# Physics GK Quiz-20

## Physics GK Quiz-20

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

### 381. A device which is used to limit the current in an electrical circuit is called a

(1) Grid
(2) Fuse
(3) Hub
(4) Conductor
381. (2) A fuse places a limit on the amount of current that can be drawn by an electric circuit by opening (blowing or melting) when the current exceeds a preset limit. This protects the circuit and the surroundings from fire or damage in the case of an overload or short circuit.

### 382. Hypermetropia or longsight can be corrected by using :

(1) Bifocal lenses
(2) Cylindrical lenses
(3) Concave lenses
(4) Convex lenses
382. (4) It can be corrected with convex lenses in eyeglasses or contact lenses. Convex lenses have a positive dioptric value, which causes the light to focus closer than its normal range.

### 383. Stars appear twinkling because of ......... of light.

(1) Reflection
(2) Scattering
(3) Emission
(4) Absorption
383. (2) The stars’ light must travel through the Earth’s multi-layered atmosphere, and as it does so it is
refracted (bent) multiple times and in random directions. The phenomenon of random refraction
appears to make the star wink as though it is in motion. It is this action that our eyes translate as
twinkling. When the light itself enters the air – or atmosphere – it is scattered, with the scattering
depending on the air temperature with warm air causing less bends and colder air causing more.

### 384. Speed of sound in air is unaffected by change in :

(1) Pressure
(2) Humidity
(3) Temperature
(4) Volume
384. (1) The speed of sound in air depends on the density of air and the density of air depends on the
temperature. The speed of sound is unaffected by pressure. If the atmospheric pressure changes, the
speed of sound in air remains constant.

### 385. The audio signals of TV are :

(1) Amplitude modulated
(2) Frequency modulated
(3) Unmodulated
(4) Velocity modulated
385. (2) In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analogue television, the sound portion of a broadcast is invariably modulated separately from the video. Most commonly, the audio and video are combined at the transmitter before being presented to the antenna, but in some cases separate aural and visual antennas can be used.

### 386. In mirrors the back surface is coated with a thin layer of :

(1) Mercury
(2) Silver
(3) Red oxide
(4) Silver nitrate
386. (2) In a spherical mirror, one of the two curved surfaces is coated with a thin layer of silver followed by a coating of red lead oxide paint. Thus, one side of the spherical mirror is opaque and the other side is a highly polished reflecting surface.

### 387. A bomb at rest explodes into a large number of tiny fragments. The total momentum of all the fragments

(1) is zero
(2) depends on the total mass of all the fragments
(3) depends on the speeds of various fragments
(4) is infinity
387. (1) In an explosion, an internal impulse acts in order to propel the parts of a system (often a single object) into a variety of directions. After the explosion, the individual parts of the system (that is often a collection of fragments from the original object) have momentum. If the vector sum of all individual parts of the system could be added together to determine the total momentum after the explosion, then it should be the same as the total momentum before the explosion. Just like in collisions, total system momentum is conserved.

### 388. An optically plane surface reflects a beam of light

(1) as a parallel beam in one direction
(2) as diffused beams in all directions
(3) as parallel beams in all directions
(4) as a diffused beam in one direction
388. (1) An optically plane surface reflects a beam of light as a parallel beam in one direction. The set of incident rays are reflected as a set of rays. In other words, a beam of light is reflected as a beam of light if the mirror is plane and its reflecting surface is smooth. This type of reflection of a set of rays is called Specular Reflection.

### 389. The slope of a velocity-time graph represents

(1) acceleration
(2) displacement
(3) distance
(4) speed
389. (1) A plot of velocity-time is used to determine the acceleration of an object (the slope). If the acceleration is zero, then the velocity-time graph is a horizontal line (i.e., the slope is zero). If the acceleration is positive, then the line is an upward sloping line (i.e., the slope is positive). If the acceleration is negative, then the velocity-time graph is a downward sloping line (i.e., the slope is negative).

### 390. The earth-wire of a cable is connected to

(1) the outer metallic body of the appliance
(2) the fuse of the appliance
(3) the filament of the appliance
(4) short circuitry of the appliance
390. (2) Earth wire prevents Electric shocks. It is connected to the electrical cable of an appliance to prevent electric shocks. Actually it has very low resistance and allows current to pass through it easily.

### 391. Water is used in car radiator because of its

(1) low density
(2) easy availability
(3) high specific heat capacity
(4) low boiling point
391. (3) Water is used in car radiators because the Specific heat capacity of water is high as a result of which it takes away the heat from engine better compared to other liquids. This is a form of coolant used in the radiator. If the radiator was to overheat the car would come to a stop.

### 392. How much mechanical work must be done to completely melt 1 gram of ice at 0°C ?

(1) 4.2 J
(2) 80 J
(3) 336 J
(4) 2268 J
392. (2) When water at 0 degree Celsius freezes, each gram of water gives off 80 calories. The temperature of the water stays at 0 degree C until every drop has frozen. When ice at 0 degree Celsius melts, it absorbs an equal amount of heat—80 calories for each gram of ice—and its temperature remains unchanged until it has completely melted.

### 393. A bullet is fired from a rifle which recoils after firing. The ratio of kinetic energy of the rifle to that of the bullet is

(1) zero
(2) one
(3) less than one
(4) more than one
393. (1) The rifle and bullet have zero momentum and zero kinetic energy to start with. When the trigger is pulled, the bullet gains some momentum in the forward direction, but this is canceled by the rifle’s backward momentum, so the total momentum is still zero. The kinetic energies of the gun and bullet are both positive scalars, however, and do not cancel. The total kinetic energy is allowed to increase, because kinetic energy is being traded for other forms of energy. Some of the initial chemical energy of the gun powder, for example, is transformed into thermal energy and sound energy. From the relationship between kinetic energy and momentum, we know that if a massive particle and a light particle have the same momentum, the light one will have a lot more kinetic energy. If a light particle and a heavy one have the same velocity, the heavy one has more kinetic energy. Since the momentum of the gun and the bullet is same, the bullet which has a lower mass has a higher kinetic energy.

### 394. Heat stored in water vapour is

(1) specific heat
(2) latent heat
(3) absolute heat
(4) relative heat
394. (2) Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. So water vapor which is the end product of evaporated water has latent heat.

### 395. The layer of atmosphere used for radio-wave transmission is

(1) chromosphere
(2) troposphere
(3) ionosphere
(4) stratosphere
395. (3) The ionosphere is a part of the upper atmosphere and is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth.

### 396. Negative feedback in amplifiers

(1) increases bandwidth and decreases noise
(2) decreases bandwidth and decreases noise
(3) increases bandwidth and increases noise
(4) decreases bandwidth and increases noise
396. (1) Voltage gain, input impedance, output impedance, bandwidth etc. are few important characteristics of an amplifier. These parameters are more or less constant for a given amplifier. Quite often, the values of these parameters are required to be controlled. This can be conveniently achieved by employing a technique, called the feedback. Some of the advantages of negative feedback include: gain stability; reduced non-linear distortion, reduced noise, increased bandwidth, increased input impedance and reduced output impedance.

### 397. Water cannot be used to extinguish fire caused by electric current, because

(1) it may cause electrocution
(2) it may cause hydrolysis
(3) it may cause electrolysis
(4) it may spoil the wiring
397. (1) Using water for putting out electric fires is risky, as the firemen can be electrocuted by the water jet. Water is a good conductor of electricity, and, the electric current flows through the water-jet directly into the hands of the person who is holding the waterhose, resulting in immediate electrocution.

### 398. A periscope works on the principle of

(1) refraction
(2) total internal reflection
(3) diffraction
(4) reflection
398. (2) Total internal reflection (TIR) is the phenomenon that involves the reflection of the entire incident light off the boundary. Glass prisms can be shaped to produce total internal reflection and as such are employed in binoculars, periscopes, telescopes, and other optical instruments. For the prism used in periscope, the light rays hit the inside surface at an angle greater than the critical angle of 42 and hence they are internally reflected.

### 399. A copper disc has a hole. If the disc is heated, the size of the hole

(1) increases
(2) decreases
(3) No change
(4) first increases and than decreases
399. (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 (sum of two radii) of the circular hole also increases.

(1) plain
(2) concave
(3) convex
(4) inverted