Electric current - part (2)

Electric current – part (2)

Total Questions: 39

1. With reference to street-lighting, how do sodium lamps differ from LED lamps? [I.A.S. (PRE) 2021]

1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.

2. As street-lights, sodium lamps have longer life span than LED lamps.

3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.

Select the correct answer using the code given below.

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

Correct Answer: (c) 1 and 3 only

Solution:Lights from sodium lamps ie. sodium vapour lights are omnidirectional. Omnidirectional lights produce light in 360 degrees. This is a large system inefficiency because at least half of the light needs to be reflected and redirected to the desired area being illuminated.

Whereas LED lamps emit light directionally i.e. over 180 degrees which means there are far fewer losses from the need to redirect or reflect light. Hence, statement 1 is correct.

LEDs have an extremely long lifespan relative to every other lighting technology (including sodium lamps). New LEDs can last 50,000 to 100,000 hours or more more than 3-4 times than sodium lamps. Hence, statement 2 is incorrect.

Sodium vapour lights have the worst colour rendering. Low pressure sodium lamps in particular are monochromatic resulting in the colours of illuminated objects being nearly indistinguishable, which means that the objects illuminated by them appear shadowy black rather than the colour you would see during daylight.

LEDs waste much less energy producing waste heat and they also provide an incredibly better variety of high colour rendering index options to the user (thus eliminating the monochromatic black appearance of objects illuminated by sodium lamps). Hence, statement 3 is correct.

2. What is the difference between a CFL and an LED Lamp? [I.A.S. (Pre) 2011]

1. To produce light, a CFL uses mercury vapour and phosphor while an material. LED lamp uses semi-conductor.

2. The average life span of a CFL is much longer than of an LED lamp.

3. A CFL is less energy-efficient as compared to an LED lamp.

Which of the statements given above is/are correct:

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

Correct Answer: (c) 1 and 3

Solution:CFL uses mercury vapour and phosphor to produce light The gases becomes excited by electricity and when combined with mercury particles, produces invisible ultraviolet light. The UV light then hits the white phosphor coating inside the bulb causing it to fluorescence and emit white light.

LED lamp uses semi-conductor material to produce light. CFLS have a life of about 6000 to 10,000 hours, while LED has a life span of about 50,000 hours. LED lamps are more energy efficient as compared to CFL.

3. What gases are filled in fluorescent tube? [U.P.P.C.S. (Pre) 1990]

(a) Neon
(b) Sodium
(c) Mercury
(d) Mercury and neon

Correct Answer: (d) Mercury and neon

Solution:Fluorescent lamps are filled with the low pressure gases specially Mercury and noble gases like Argon, Neon, Xenon and Krypton.

4. In comparison to an electric bulb, a fluorescent tube is preferred because - [U.P.P.C.S.(pre) 2009]

(a) it has greater external outer surface to radiate emit the light.
(b) there is no effect of voltage fluctuation on it.
(c) in the tube electric energy almost converted into light energy.
(d) none of these.

Correct Answer: (d) none of these.

Solution:In comparison to electric bulb, fluorescent tubes transform more electric energy into light energy. A bulb of 100 watt converts only 5 percent of electric energy into light energy but fluorescent tubes convert approximately 22% of electric energy into light energy. Thus, none of the statement is true.

5. How does the white light produce in the tube? [47th B.P.S.C. (PRE) 2005]

(a) By heating up the copper wire.
(b) By heating the up the filament.
(c) By accelerating the atoms
(d) By oscillating the molecules.

Correct Answer: (d) By oscillating the molecules.

Solution:A tube light or fluorescent lamp is a kind of light that uses a combination of electricity and mercury vapour to create light energy. The electricity causes electrons to travel at high speed between the poles. These electrons hit the mercury gas molecules and oscillate them.

The mercury gas is then ionized by the current flowing within, producing ultraviolet radiation. Since the human eye is unable to see this radiation, the glass tube is coated with a fluorescent material which changes it into white light. Thus, among the given options, option (d) is the correct answer.

6. Which is the gas inside an electric bulb? [M.P.P.C.S. (Pre) 2000]

(a) Oxygen
(b) Air
(c) Carbon dioxide
(d) None of the above

Correct Answer: (d) None of the above

Solution:The electric bulb was invented by Edition. A coiled filament of tungsten metal with a high melting point fits inside it. To prevent the metal from oxidation,Nitrogen and Argon gases are filled in the bulb instead of vacuum.

7. Fluorescent tubes are fitted with choke. The choke coil- [I.A.S.(Pre) 2000]

(a) Reduces current in the circuit.
(b) Steps down the line voltage.
(c) Steps up the line voltage.
(d) Chokes low frequency current.

Correct Answer: (a) Reduces current in the circuit.

Solution:The choke coil in a fluorescent tube lighting system is responsible for reducing the current in the circuit. It is designed to limit the current flowing through the tube and prevent it from exceeding safe levels.

The choke coil works by creating a magnetic field that opposes changes in current, which effectively limits the current flow. Therefore, the correct answer is: Reduce current in the circuit

8. Transformer is used for : [U.P.P.C.S. (PRE) 2006]

(a) Converting AC into DC
(b) Converting DC into AC
(c) To step-up DC voltages
(d) To step-up or step-down AC voltages

Correct Answer: (d) To step-up or step-down AC voltages

Solution:The transformer is used to step- up and step down the alternating current. There are two types of coils in transformer (i) Primary coil (2) Secondary coil. Alternating current flows through the primary coil which inflicts the potential in the secondary coil.

9. Which of the following of the purpose for which a transformer is uses? [U.P.P.C.S.(MAINS) 2011]

(a) To convert AC into DC
(B) To convert DC into AC
(c) To steps-up or step-down AC voltage
(D) TO steps-up DC voltage

Correct Answer: (c) To steps-up or step-down AC voltage

10. A mobile phone charger is : [U.P.P.C.S (Pre) 2017]

(a) An inverter
(b) A UPS
(c) A step up transformer
(d) A step down transformer

Correct Answer: (d) A step down transformer

Solution:A mobile phone charger works on the simple principle of conversion of AC (alternating current) to DC (direct current). As we connected charger to 220 volt AC switchboard, the first job of the charger is to step down the high 220 v in 9v or 10v before it's converted to the DC voltage needed by the phone's battery. A step down transformer is used in mobile phone charger.