Physics (Part-II) (UPSC)

Total Questions: 60

31. The following item consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this item using the code given below. [1998]

Assertion (A): The temperature of a metal wire rises when an electric current is passed through it.

Reason (R): Collision of metal atoms with each other releases heat energy.

Correct Answer: (a) Both A and R are true and R is the correct explanation of A
Solution:A metallic conductor has a large number of free electrons in it. When a potential difference is applied across the ends of a metallic wire, the free electrons begin to drift from a region of low potential to a region of high potential. These electrons collide with the positive ions (the atoms which have lost their electrons). In these collisions, the energy of the electron is transferred to the positive ions and they begin to vibrate more violently. As a result, heat is produced. The greater the number of electrons flowing per second, the greater will be the rate of collisions and so greater is the heat produced.

32. Consider the following statements: [1999]

An ordinary light bulb has a rather short life because the:

1. filament wire is not uniform.

2. bulb cannot be evacuated completely.

3. wires supporting the filament melt at high temperatures.

Which of the above statements are correct?

Correct Answer: (d) 1, 2 and 3
Solution:The electric light bulb uses a glowing wire filament heated by electrical resistance to white to generate light (a process known as thermal radiation). The 'bulb' is the glass enclosure which keeps the filament in a vacuum or low-pressure noble gas. Complete evacuation of bulb is not possible. A small quantity of gases left behind which expands on heating by heat through filament. Moreover, the wires supporting the filament sometimes melt due to high heat generated as a result of high resistance of filament. Thus, these factors leads to short life span of ordinary bulb.

33. Consider the following statements regardingamotorcar battery: [1999]

1. The voltage is usually 12 V.

2. Electrolyte used is hydrochloric acid.

3. Electrodes are lead and copper.

4. Capacity is expressed in ampere-hour.

Which of the above statements are correct?

Correct Answer: (d) 1 and 4
Solution:In case of motor car battery or automotive batteries a nominal 12-volt potential difference is provided by connecting six galvanic cells in series. Capacity of these batteries is expressed in ampere-hour. Electrolyte used is a solution of about 35% sulphuric acid and 65% water and electrodes used are plates of lead and separate plates of lead dioxide.

34. Fluorescent tubes are fitted with a choke. The choke coil: [2000]

Correct Answer: (d) chokes low frequency current
Solution:Chokes, or inductive coils, in fluorescent tubes limit current flow and provide the high voltage needed to start the lamp. They impede high-frequency current, which creates a voltage spike to initiate discharge. The choke does not step up or step down voltage; instead, it regulates and limits current flow. While it helps control current, its main function is not solely to reduce it. Thus, the most accurate description of the choke's role is to choke low-frequency current, aiding in the creation of the necessary high voltage for lamp ignition.

35. Two wires have their lengths, diameters and resistivities all in the ratio of 1: 2. If the resistance of the thinner wire is 10 ohm, the resistance of the thicker wire is: [2001]

Correct Answer: (b) 10 ohm
Solution:Resistance (R) of a length /, resistivity p and area of cross-section A is given by:

36. The following item consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this item using the code given below. [2004]

Assertion (A): In our houses, the current in A.C. electricity line changes direction 60 times per second.

Reason (R): The frequency of alternating voltage supplied is 50 Hz.

Correct Answer: (d) A is false but R is true
Solution:An alternating current (a.c.) is one which is constantly changing direction. Alternating current have two cycles positive half cycle and negative half cycle. Mains electricity is an a.c. supply which is supplied to homes in India at 220 V AC at 50 Hz. Thus, in our houses the current in A.C. electricity line changes direction = 50 * 2 = 100 times per second.
Alternating Current (AC)
In alternating current, the electric charge flow changes its direction periodically. AC is the most commonly used and most-preferred electric power for household equipment, office, buildings, etc. It was first tested based on the principles of Michael Faraday in 1832 using a Dynamo Electric Generator.
Alternating current can be identified in a waveform called a sine wave. In other words, it can be referred to as a curved line. These curved lines represent electric cycles and are measured per second. The measurement is read as Hertz (Hz). AC is used in powerhouses and buildings because generating and transporting AC across long distances is relatively easy. AC is capable of powering electric motors which are used in refrigerators, washing machines, etc.
Direct Current (DC)
Unlike alternating current, the flow of direct current does not change periodically. The current electricity flows in a single direction in a steady voltage. The major use of DC is to supply power to electrical devices and also to charge batteries. Example: mobile phone batteries, flashlights, flat-screen television and electric vehicles. DC has the combination of a plus and a minus sign, a dotted line or a straight line.
Everything that runs on a battery and uses an AC adapter while plugging into a wall or uses a USB cable for power relies on DC. Examples would be cellphones, electric vehicles, flashlights, flat-screen TVs (AC goes into the TV and is converted into DC).

37. What is the order of magnitude of electric resistance of the human body (dry)? [2005]

Correct Answer: (b) 10^4 ohm
Solution:Human body has different resistance when human body dry then the resistance is 10⁵ ohms, and when human body wet because of water or sweet then the resistance is 1000 ohms, so for dry skin the resistance is 10⁴ ohms. Hence option (b) is correct.
What is Resistance?
Resistance is a material’s tendency to resist or oppose the flow of an electric current. The measure of a material’s resistance is called resistivity, and it quantifies how strongly a material opposes the flow of electric current.
The conductivity of a material is the inverse of its resistivity. In other words, materials with high resistivity have low conductivity and vice versa.
The resistance of a material is affected by temperature. As temperature increases, the atoms in a material vibrate more, which makes it harder for electrons to flow freely through the material. This increase in resistance with temperature is called positive temperature coefficient (PTC).
Some materials have a negative temperature coefficient of resistance (NTC), which means that their resistance decreases with increasing temperature. These materials are used in heat-sensitive devices, such as thermistors.

38. Which one of the following is printed on a commonly used fluorescent tube light? [2006]

Correct Answer: (c) 6500 K
Solution:A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapour. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. 6500 K is usually printed on a used fluorescent tubelight.
Fluorescent lighting
  • A fluorescent lamp is a low pressure mercury-vapor gas-discharge lamp.
  • Electrons that are bound to mercury atoms are excited to states where they will radiate ultraviolet light as they return to a lower energy level.
  • This emitted ultraviolet light is converted into visible light as it strikes the fluorescent coating on the bulb (phosphor coating).

39. Electrically charged particles from space travelling at speeds of several hundred km/sec can severely harm living beings if they reach the surface of the Earth. What prevents them from reaching the surface of the Earth? [2012-1]

Correct Answer: (a) The Earth's magnetic field diverts them towards its poles
Solution:Earth's magnetic field diverts them torwards its poles.
About The Earth’s magnetic field
  • Earth’s magnetic field, or as some might call it, geomagnetic field, is an attractive force that surrounds the world, beginning in its core and extending into space.
  • Created by molten iron and nickel churning away in the outer core through the geodynamo process, it shields Earth from the solar winds and cosmic radiations.
  • The magnetic field is also much responsible for the auroras, these captivating displays of lights near the polar regions caused by charged particles interacting with the atmosphere.
  • It is fundamentally important for navigation, as it guides compasses and influences the migratory behavior of animals like birds and sea turtles. It is dynamic and everlastingly in change, with occasional reversals during which magnetic poles interchange their places.
  • The Earth’s magnetic field must stand for Earth and help in stabilizing the environment, protect technological systems such as satellites, and all in all assist in making Earth liveable. This is what makes Earth’s magnetic field a core of the Earth’s ecosystem.

40. The known forces of nature can be divided into four classes, viz., gravity, electromagnetism, weak nuclear force and strong nuclear force. With reference to them, which one of the following statements is not correct? [2013-1]

Correct Answer: (a) Gravity is the strongest of the four
Solution:Gravity is the feeble force among the four.
Gravitational Force
 The gravitational force is weak but very long-ranged. Furthermore, it is always attractive. It acts between any two pieces of matter in the Universe since mass is its source.
Weak Nuclear Force
The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and. As its name indicates, it is very weak. The weak force causes Beta-decay ie. the conversion of a neutron into a proton, an electron, and an antineutrino.
Electromagnetic Force
The electromagnetic force causes electric and magnetic effects such as the repulsion between electrical charges or the interaction of bar magnets. It is long-ranged but much weaker than the strong force. It can be attractive or repulsive and acts only between pieces of matter carrying an electrical charge. Electricity, magnetism, and light are all produced by this force.
Strong Nuclear Force
 The strong interaction is very strong but very short-ranged. It is responsible for holding the nuclei of atoms together. It is basically attractive but can be effectively repulsive in some circumstances. The strong force is ‘carried’ by particles called gluons; that is, when two particles interact through the strong force, they do so by exchanging gluons. Thus, the quarks inside of the protons and neutrons are bound together by the exchange of the strong nuclear force.