# Physics GK Quiz-13

## Physics GK Quiz-13

Physics Multiple Choice Questions (MCQs) Quiz for State and UPSC Civil Services Examinations. Objective Questions on Physics for competitive examinations.

### 241. The unit of electrical power is

(1) Bolt
(2) Watt
(3) Kilowatt hour
(4) Ampere
241. (2) Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Watt is named after the Scottish engineer James Watt who invented the Newcomen Steam Engine.

### 242. The scientist who first sent electromagnetic waves to distant places is

(1) James Clerk Maxwell
(2) Heinrich Hertz
(3) Thomas Alva Edison
(4) John Logie Baird
242. (3) Radio is the transmission of signals through free space by electromagnetic radiation of a frequency significantly below that of visible light, in the radio frequency range, from about 30 kHz to 300 GHz. These waves are called radio waves. Experiments were undertaken by Thomas Edison and his employees of Menlo Park. Edison applied in 1885 to the U.S. Patent Office for a patent on an electrostatic coupling system between elevated terminals. The patent was granted as U.S. Patent 465,971 on December 29, 1891. The Marconi Company would later purchase rights to the Edison patent to protect them legally from lawsuits.

### 243. The centre of gravity of a sprinter during the race lies

(2) behind his feet
(3) at the centre of the body
(4) to the left side of the body
243. (1) Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground. This is in contrast to walking, where one foot is always in contact with the ground, the legs are kept mostly straight and the center of gravity vaults over the legs in an inverted pendulum fashion. A characteristic feature of a running body from the viewpoint of spring-mass mechanics is that changes in kinetic and potential energy within a stride occur simultaneously, with energy storage accomplished by springy tendons and passive muscle elasticity.

### 244. Cloudy nights are warmer compared nights, because clouds mainly

(1) absorb heat from the atmosphere send it towards earth
(2) prevent cold waves from the sky descending on earth
(3) reflect back the heat given by earth
(4) produce heat and radiate it toward earth
244. (3) It is because the night side of Earth will radiate infra-red radiation (heat) back into space. When there is cloud cover, the clouds act like a blanket and trap the heat close to the ground just like a blanket traps heat close to our body.

### 245. A metal plate with a circular hole at the centre is heated. What will happen to the area of the hole ?

(1) Increase
(2) Dacrease
(3) Remain constant
(4) Will increase first and then dcrease
245. (1) Assuming the disc is uniform and isotropic (the same in different directions), the hole will expand in the same ratio as the metal. It is because the thermal expansion equation applies to all lengths associated with the metal, including the circumference of the hole, since the edge of the hole is made out of metal. And if the circumference of the hole expands, so does the diameter.

### 246. The period of revolution of a geostationary satellite is

(1) 365 days
(2) 30 days
(3) 24 hours
(4) changing continuously
246. (3) A geosynchronous satellite is a satellite in geosynchronous orbit, with an orbital period the same as the Earth’s rotation period. Such a satellite returns to the same position in the sky after each sidereal day, and over the course of a day traces out a path in the sky. A special case of geosynchronous satellite is the geostationary satellite, which has a geostationary orbit – a circular geosynchronous orbit
directly above the Earth’s equator.

### 247. A piece of paper and a cricket ball are dropped from the same height. Under which of the following conditions do both reach the surface simultaneously ?

(1) They must have the same volume
(2) They must have the same density
(3) They must have the same mass
(4) They must be dropped in vacuum
247. (4) When a piece of paper and a cricket ball are dropped from the same height, they reach the surface at different time because the shape of the paper is more flat and it behaves like a parachute causing more air resistance acting on it with respect to the ball. But, in order to reach the surface at the same time by both the articles, they must be dropped in vacuum. It is because in vacuum there is no other force other than force of gravity occurring on them and this leads to a conclusion that both the article reaches at the same time.

### 248. The frequency of ultrasound wave is typically

(1) Above 20 kHz
(2) Above 20,000 kHz
(3) Below 20 kHz
(4) Below 02 kHz
248. (3) Ultrasound is a cyclic sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound is thus not separated from “normal” (audible) sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz.

### 249. Which type of reaction produces the most harmful radiation ?

(1) Fusion reaction
(2) Fission reaction
(3) Chemical reaction
(4) Photo-Chemical reaction
249. (2) Radiation is one of the byproducts of radioactive decay of unstable atomic nuclei. Nuclear fission is the process on which the operation of most nuclear power plants is based. When certain nuclei (for example that of uranium-235) are bombarded with neutrons, the nucleus splits into two smaller nuclei of roughly (though not exactly) equal sizes. Because of the binding characteristics of the neutrons and protons in the original and resultant nuclei (and also the famous mass-energy relation that Einstein discovered), the result of this split is the liberation of a large amount of energy, manifested in the kinetic energy of the resultant nuclei.

### 250. Optical fibres are based on the phenomenon of

(1) Interference
(2) Dispersion
(3) Diffraction
(4) Total Internal Reflection
250. (4) An optical fiber (or optical fibre) is a flexible, transparent fiber made of glass (silica) or plastic,
slightly thicker than a human hair. It functions as a waveguide, or “light pipe”, to transmit light between
the two ends of the fiber. Optical fibers typically include a transparent core surrounded by a
transparent cladding material with a lower index of refraction. Light is kept in the core by total internal
reflection. This causes the fiber to act as a waveguide. Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, no light can pass through and all of the light is reflected.

### 251. ‘Mirage’ is an example of

(1) refraction of light only
(2) total internal, reflection of light only
(3) refraction and total internal reflection of light
(4) dispersion of light only
251. (3) A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce
a displaced image of distant objects or the sky. In contrast to a hallucination, a mirage is a real optical
phenomenon which can be captured on camera, since light rays actually are refracted to form the false image at the observer’s location. As light passes from colder air across a sharp boundary to significantly warmer air, the light rays bend away from the direction of the temperature gradient. When light rays pass from hotter to cooler, they bend toward the direction of the gradient. If the air near the ground is warmer than that higher up, the light ray bends in a concave, upward trajectory. Once the rays reach the viewer’s eye, the visual cortex interprets it as if it traces back along a perfectly straight “line of sight”. This line is however at a tangent to the path the ray takes at the point it reaches the eye.

### 252. The phenomenon of light associated with the appearance of blue colour of the sky is

(1) Interference
(2) Reflection
(3) Refraction
(4) Scattering
252. (4) During daylight, the sky appears to be blue because air scatters blue sunlight more than it
scatters red. At night, the sky appears to be a mostly dark surface or region scattered with stars. Except
for light that comes directly from the sun, most of the light in the day sky is a result of scattering, which
is dominated by a small-particle limit called Rayleigh scattering. The scattering due to molecule sized
particles (as in air) is greater in the forward and backward directions than it is in the lateral direction.
Scattering is significant for light at all visible wavelengths, but it is stronger at the shorter (bluer)
end of the visible spectrum, meaning that that the scattered light is more blue than its source, the sun.

### 253. Lens is made up of

(1) Pyrex glass
(2) Flint glass
(3) Ordinary glass
(4) Cobalt glass
253. (2) Flint glass is optical glass that has relatively high refractive index and low Abbe number (high
dispersion). A concave lens of flint glass is commonly combined with a convex lens of crown glass to
produce an achromatic doublet lens because of their compensating optical properties, which reduces
chromatic aberration (colour defects).

### 254. The time period of a pendulum when taken to the Moon would:

(1) remain the same
(2) decrease
(3) become zero
(4) increase
254. (4) Since the pendulum rate will increase with an increase in gravity, and local gravity varies with
latitude and elevation on Earth, pendulum clocks must be readjusted to keep time after a move. For
example, a pendulum clock moved from sea level to 4000 feet will lose 16 seconds per day. Even moving a clock to the top of a tall building will cause it to lose measurable time due to lower gravity. Time period of a pendulum taken to the moon will be longer than the period of the same pendulum on earth.

### 255. The atmospheric air is held to the Earth by :

(1) gravity
(2) winds
(3) clouds
(4) rotation of the Earth
255. (1) The atmosphere is an ocean of air held in place by gravity, extending from the surface to an altitude of hundreds of kilometers, the edge of space. Energy from the sun heating the air and land surface to different degrees, drives atmospheric circulation. Patterns of circulation are also influenced by Earth’s rotation, latitude and the distribution of land, ocean and ice. Surface gravity, the force that holds down an atmosphere, differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter is able to retain light gases such as hydrogen and helium that escape from lower gravity objects.

### 256. The function of ball bearings in a wheel is :

(1) to increase friction
(2) to convert kinetic friction into rolling friction
(3) to convert static friction into kinetic friction
(4) just for convenience
256. (2) A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. In general, it’s main function is to convert kinetic friction into rolling friction. It achieves this by using at least two races to contain the balls and transmit the loads through the balls.

### 257. 'Shock-absorbers' are usually made of steel as it :

(1) is not brittle
(2) has lower elasticity
(3) has higher elasticity
(4) has no ductile property
257. (3) A shock absorber is a mechanical device designed to smooth out or damp shock impulse, and dissipate kinetic energy. Steel is an alloy made by combining iron and other elements, the most common of these being carbon. When carbon is used, its content in the steel is between 0.2% and 2.1% by weight, depending on the grade. Varying the amount of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the higher elasticity,
hardness, ductility, and tensile strength of the resulting steel.

### 258. Among the following materials sound travels fastest in

(1) Steel
(2) Air
(3) Vacuum
4) Water
258. (1) The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68 °F), the speed of sound is 343.2 metres per second (1,126 ft/s). the speed of sound varies from substance to substance. Sound travels faster in liquids and nonporous solids than it does in air. It travels about 4.3 times as fast in water (1,484 m/s), and nearly 15
times as fast in iron (5,120 m/s), than in air at 20 degrees Celsius. Sound waves in solids are composed
of compression waves (just as in gases and liquids), but also exhibit a different type of sound wave called a shear wave, which occurs only in solids.

### 259. Lambert’s law is related to

(1) Reflection
(2) Refraction
(3) Interference
(4) Illumination
259. (4) In optics, Lambert’s cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle between the observer’s line of sight and the surface normal. The law is also known as the cosine emission law or Lambert’s emission law. A surface which obeys Lambert’s law is said to be Lambertian, and exhibits Lambertian reflectance. Such a surface has the same radiance when viewed from any angle. This means, for example, that to the human eye it has the same apparent brightness (or luminance).

### 260. One can distinguish a telescope from a microscope by observing

(1) length
(2) colour
(3) size of the lens
(4) length and size of the lens