Chapter 18, Problem 13Q

When electric lights are operated on low-frequency ac (say, 5 Hz),they flicker noticeably. Why?

To determine

To explain:

The reason why the flickering becomes noticeable when some low frequency ac is used to operate the electric lights

Answer to Problem 13Q

Solution:

It happens because of the burning out frequency of the lights come quite under the maximum observable limit for a human eye.

Explanation of Solution

An alternating current, keeps on changing its direction of flow from one side to another. Therefore in every cycle, the current becomes zero two times, i.e. the bulb will stop burning out in the cycle two times. Now suppose, the frequency of ac is n, it means, the bulb will stop burning out 2n times in a second.

Many studies say that maximum observable rate for a human eye is about 20-50 hz. So, when the frequency of ac is so reduced like 5 Hz, it means the bulb will stop burning out 10 times in a second, i.e. the frequency of bulb being on and off being 10 Hz, which is quite under the maximum observable limit for a human eye. Hence, in low-frequency ac (say, 5 Hz), the electric lights flicker noticeably.