# Physics GK Quiz-9

## Physics GK Quiz-9

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

### 161. Energy that is produced commercially from coal is called

(1) Light energy
(2) Kinetic energy
(3) Thermal energy
(4) Potential energy
161. (3) Thermal energy, but in most cases coal is used for electricity. The steel industry uses coal (or coke rather) in blast furnaces. Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system temperature. This quantity may be difficult to determine or even meaningless unless the system has attained its temperature only through heating, and not been subjected to work input or output, or any other energy-changing processes.

### 162. We always see the same face of the moon, because

(1) it is smaller than the earth
(2) it revolves on its axis in a direction opposite to that of the earth
(3) it takes equal time for revolution around the earth and rotation on its own axis
(4) it rotates at the same speed as the earth around the sun
162. (3) It is tidal locking that causes the synchronous rotation which causes the Moon to present “just one side” to the Earth all the time. The Moon rotates (spins) but very slowly, making one turn on its axis in the same time it takes to revolve (orbit) around the Earth. So it maintains a “constant face” in our direction. We actually see slightly more than half of its surface as it turns. Tidal locking (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Earth’s Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This causes one hemisphere constantly to face the partner body.

### 163. Water pipes in hilly areas often burst on a cold frosty night, because

(1) the material of which pipes are made contracts due to cold and so breaks
(2) water in the pipes freezes and on freezing water expands, so pipes break
(3) frost makes the pipes rusty, so they break
(4) None of these
163. (2) The temperature may fall below 00 C during cold frosty nights which converts the water inside the pipes into ice, resulting in an increase in volume. This exerts great force on the pipes and as a result,
they burst.

### 164. A parachute descends slowly whereas a stone dropped from the same height falls rapidly, because

(1) stone is heavier than parachute
(2) special mechanisms are present in parachute
(3) a parachute has a larger surface area and air resistance is more
(4) None of these
164. (3) Gravity is pulling the man down while friction with the air slows him down. With the parachute out it adds more friction slowing him down because air resistance works against the very large surface area of the parachute. A free falling skydiver that has not yet opened his parachute can make his downward speed vary between about 110 mph and 225 mphusing air resistance. A spread eagled position presents the maximum area, and we fall the slowest.

### 165. Which of the following instruments is used to measure humidity ?

(1) Kata Thermometer
(2) Anemometer
(3) Sling Psychrometer
(4) Clinical Thermometer
165. (3) A psychrometer consists of two thermometers, one which is dry and one which is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the
wet-bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb
thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers.
The sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes, is sometimes used for field measurements, but is being replaced by more convenient electronic sensors.

### 166. The type of mirrors used in the headlamp of cars is

(1) parabolic concave
(2) plane
(3) spherical convex
(4) cylindrical concave
166. (1) Curved mirror that reflects light from its inner surface, the curve being inward. It may be either
circular or parabolic in section. A concave mirror converges parallel light rays inward to the point of
principal focus. The image formed by a concave mirror is real (reduced and inverted) if the object is not too close to the mirror. A real image is formed at the point of convergence. If the object is close to the
mirror then the image formed will be virtual, enlarged, and upright, as the rays of light cannot converge to a point. Only a parabolic concave mirror has a true, single-point principal focus for parallel rays. For this reason, parabolic mirrors are used as reflectors to focus light in telescopes, or to focus microwaves in satellite communication systems.

### 167. The reason for a swimming pool to appear less deep than the actual depth is

(1) refraction
(2) light scattering
(3) reflection
(4) interference
167. (1) The apparent depth will look less that its real depth due to the refraction of light. Refraction is the bending of a wave when it enters a medium where it’s speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell’s Law.

### 168. Alternating current is converted into direct current by a

(1) transformer
(2) dynamo
(3) oscillator
(4) rectifier
168. (4) A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-arc valves, solid-state diodes, silicon-controlled rectifiers and other silicon-based semiconductor switches.

### 169. Which of the following is most elastic?

(1) Rubber
2) Wet clay
(3) Steel
(4) Plastic
169. (3) In physics, elasticity is a physical property of materials which return to their original shape after the stress that caused their deformation is no longer applied. For very small deformations, most elastic materials, such as springs, exhibit linear elasticity. This means that they are characterized by a linear relationship between stress and strain (the relative amount of deformation). To describe elastic properties of linear objects like wires, rods, or columns which are stretched or compressed, a convenient parameter is the ratio of the stress to the strain, a parameter called the “Young’s modulus” or “Modulus of Elasticity” of the material. Young’s modulus can be used to predict the elongation or compression of an object as long as the stress is less than the yield strength of the material. Among rubber, steel, wet clay and plastic; steel has the maximum elasticity ranging between 180-200 gpa.

### 170. When a person walking in bright Sunlight enters a dark room, he is not able to see clearly for a little while because

(1) the eye muscles cannot immediately adjust the focal length of the eye lens.
(2) the retina retains the bright images for sometime and becomes momentarily insensitive.
(3) the iris is unable to contract the pupil immediately.
(4) the iris is unable to dilate the pupil immediately.
170. (4) We need light to see what is around us and to see colour. Light bounces off the objects we look at. These reflect different amounts of light which we see as different colours. Our eyes need light to work. Light entering the eye is collected by the retina and processed by the brain to obtain the pictures that we need to see. Light is an essential part of this process, for example it is difficult to read when light levels are low. Sometimes light can cause problems for our vision. Usually our eyes adjust to the new source of light and we are able to see clearly again after a few seconds. The eye adjusts to the new level of light by making our pupil smaller (constricting). When a person walking in bright light enters a dark room, he is not able to see clearly because the iris is unable to dilate the pupil immediately. This
phenomenon is called as photophobia.

### 171. The swing of a spinning cricket ball in air can be explained on the basis of

(1) Sudden change in wind direction.
(2) Buoyancy of air.
(3) Turbulence caused by wind.
(4) Bernoulli’s theorem.
171. (3) The main theory of swing bowling surrounded turbulent and laminar airflow. Laminar air separates from the surface of the ball earlier than turbulent flow air, so that the separation point moves toward the front of the ball on the laminar side. On the turbulent flow side it remains towards the back; inducing a greater lift force on the turbulent airflow side of the ball. The calculated net lift force is not enough to account for the amount of swing observed: Additional force is provided by the pressure-gradient force.

### 172. Why two thin shirts can keep us warmer than a single thick shirt in winter ?

(1) Two thin shirts become thicker so prevent transmission of heat
(2) Air layer between two shirts works as good conductor
(3) Air layer between two shirts behaves like insulating media
(4) No radiation of heat takes place
172. (3) Air is a good insulator. Therefore, we feel warmer as the heat can’t conduct away as easily. The reason layers work to keep us warm is that they form air pockets in between each layer which is insulated by our body heat. The more pocket of air we have, the more insulation we have, thus the warmer we are.

### 173. Which layer of the earth’s atmosphere reflect back the radio waves to the earth’s surface ?

(1) ionosphere
(2) stratosphere
(3) mesosphere
(4) exosphere
173. (1) Long distance propagation of radio waves depends on an invisible layer of charged particles, which envelops the Earth. This layer of charged particles known as the ionosphere has been in existence for millions of years. For those, who pioneered the long distance radio communication during the early part of the twentieth century, the ionosphere came as a boon.

### 174. Sound cannot pass through

(1) water
(2) steel
(3) air
(4) vacuum
174. (4) Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing. Sound also travels through plasma. Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). Sound cannot travel through a vacuum because it does not provide the medium to propagate.

### 175 In a photocell light energy is converted into

(1) potential energy
(2) chemical energy
(3) heat energy
(4) electrical energy
175. (4) Photocells are scientifically known as photoresistors. A photo-resistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. A photo-resistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron conduct electricity, thereby lowering resistance. Photo-resistors are basically photocells.

### 176. A storm is predicted if atmospheric pressure

(1) rises suddenly
(3) falls suddenly
176. (3) Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, so the level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. In areas of low pressure, air is rising away from the surface of the
earth more quickly than it can be replaced by air flowing in from surrounding areas. This reduces the
weight of air above the reservoir so the mercury level drops to a lower level. A sudden drop in atmospheric pressure over a few hours often forecasts an approaching storm, with heavy rain and strong winds.

### 177. An egg sinks in soft water but floats in a concentrated solution of salt because

(1) egg absorbs salt from the solution and expands
(2) albumin dissolves in salt solution and egg becomes lighter
(3) the density of salt solution exceeds the density of eggs
(4) water has high surface tension
177. (3) Following the density principle, the density of the egg compared to the density of the tap water is lesser (the density principle states that an object or substance with lesser density will float on the surface of the substance or object which has the greater density). Now, with salt, salt makes the density of the tap water to increase and increases also the salinity of the water making the egg float because
the density of the water with salt is now greater than the density of the egg and that is why an egg sinks in soft water but floats in a concentrated solution of salt.

### 178. What should a person on a freely rotating turn table do to decrease his (angular) speed ?

(1) Bring his hands together
(2) Raise his hands up
(4) Sit down with raised hands
178. (3) The answer is related to the conservation of angular momentum. The person on the rotating table will maintain approximately the same angular momentum during the spin. If he moves his arms in, it will reduce the rotational inertia by reducing the distance of the mass of her arms and hands from the axis of rotation. In order to maintain the same angular momentum, angular rotation is increased. However, when he spreads his hands outward, the angular rotation is decreased. The spin becomes slower.

### 179. Which of the following celestial bodies contains abundant quantities of helium-3, a potential source of energy ?

(1) Earth
(2) Moon
(3) Venus
(4) Saturn
179. (2) The Moon and other celestial bodies contain a vast amount of natural resources. One, and in
abundance, is helium-3. It is estimated that forty tons of it can meet twenty five percent of the global
energy demand for one year. This clean, nonradioactive and safe source of energy scarcely exists on Earth. Helium-3 derives from the dismantlement of old and outdated nuclear warheads. Helium-3 is
used within medicine, scientific research, and well logging operations in the oil and gas industry, and
for homeland security. Major space nations such as Russia, China, India, and Japan appear to have
integrated within their space programmes to commercially exploit the Moon for helium-3 in the future.

### 180. Smog is a combination of

(1) Air and water vapours
(2) Water and smoke
(3) Fire and water
(4) smoke and fog