# Physics GK Quiz-15

## Physics GK Quiz-15

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

### 281. A piece of wood is held under water. The upthrust on it will be:

(1) equal to the weight of the wood
(2) less than weight of the wood
(3) more than weight of the wood
(4) Zero
281. (2) According to the Archimedes’ principle, a floating object will experience an upthrust force from water, equal to the weight of water displaced (pushed aside). It will sink into the water until it reaches the point where the weight of the water pushed aside equals its own weight. For an object that is floating, the mass of the material equals the mass of water that is displaced by the object (1 kg = 1 L of water). Dense objects cannot displace enough water to provide an upthrust force to counterbalance their weight, so they plummet below the surface. Objects made of material denser than water (e.g. a boat made of iron) can still float if they contain air so that the mean density is less than that of water. The upthrust is often called the loss in weight of the object. Upthrust = apparent loss of weight of object = weight in air - weight in liquid.

### 282. Persistance of vision is the principle behind

(1) Camera
(2) Spectroscope
(3) Cinema
(4) Periscope
282. (3) Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina. Persistence of vision is still the accepted term for this phenomenon in the realm of cinema history and theory. In the early days of film innovation, it was scientifically determined that a frame rate of less than 16 frames per second (frame/s) caused the mind to see flashing images.

### 283. In a nuclear reactor, one of the following is used as a fuel.

(1) Coal
(2) Uranium
(4) Diesel
283. (2) Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 has the distinction of being the only naturally occurring fissile isotope. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology.

### 284. The density of the liquid when heated

(1) decreases
(2) increases
(3) does not change
(4) may increase of decrease depending on pressure
284. (1) The mass density of a material varies with temperature and pressure. Increasing the temperature of a substance (with some exceptions) decreases its density by increasing the volume of that substance. In most materials, heating the bottom of a fluid results in convection of the heat from bottom to top of the fluid due to the decrease of the density of the heated fluid.

### 285. The distance between node and adjacent antinode is 30 cm. The wavelength is

(1) 30 cm.
(2) 90 cm.
(3) 120 cm.
(4) 60 cm.
285. (4) A node is a point along a standing wave where the wave has minimal amplitude. The opposite of a node is an anti-node, a point where the amplitude of the standing wave is a maximum. The distance between a node and anti-node is 1/2 of a wavelength; since a wavelength is from one node to another (or any point to its next “identical” point). So if w is the wavelength, then 1 2 = 30. So, w = 30 × 2 = 60 cm

### 286. The nature of fuse wire is

(1) high resistance and low melting point.
(2) high resistance and high melting point.
(3) low resistance and high melting point.
(4) low resistance and low melting point.
286. (1) A fuse or fusable link works by opening an electrical circuit when the current becomes too high. As the current increases, the temperature of the wire increases. So a fuse wire should have a low melting point and should be connected in series with the appliance. Besides, it should have a comparatively high resistance.

### 287. In a railway track, two rails are joined end to end with a gap in between them because

(1) steel can be saved
(2) accidents due to contraction in winter can be avoided
(3) air-gaps are necessary for bearing the weight of running train
(4) accidents due to expansion in summer can be avoided.
287. (4) The volume of materials changes depending on current temperature. Usually heat makes them expand, and cold leads them to contract. There have to be gaps to avoid strain on the tracks. The same applies in many other fields of engineering, i.e. bridge building.

### 288. Vehicle tyres are inflated properly

(1) to ensure smooth running.
(2) to allow the vehicle to take more load.
(3) to avoid skidding and to minimise friction
(4) to go fast and save fuel.
288. (3) Under-inflated tyres create more friction with the road, increasing fuel consumption by up to ten per cent and are prone to skidding on either wet or slippery surfaces. Low pressures also lead to premature wear and risk of tyre blowout or bulging.

### 289. Refrigeration is a process which

(1) kills bacteria
(2) slows down the bacterial growth
(3) Inactivates the bacteria
(4) Plasmolyses the bacteria
289. (2) Bacteria thrive in warm, moist conditions. Refrigeration allows the temperature to be lowered
so far that food can be stored for days or even months. Keeping food cold (at or below 4°C) slows down bacterial growth which helps reduce the risk of foodborne illness.

### 290. Air coolers are more suitable for

(1) hot and humid climate
(2) hot and dry climate
(3) cool and humid climate
(4) cool and dry climate
290. (2) An air cooler works on the simple principle of evaporation. It does not yield good results when
humidity is high. On the contrary, the temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation), which can cool air using much less energy than refrigeration.

### 291. A transformer

(1) converts DC to AC
(2) is used to decrease or increase AC voltage
(3) converts AC voltage to D.C. voltage
(4) converts electrical energy into mechanical energy
291. (2) Transformers have made long-distance transmission of electric power a practical reality, as AC voltage can be “stepped up” and current “stepped down” for reduced wire resistance power losses along power lines connecting generating stations with loads. At either end (both the generator and at the loads), voltage levels are reduced by transformers for safer operation and less expensive equipment.

### 292. When hot-water is sprinkled on a hotter glass tumbler, it breaks because

(1) glass suddenly expands
(2) glass suddenly contracts
(3) water evaporates
(4) glass reacts chemically with water
292. (2) If the poured liquid is relatively hotter or colder in comparison to the tumbler, it will break. When hot water is poured into a tumbler there is an uneven increase in the thermal expansion of the walls of the vessel. If the expansion is uneven enough and the strength of the glass is not strong enough, the tumbler breaks. However, if the poured water is colder in comparison to the tumbler, it breaks due to contraction and unequal temperature difference between the layers.

### 293. Electricity is transmitted over long distances

(1) it is fast
(2) there is less wastage of energy
(3) it is safer
(4) it is cheaper
293. (2) Long distance means higher wastage of energy. So High-voltage direct-current (HVDC) technology is used for greater efficiency in very long distances. Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in long-distance transmission.

### 294. The material used in the fabrication of a transistor is

(1) Aluminium
(2) Copper
(3) Silicon
(4) Silver
294. (3) A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. The first transistors were made from germanium (Ge). Silicon (Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor material gallium arsenide (GaAs) and the semiconductor alloy silicon germanium (SiGe).

### 295. If there is a circular hole in a metal plate, what happens to the radius of the hole when the plate is heated?

(1) Increases
(2) Decreases
(3) Unchanged
(4) Depends upon the metal
295. (1) When a metal plate with a circular hole at its centre is heated, definitely along with the areal expansion of the plate the diameter of the circular hole also increases. Using the differential equations of coefficients of expansions, we find that the hole radius increases at the same linear rate as the metal.

### 296. The velocity of sound in air

(1) decreases with increase of temperature
(2) increases with decrease of temperature
(3) does not depend on temperature
(4) decreases with decrease of temperature
296. (4) Assuming air to be an ideal gas, the speed of sound c depends on temperature only, not on the pressure or density. As air temperature increases, so does the speed of sound and vice versa. This is due to the individual air particles having extra kinetic energy at higher temperatures and lesser at lower temperatures.

### 297. Electric current in a metal wire is due to the flow of—

(1) Electrons
(2) Protons
(3) Ions
(4) Holes
297. (1) A solid conductive metal contains mobile, or free electrons, originating in the conduction electrons. When a metal wire is connected across the two terminals of a DC voltage source such as a battery, the source places an electric field across the conductor. The moment contact is made, the free
electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the charge carrier in a typical solid conductor.

### 298. The device which converts sound energy into electrical energy is called

(1) Amplifier
(2) Speaker
(3) Microphone
(4) Transmitter
298. (3) A microphone converts sound energy to electrical energy this energy level is used as an output using same energy level amplified or it could be used to record those same energy output and pattern too be played again. Sound waves strike a plastic or thin metal diaphragm, causing in to move an attached coil within a strong magnetic field. The induced current duplicates the frequency of the sound wave.

### 299. The light rays used for eliminating bacteria

(1) X-rays
(2) infrared rays
299. (4) Ultraviolet radiation is used to kill microorganisms, molds and fungus in various environmental applications. UV sterilization is used for air-purification systems, water purification, aquarium and pond maintenance, laboratory hygiene and food and beverage protection.

### 300. Mud houses are cooler in summers and warmer in winters as compared to brick houses because

(1) mud is a good conductor
(2) mud is a bad conductor
(3) mud is a poor insulator
(4) evaporation of water causes cooling in summers and sunlight coming through holes causes warming in winters