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.

(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

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?

(a) 1 and 3
(b) 2 and 3
(c) 1 and 2
(d) 1, 2 and 3

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?

(a) 1 and 2
(b) 2 and 3
(c) 3 and 4
(d) 1 and 4

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]

(a) steps up the line voltage
(b) steps-down the line voltage
(c) reduces current in the circuit
(d) chokes low frequency current

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]

(a) 5 ohm
(b) 10 ohm
(c) 20 ohm
(d) 40 ohm

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.

(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R are individually true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

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.

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

(a) 10^2 ohm
(b) 10^4 ohm
(c) 10^6 ohm
(d) 10^8 ohm

Correct Answer: (b) 10^4 ohm

Solution:Human body has different resistance when human body dry then the resistance is 10 ^ 5 ohms, and when human body wet because of water or sweet then the resistance is 1000 ohms, so for dry skin the resistance is 10 ^ 4 ohms. Hence option (b) is correct.

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

(a) 220 K
(b) 273 K
(c) 6500 K
(d) 9000 K

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.

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]

(a) The Earth's magnetic field diverts them towards its poles
(b) Ozone layer around the Earth reflects them back to outer space
(c) Moisture in the upper layers of atmosphere prevents them from reaching the surface of the Earth
(d) None of the statements (a), (b) and (c) given above is correct

Correct Answer: (a) The Earth's magnetic field diverts them towards its poles

Solution:Earth's magnetic field diverts them torwards its poles.

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]

(a) Gravity is the strongest of the four
(b) Electromagnetism acts only on particles with an electric charge
(c) Weak nuclear force causes radioactivity
(d) Strong nuclear force holds protons and neutrons inside the nucleus of an atom

Correct Answer: (a) Gravity is the strongest of the four

Solution:Gravity is the feeble force among the four.

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