What Do You Think? The purpose of this activity is to compare the variation in i

Question

What Do You Think? The purpose of this activity is to compare the variation in intensity of light from light sources. How does the light from a DC light source from an AC light source? How does the light from a fluorescent tube compare to the light from an incandescent bulb? Data Table Questions 1. How does the frequency of light intensity variation for the fluorescent AC bulb and the incandescent AC bulb compare to the accepted value for AC frequency? 2. How does the fluctuation of an incandescent bulb run on 60 Hz (or 50 Hz) AC differ from the fluorescent bulb? 3 How does the fluctuation of a light bulb run on 60 Hz (or 50 Hz) AC differ from the incandescent bulb powered by DC?

Explanation / Answer

1. The AC voltage and current fluctuates at 60 cycles per second in US and 50 cycles per second in India, UK etc.Therefore the amplitude of current becomes highest 120 times per second in US and 100 times per second in India, UK etc.Maximum brightness occurs at maximum amplitude. Hence an AC flurescent lamp light intensity variation is 120 times per second in US and 100 times per second in India, UK etc.

In case of an AC filament (also called incandescent lamp) the same rate of variation takes place in the intensity of light that is at 120 or 100 times per second. However to complicate matters, in case of the AC lamps of both fluresent and incandecent types, voltage (and current) becomes "zero" at 120/100 times per second. In other words, light intensity variation for both flurescent AC bulb and incadescent AC bulb happen at twice the rate of accepted rate of fluctuation of AC. So the light intensity variation becomes even less noticeable.

2. In case of incandesent lamps, the intensity of light depends on the temperature of the filament. The temperature of course depends on the square of the current. The filament does not get time to cool off in such a short time of say 1/120th of a second when the current becomes zero every cycle and the light intensity remains fairly steady. Thereore the incandesnt bulbs do not appear to fluctuate so much in intensity of light output after the initial heating up period. So to the naked eye the fluctuation in light intensity is less for AC incandescent bulbs than for the AC flurescent bulbs.

3. In DC, there is no fluctuation in applied voltage or current. Therefore in DC the incandescent bulb emits an absolutely steady intensity of light. Though the human eyes can not detect it, as explained above, the incandescent bulb does emit a fluctuating light intensity under AC supply because the current heating the filment fluctuates. If the supply is 60 Hz, the maximum intensity is after every 1/120th or (1/100th ) of a second, too quick for the eyes to detect the fluctuation. On top of that there is that stabilizing effect on the intensity of light due to the steady-state temperature attained by the filament in the incandescent lamps.

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.