I read that current enters an appliance through live wire and leaves through the neutral wire.Consider the appliance to be a light bulb.Now my question:

1.Shouldn't the current first travel to the appliance though the live wire,become zero,and then flow from neutral to live? Because AC reverses direction?

2.The current is alternating in magnitude.But why is the bulb glowing with the same power? Should not the bulb first glow very brightly,then dim and finally gets off and then again starts glowing??

  • \$\begingroup\$ First question does not make any sense. The answer to the second - it is doing that. But very quickly. \$\endgroup\$
    – Eugene Sh.
    Mar 27, 2015 at 14:36
  • \$\begingroup\$ Please re-state question 1 because it really doesn't make any sense. \$\endgroup\$
    – Andy aka
    Mar 27, 2015 at 15:09
  • \$\begingroup\$ What's wrong with question 1? The voltage across the lightbulb is positive, then zero, then negative (as a sine wave should), and OP is asking if the current does the same. \$\endgroup\$
    – Greg d'Eon
    Mar 27, 2015 at 15:55

1 Answer 1


First of all, whatever it was that you read, was (partially) wrong. During half of the AC cycle, it flows one way, then during the other half, it flows the other way. So your #1 is correct.

For #2, the bulb is really more of a heater that simply gets so hot that it glows. (assuming incandescent; CFL's and LED's are much more complicated) Temperature takes a relatively long time to change in most cases, so it easily rides through the AC zero-crossings. Keep in mind that these zero-crossings happen 120x per second for 60Hz line frequency, so even traditional fluorescent bulbs that do turn off at zero are difficult to see doing that without something moving to complete the strobe effect.


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