# Physics GK Quiz-40

## Physics GK Quiz-40

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

### 781. The point where total mass of a body is supposed to be concentrated is known as

(2) Centre of mass
(3) Centre of gravity
(4) Centre of motion
781. (2) The centre of mass of a body or a system of particles is defined as a single point at which the whole mass of the body or system is imagined to be concentrated and all the applied forces acts at that point. It is that point that moves when forces are applied on the body.

### 782. Where do the electrical charges reside in a charged conductor?

(1) at the core
(2) throughout the body
(3) depended upon the nature of the body
(4) outer surface of the body
782. (4) In all conductors, charges reside on the surface. The reason for this is that conductors have free electrons, that is, the electrons are loosely attached to the nucleus of the atoms in the conductors. When a solid conductor in equilibrium carries a net charge, the charge resides on the outer surface of the conductor. Besides, the electric field just outside the conductor is perpendicular to the surface and that
the field inside is zero.

### 783. The colour of the ocean appears to be blue because the sunlight falling on it is

(1) reflected
(2) refracted
(3) diffracted
(4) scattered
783. (4) The reason the ocean is blue is due to the absorption and scattering of light. The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue, thus red light is absorbed quickly in the ocean leaving blue. The red, yellow, and green wavelengths of sunlight are absorbed by water molecules in the ocean.

### 784. Which of the following is not true about X-rays?

(1) have low penetrating power
(2) travel with the speed of light
(3) Can be reflected or refracted
(4) can affect photographic plates
784. (1) X-rays and gamma rays are photons, i.e. highenergy light-waves. When emitted by a source, for example, radium or cobalt, located outside the body, they easily pass through the body, hence they are usually called penetrating radiation. Because X-rays are penetrating, they can be used in diagnostic medicine to image human bones or human organs made opaque by a dye.

### 785. What type of force acts on a car moving around a curve?

(1) Centrifugal force
(2) Cohesive force
(3) Centripetal force
(4) Gravitational force
785. (3) Any object moving in a circle (or along a circular path) experiences a centripetal force. It is this physical force that pushes or pulls the object towards the center of the circle. In the case of a car moving along a curve, as it makes a turn, the force of friction acting upon the turned wheels of the car provides centripetal force required for circular motion. The net force on a car travelling around a curve is the centripetal force, Fc = m v2 / r, directed toward the center of the curve.

### 786. Which phenomenon is responsible for the echo of sound wave?

(1) Reflection
(2) Refraction
(3) Interference
(4) Polarisation
786. (1) Reflection of sound waves off of surfaces lead to one of two phenomena - an echo or a reverberation.The echo is produced due to hitting of the sound waves with the obstacles which makes the sound to reflect back.Echoes occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard.

### 787. Which of the metals has the maximum thermal conductivity?

(1) Iron
(2) Aluminium
(3) Silver
(4) Copper
787. (3) Silver has the best thermal conductivity of 429 W/mK at RT (295K). It is followed by copper, gold and aluminium. Metals generally have very good electrical conductivity, that leads to high thermal conductivity. There is a rule governing the relation between electrical conductivity and thermal conductivity, called Wiedemann-Franz Law.

### 788. Which of the following is the cleanest source of energy?

(1) Biofuel
(2) Fossil fuel
(3) Nuclear power
(4) Wind energy
788. (4) Solar, wind and hydro power are examples of cleanest energy sources. According to a new US study, Greenhouse gases produced over the lifetime of a wind turbine are less than that of fossil-fuel based energy sources and most other renewables. Only ocean energy (wave and tidal) and hydropower have lower emissions than wind. As a clean, fuel-free source of energy, wind turbines create energy without generating the damaging pollutants.

### 789. Which one of the following is an ohmic conductor?

(1) Germanium
(2) Silicon
(3) Carbon
(4) Silver
789. (4) Ohmic conductors are conductors that obey Ohm’s law, that is Voltage/Current ratio is constant; in other words, the equation Resistance = Voltage/Current is obeyed. An Ohmic conductor’s resistance does not change with varying current. Metallic conductors such as silver are Ohmic conductors.

### 790. The spring balance works on the principle of __________ .

(1) Boyle’s Law
(2) Hooke’s Law
(3) Bernoulli’s principle
(4) Pascal’s Law
790. (2) A spring scale or spring balance or Newton meter is a type of weighing scale. It consists of spring fixed at one end with a hook to attach an object at the other. It works by Hooke’s Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position.

### 791. If there were no gravity, which of the following will not be there for a fluid?

(1) Viscosity
(2) Surface Tension
(3) Pressure
(4) Upward Thrust
791. (4) If there is no gravity, there will not be any upward thrust to keep a liquid buoyant. Whenever a body is placed in a fluid, the fluid applies resultant up ward directional force on the body and it is called up thrust. It’s numerical value which is the product of volume of the fluid displaced, density of the fluid and acceleration due to gravity at the given place. Up thrust increases with increase in acceleration due to gravity and vice versa.

### 792. Sun appears red in colour at sunrise and sunset due to

(1) the fact that sun emits only red colour at that time
(2) red light having longer wave length scatters away
(3) that sun comes out of the mountains
(4) that all other colours scatter away except red
792. (4) According to Rayleigh’s law, the intensity of scattered light varies inversely as the fourth power of its wavelength. Sunlight consists of seven colours. Of these, red has the maximum wavelength. During sunrise and sunset, the rays have to travel a larger part of the atmosphere because they are very close to the horizon. Therefore, light other than red is mostly scattered away. Most of the red light, which is the least scattered, enters our eyes. Hence, the sun and the sky appear red.

### 793. At hill stations, the boiling point of water will be

(1) same as at sea level
(2) less than that at sea level
(3) more than that at sea level
(4) equal to the melting point of ice
793. (2) The boiling point is the temperature at which the vapour pressure of the liquid equals the environmental pressure surrounding the liquid. Atmospheric pressure is due to air above any given point. The atmospheric pressure at high altitudes like hill station is less than at the sea level.Thus, vapour pressure will equal atmospheric pressure at a comparatively low temperature. Thus, the boiling point of water is reduced less than at sea level.

### 794. When we see an object, the image formed on the retina is

(1) Real and inverted
(2) Real and erect
(3) Virtual and erect
(4) Virtual and inverted
794. (1) The retina is the thin light-sensitive membrane lining the inner eyeball-towards the back of the eye. As our eye lens are convex in nature, they form mostly real and inverted images on the retina. When the images formed are real and inverted, the message sent by the optic nerve are chemical impulses which turn them into erect images that we see.

### 795. Kinetic energy depends on

(1) the velocity or speed of the moving body.
(2) the mass of the moving body
(3) the pressure of the moving body
(4) both mass and velocity of the moving body
795. (4) Kinetic energy is the energy associated with the movement of objects. The amount of kinetic energy that an object has depends upon two variables: the mass (m) of the object and the speed (v) of the object.

### 796. In which form is the supplied heat energy stored during change in temperature of substance?

(1) Heat energy
(2) Kinetic energy
(3) Potential energy
(4) Both kinetic and potential energy
796. (2) Temperature is used as a measure for heat in an object by measuring the amount of kinetic energy in the molecules that make up the object. It is a measure of the average kinetic energy of the random molecular motion. Heat is a form of energy that can be transferred from the heat source to the molecules, and it can also change its form from heat to movement. The molecules that gain extra energy will have a higher tendency to move more than the molecules that have not gained the extra energy. Energy of motion is called kinetic energy. When the molecules move or vibrate more, they have a higher kinetic energy, and this is recorded as an increase in temperature. In nutshell, as a body gains or loses kinetic energy its temperature will increase or decrease.

### 797. If ice floating on water in a vessel melts, the water level in the vessel _______.

(1) increases
(2) does not change
(3) first increases before decreasing
(4) decreases
797. (2) When ice floating on water in a vessel melts, the water level in the vessel does not change. This is because when floating, the ice displaces an amount of water equal to its mass, and when melted, it becomes an amount of water equal to its mass. It is accordance with Archimedes Principle, which states
that the mass of the liquid displaced by a floating object is equivalent to the mass of the object.

### 798. A galvanometer can be converted to a voltmeter by connecting

(1) a high resistance in parallel
(2) a high resistance in series
(3) a low resistance in series
(4) a low resistance in parallel
798. (2) In order to convert a Galvanometer into voltmeter, a very high resistance known as “series resistance” is connected in series with the galvanometer. The conversion is made for enabling the galvanometer to measure the potential difference across any component in a circuit.

### 799. The frequency of direct current is _______

(1) Zero
(2) 50 HZ
(3) 60 HZ
(4) 100 HZ
799. (1) Direct Current (DC) refers to power systems that use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly varying local mean value of a voltage or current. It is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, power supplies, thermocouples, solar cells, or dynamos.

### 800. For photoelectric effect to take place, the metal used as the cathode should have______

(1) high melting point
(2) low melting point
(3) low work function
(4) low resistance