Electric current - part (1)

Electric current – part (1)

Total Questions: 50

11. Which of the following metals is used as filaments in lighting bulbs ? [44th B.P.S.C. (PRE) 2000]

(a) iron
(b) molybdenum
(c) silver
(d) tungsten

Correct Answer: (d) tungsten

Solution:The metal used as filaments in lighting bulbs, specifically in incandescent light bulbs, is tungsten. Here some reasons for using it:

High Melting Point: Tungsten has the highest melting point of all metals, around $3422^{\circ} C$ ( $6191.6^{\circ} F$ ). This is crucial because the filament in a light bulb needs to heat up to very high temperatures (around $250 0^{\circ} C$ ) to glow and produce light without melting or vaporizing quickly.

Low Vapor Pressure: Even at high operating temperatures, tungsten has a low rate of evaporation. This property ensures that the filament lasts for a reasonable amount of time before thinning and breaking.

Durability: Tungsten can withstand the thermal and mechanical stresses that occur during the on-off cycles of a light bulb.

Cost-Effective: While other materials might have some of these properties, tungsten offers a good balance of performance and cost for mass production of incandescent light bulbs.

12. The filament of an electric bulb is made of - [U.P.P.C.S.(pre) 1990,2005,2011]

(a) magnesium
(b) iron
(c) nichrome
(d) tungsten

Correct Answer: (d) tungsten

Solution:Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first isolated as a metal in 1783.Its important ores include scheelite and wolframite, the latter lending the element its alternative name.

Tungsten has the highest melting point of all metals, around $3422^{\circ} C$ ( $6191.6^{\circ} F$ ). This is crucial because the filament in a light bulb needs to heat up to very high temperatures (around $250 0^{\circ} C$ ) to glow and produce light without melting or vaporizing quickly.

13. Filament of electric bulb is made of : [Uttarakhand P.C.S. (PRE) 2012]

(a) tungsten
(b) nichrome
(c) lead
(d) aluminium

Correct Answer: (a) tungsten

Solution:Tungsten metal is used as filament in lighting bulbs in 1781 Carl Wilhelm Scheele and Torbern Bergman suggested that is might be possible to obtained a new metal by reducing tungsten acid two year later in 1783 juan jose and fausto elhuyer were the first to isolate tungsten through the reduction of this acid with charcol for this reason they are credited with the discovery of this element its melting point is 3422 degree celsius and its boiling point is 5555 degree celsius.

14. Consider the following statements : [I.A.S. (PRE) 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 statement 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:An ordinary light bulb has a rather short life because the :-
(1) filament wire is not uniform (2) bulb cannot be evacuated completely and (3) wires supporting the filament melt at high temperature.

1.Filament wire is not uniform: While there might be minor variations in the thickness of the filament wire, even slight imperfections can lead to "hot spots" where the resistance is higher, causing localized overheating and faster evaporation. So, non-uniformity can contribute to a shorter life.

2. Bulb cannot be evacuated completely: Achieving a perfect vacuum is practically impossible. Even trace amounts of residual gas can contribute to filament degradation through chemical reactions and increased heat loss. This is a significant factor in limiting the bulb's lifespan.

3.Wires supporting the filament melt at high temperatures: Moreover the wires supporting the filament sometimes melt due to high heat generated as a result of high resistance of filament.

15. The temperature of the filament of lighted electric bulb is generally : [R.A.S./R.T.S.(PRE)2007]

(a) 100°C to 500°C
(b) 1000°C to 1500°C
(c) 2000°C to 2500°C
(d) 3000°C to 3500°C

Solution:Tungsten metal is used as filament in electric bulb. when electric current flow through it, its temperature goes up to 1500°C to 2500°C. Generally electric bulbs convert only 5 percent to 10 percent of electric energy into light.

16. Lightning can even burn a tree because it contains tremendous amount of: [M.P.P.C.S.(PRE) 2017]

(a) heat energy
(b) electric energy
(c) chemical energy
(d) nuclear energy

Correct Answer: (b) electric energy

Solution:Lightning energy can even burn the tree because it contains tremendous amount of electrical energy. It's a sudden flow of electric current between electrically charged regions, typically within a cloud, between clouds, or between a cloud and the ground.

When this massive electrical energy flows through a tree (which contains water and conductive materials), it causes rapid heating due to the resistance of the tree. This intense heat can vaporize the water inside the tree, leading to a steam explosion that can split the trunk, shatter branches, and even ignite the tree.

17. Consider the following statement: [Uttarakhand P.C.S. (mains) 2002, U.P.P.C.S.(pre) 1999]

Assertion (A) : lightning conductor prevent building from the damage in the event of lightning strike.
Reason (R) :the electric charge conduct to ground through the wire instead of passing through the structure.
select the correct answer using the codes given below :

(a) both (A) and (R) are correct and (R) is the correct explanation of (A).
(b) both (A) and (R) are correct and (R) is not the correct explanation of (A).
(c) (A) is the correct but (R) is wrong.
(d) (A) is the wrong but (R) is correct.

Correct Answer: (a) both (A) and (R) are correct and (R) is the correct explanation of (A).

Solution:A lighting conductor is a metal rod mounted on the top of an elevated structure electrically bonded using a wire or electrical conductor to interface with the ground through an electrode to protect the structure in the event of lighting strike.

If the lightning hits the structure it will preferentially strike the rod and would be conducted to ground through wire instead of passing through the structure hence the building will be saved by using this lightning conductor.

18. In a three pin electrical plug longest pin should be connected to - [U.P.P.C.S.(pre) 2007]

(a) ground terminal.
(b) live terminal.
(c) neutral terminal.
(d) any terminal.

Correct Answer: (a) ground terminal.

Solution:In the three pin plug there is the ends namely live, neutral and earth the earth pin on a plug is longer than the line or neutral pins. This means the earth pin is the first to connect and last to disconnect for safety of electrical device, it provide an alternative and easy path for leakage or fault current flow.

19. In a electric apparatus earth is used - [U.P.P.C.S.(pre) 2002]

(a) to reduce the expenditure.
(b) because the apparatus used 3-phase.
(c) for safety.
(d) in the form of fuse.

Correct Answer: (c) for safety.

20. The electric current does not flow the between two properly connected charged bodies if they are having: [U.P.P.S.C. (R.I.) 2014]

(a) same charge.
(b) same capacity.
(c) same resistivity.
(d) same potential.

Correct Answer: (d) same potential.

Solution:According to ohm's law if there is a potential difference (V) across a resistor then there is a current (I) flowing through it, current flow in a circuit as a result of difference in potential between two points in the circuit .

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Electric current – part (1)