Physics

21. A 100 W electric bulb is used for 8 hrs/day. What would be the units o

A 100 W electric bulb is used for 8 hrs/day. What would be the units of energy consumed in the month of April ?

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24 units
16 units
8 units
0·8 units
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To calculate the energy consumed, we use the formula: Energy = Power × Time.
Power = 100 W = 0.1 kW (kilowatt).
The bulb is used for 8 hours per day.
The month of April has 30 days.
Total usage time in April = 8 hours/day × 30 days = 240 hours.
Energy consumed in kWh = Power (kW) × Total time (hours) = 0.1 kW × 240 hours = 24 kWh.
One unit of electrical energy is equivalent to 1 kWh.
Therefore, the energy consumed is 24 units.
– Convert power from Watts to Kilowatts (1 kW = 1000 W).
– Calculate the total usage time in hours for the month.
– Use the formula: Energy (kWh) = Power (kW) × Time (hours).
– 1 unit of energy = 1 kWh.
Electrical energy consumption is typically measured in kilowatt-hours (kWh) by electricity meters. This unit is often referred to as a “unit” in common parlance for billing purposes.

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22. Which one of the following statements is correct?

Which one of the following statements is correct?

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Weight of an object may vary from place to place but its mass remains constant.
Mass of an object may vary from place to place but its weight remains constant.
Both weight and mass of an object do not vary from place to place.
Both weight and mass of an object vary from place to place.
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Mass is an intrinsic property of an object, representing the amount of matter it contains. It remains constant regardless of location. Weight, on the other hand, is the force exerted on an object due to gravity (Weight = Mass x gravitational acceleration). Since gravitational acceleration varies slightly depending on altitude, latitude, and local geological features, and significantly on different celestial bodies, the weight of an object can vary from place to place, while its mass remains constant.
– Mass is the amount of matter; it is constant.
– Weight is the force of gravity (mass x gravity); it varies with gravity.
– Gravitational acceleration varies from place to place.
On the surface of the Earth, the variation in gravitational acceleration is small, so the variation in weight is also relatively small. However, the distinction between mass and weight is fundamental in physics. On the Moon, an object has the same mass as on Earth but weighs significantly less because the Moon’s gravity is weaker.

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23. Gases can be liquefied by

Gases can be liquefied by

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reducing pressure and temperature.
applying pressure and reducing temperature.
reducing pressure and raising temperature.
applying pressure and raising temperature.
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Gases can be liquefied by increasing the intermolecular forces or reducing the kinetic energy of the gas particles so that they can come closer together and form a liquid state. Applying pressure forces the particles closer, increasing intermolecular interactions. Reducing temperature decreases the kinetic energy of the particles, making it easier for them to form liquid bonds. Liquefaction occurs when the temperature is at or below the critical temperature and sufficient pressure is applied. The most effective way is typically applying pressure and reducing temperature.
Liquefaction of gases requires increasing pressure and/or decreasing temperature to bring molecules closer and reduce their kinetic energy.
For every gas, there is a critical temperature above which it cannot be liquefied by pressure alone. Below the critical temperature, increasing pressure can cause liquefaction. Reducing the temperature makes liquefaction easier at lower pressures. Therefore, applying pressure and reducing temperature together is the standard method for liquefying gases.

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24. In an atomic gas, the motion of particles (atoms) is governed by the c

In an atomic gas, the motion of particles (atoms) is governed by the collisions. If the gas is ionized, then the motion of created particles may be mainly governed by

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gravitational force.
collisions.
scattering of particles.
electromagnetic force between the particles.
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In an atomic gas, the atoms are electrically neutral, and their motion is primarily governed by collisions between them. When the gas is ionized, atoms lose or gain electrons, becoming charged particles (ions and free electrons). These charged particles exert strong electrostatic (electromagnetic) forces on each other over relatively long distances compared to the short-range forces involved in neutral collisions. Therefore, the motion of particles in an ionized gas (plasma) is mainly governed by the long-range electromagnetic forces between these charged particles, rather than just collisions.
In a neutral atomic gas, particle motion is dominated by collisions. In an ionized gas (plasma) containing charged particles, the motion is dominated by long-range electromagnetic forces between these charges.
An ionized gas is also known as plasma, which is often considered the fourth state of matter. The collective behavior of charged particles under the influence of electromagnetic fields is a key characteristic of plasma physics. While collisions still occur in plasma, their influence on overall motion is often less dominant than the electromagnetic forces, especially in hot, tenuous plasmas.

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25. The transfer of thermal energy carries which of the following

The transfer of thermal energy carries which of the following phenomena?

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Conduction and convection only
Only conduction
Conduction, convection and radiation
Only radiation
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Thermal energy, or heat, can be transferred from one place to another through three fundamental mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact of particles, primarily in solids. Convection is the transfer of heat through the movement of fluids (liquids or gases). Radiation is the transfer of heat through electromagnetic waves, which does not require a medium and can occur through a vacuum.
The three main modes of thermal energy transfer are conduction, convection, and radiation.
Examples: Conduction transfers heat through a metal rod when one end is heated. Convection transfers heat in boiling water or rising hot air. Radiation transfers heat from the sun to the Earth, or from a fire to your hands. All three phenomena contribute to the transfer of thermal energy in different situations.

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26. An electric refrigerator rated 400 W operates 10 hours/day. What is th

An electric refrigerator rated 400 W operates 10 hours/day. What is the cost of the energy to operate it for 30 days at ₹ 3.00 per kWh?

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₹ 360
₹ 3,600
₹ 36
₹ 400
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The power rating of the refrigerator is 400 W, which is equal to 0.4 kW (since 1 kW = 1000 W). The refrigerator operates for 10 hours per day. Over 30 days, the total operation time is 10 hours/day * 30 days = 300 hours. The total energy consumed is the power multiplied by the time: Energy (kWh) = Power (kW) * Time (hours) = 0.4 kW * 300 hours = 120 kWh. The cost of energy is ₹ 3.00 per kWh. Total cost = Energy consumed * Cost per kWh = 120 kWh * ₹ 3.00/kWh = ₹ 360.
Energy consumed is calculated as Power (in kW) multiplied by Time (in hours). The total cost is the energy consumed multiplied by the rate per unit of energy (kWh).
The unit of energy used for billing is typically the kilowatt-hour (kWh), often called a ‘unit’ of electricity. Power in watts needs to be converted to kilowatts before calculating energy in kWh if time is in hours.

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27. An object is kept at infinity from the position of a concave (spherica

An object is kept at infinity from the position of a concave (spherical) mirror. Which one is *not* true about the image of the object?

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Position of image is at the focus of the mirror
Size of image is the same as that of the object
Image is real
Image is inverted
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When an object is placed at infinity from a concave spherical mirror, the light rays from the object are considered to be parallel to the principal axis. After reflection from the concave mirror, these parallel rays converge at the principal focus (F) of the mirror. The image formed at the focus is real, inverted (relative to the infinitely distant object), and highly diminished (essentially a point image).
For an object at infinity from a concave mirror, the image is formed at the focus, is real, inverted, and highly diminished (point-sized).
Option A states the position of the image is at the focus, which is true. Option C states the image is real, which is true for converging rays formed on the same side as the object. Option D states the image is inverted, which is true for real images formed by a concave mirror (even though a point image doesn’t visually appear inverted). Option B states the size of the image is the same as that of the object, which is false; the image is highly diminished. Therefore, the statement that is *not* true is B.

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28. Tyndall effect appears due to which one of the following properties of

Tyndall effect appears due to which one of the following properties of light?

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Reflection of light
Diffraction of light
Polarization of light
Scattering of light
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The Tyndall effect is the phenomenon where the path of a beam of light becomes visible as it passes through a colloidal dispersion or a fine suspension. This effect occurs because the larger particles in the colloid or suspension scatter the light in all directions when it strikes them.
Tyndall effect is the scattering of light by particles in a colloid or a very fine suspension.
Reflection occurs when light bounces off a surface. Diffraction is the bending of light waves as they pass around the edge of an obstacle or through a narrow slit. Polarization is the restriction of the vibration of light waves to a single plane. Scattering is the process by which light is deflected in various directions as it interacts with a medium or particles within it, which is the principle behind the Tyndall effect.

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29. The horizontal component of the earth’s magnetic field is zero at

The horizontal component of the earth’s magnetic field is zero at

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magnetic equator
magnetic poles
South and North Poles
nowhere
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The horizontal component of the earth’s magnetic field is zero at magnetic poles.
The Earth’s magnetic field lines emerge from near the geographic South Pole (which is the North magnetic pole) and enter near the geographic North Pole (which is the South magnetic pole).
– At the magnetic poles (where a compass needle points vertically downwards or upwards), the magnetic field lines are essentially perpendicular to the Earth’s surface. Therefore, the magnetic field vector has only a vertical component, and the horizontal component is zero.
– At the magnetic equator, the magnetic field lines are approximately parallel to the Earth’s surface. Therefore, the magnetic field vector is primarily horizontal, and the vertical component is zero.
– The geographic poles (South and North Poles) are points on the Earth’s rotational axis and do not necessarily coincide with the magnetic poles. The horizontal component is generally not zero at the geographic poles unless they happen to coincide perfectly with the magnetic poles (which they do not).
The Earth’s magnetic poles are not fixed and drift over time. The angle between the magnetic north and geographic north is called the magnetic declination. The angle between the horizontal plane and the Earth’s magnetic field line is called the magnetic dip or inclination; the dip is 90 degrees at the magnetic poles and 0 degrees at the magnetic equator.

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30. Stars twinkle in the sky at night because

Stars twinkle in the sky at night because

refractive index of the atmosphere changes due to the change of temperature
stars emit light in the form of pulses
of interference of light coming from different stars
of diffraction of light
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Stars twinkle in the sky at night because refractive index of the atmosphere changes due to the change of temperature.
Twinkling of stars (scintillation) is caused by atmospheric refraction.
– Light from distant stars travels through the Earth’s atmosphere before reaching our eyes.
– The atmosphere is not uniform; it consists of layers with varying temperatures and densities.
– Variations in temperature and density cause variations in the refractive index of the air.
– As light from a star passes through these turbulent layers with changing refractive index, it undergoes continuous refraction in random directions.
– This causes fluctuations in the apparent position and brightness of the star as seen from Earth. These rapid fluctuations are perceived as twinkling.
– Planets, being much closer, appear as extended sources of light rather than point sources. The light from different parts of a planet’s disc undergoes similar but independent variations, which average out, so planets do not twinkle noticeably.

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Option A correctly identifies the cause: changes in atmospheric refractive index due to temperature variations (and hence density variations) lead to varying refraction of starlight.
Option B is incorrect; stars emit light continuously.
Option C is incorrect; twinkling is an effect on light from a single star due to atmospheric effects, not interference from different stars.
Option D is incorrect; while diffraction occurs, twinkling is primarily an effect of refraction due to atmospheric turbulence.

Atmospheric refraction is also responsible for phenomena like the apparent flattening of the sun at sunrise/sunset and the fact that we can see the sun just before it rises and just after it sets. The degree of twinkling is affected by atmospheric conditions (turbulence).

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