces an electromotive force (EMF), which can drive an electric current. Faraday’s laws quantify this phenomenon.

A generator utilizes mechanical energy (to rotate a coil or magnetic field) to cause a change in magnetic flux through a coil, thereby inducing an electric current according to Faraday’s laws. Ohm’s law relates voltage, current, and resistance in a circuit. Joule’s law of heating describes the heat produced by current flowing through a resistor. A motor works on the principle of the force experienced by a current-carrying conductor in a magnetic field.