Electric Current (Physics) Part-II

Total Questions: 39

1. 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 LED lamp uses semi-conductor material.

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?

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.

2. 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.

Correct Answer: (c) 1 and 3 only
Solution:

Lights from sodium lamps i.e. 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 i.e. 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.

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

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]

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]

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]

Correct Answer: (d) None of the above
Solution:

The electric bulb was invented by Edison. 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 a choke. The choke coil- [I.A.S. (Pre) 2000]

Correct Answer: (a) Steps up the line voltage
Solution:

Starter in tube lights increases the current and sends it to the choke. Choke coil steps up the voltage of the transmitted current and send it back to tubelight resulting tubelight to glow.

After this work of the starter gets finished and choke is used to step up and step down the voltage thus prevent the tubelight from any damage.

8. With reference to the electric potential which of the following statements is/are correct? [U.P.P.C.S. (Pre) 2021]

1. It is a scalar quantity.

2. It is a vector quantity.

Select the correct answer using the codes given below.

Codes:

Correct Answer: (a) Only 1
Solution:

Electric potential is a scalar quantity. It is the amount of work needed to move a unit charge from a reference point to a specific point against an electric field.

9. If potential difference and current flowing through a wire is increased twice each then electric power: [Chhattisgarh P.C.S. (Pre) 2015]

Correct Answer: (b) Will increase four times
Solution:

Power P = V × I
where V = Voltage
    I = Current

Since both the voltage and current are increased twice

P' = (2V) (2I)

P' = 4 V × I = 4P

Hence, the electric power will increase four times.

10. 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 : [I.A.S. (Pre) 2001]

Correct Answer: (b) 10 ohm
Solution: