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AC Circuit

  1. Nov 7, 2012 #1
    Recently, I've been studying AC circuits and have developed a couple questions. I think I worked out an answer to the first, but I don't know about the second.

    First, since light bulbs in houses are powered by AC current, why do we not see them flicker? Assuming they obey Ohm's law, the current through them is in phase with voltage, and so it must be 0 twice every period. My guess is that 60 Hz or 50 Hz used in most countries is simply so fast that our eyes don't register the flicker?

    Second, this question concerns complex impedance and inductors/ resistors in circuits. Let's say we have a variable inductor in series with a resistor. Supposedly this is an example of a light-dimmer, because if we increase the inductance, then there is a larger voltage drop across it because its reactance is jωL. What I don't understand is that since the voltage is AC, and the voltage across an inductor is L*dI/dt, wouldn't there be points in the cycle that the voltage across the inductor drops to 0, and then the voltage across the light bulb should increase a whole lot! So at least momentarily, why don't light bulbs change brightness oscillating between lighter and dimmer?

    I guess that question also implies the question as to how impedance can be jωL (a constant) even though there are different voltage drops at different times across the inductor.

  2. jcsd
  3. Nov 7, 2012 #2

    Simon Bridge

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    For 1. The filament in the bulb heats up, and it takes longer than 1/60th of a second to cool down again. The light it gives off depends on it's temperature - so it does not dim right away. You can see this when you switch off a very bright light (don't look at it) the illumination takes a short while to fade away. Lights that use other methods that respond faster to changes in current show an appreciable flicker.

    For 2. You get the impedence from the relationship between voltage and current, not the voltage alone.
  4. Nov 8, 2012 #3

    Philip Wood

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    It's important to realise that ωL is not equal to Vt/It, in which Vtand It are instantaneous values, but to Vrms/Irms (or to Vpeak/Ipeak).
  5. Nov 8, 2012 #4
    Simon has is. The filament has thermal mass which tends to smooth out the flicker to a significant extent. Likewise the response of the eye/brain also tends to mask flicker although some people are more sensitive than others.

    The light output from an LED responds much faster to changing input so depending on how they are driven there can be more of a flicker problem with those. Perhaps worth a read..

  6. Nov 10, 2012 #5
    1.A regular bulb lights up because inside the bulb, a thin wire called a filament gets very hot and glows. When the alternating current turns off for a fraction of a second, the filament does not cool off enough to stop glowing. That is why you cannot see the flicker with this kind of bulb.
  7. Nov 10, 2012 #6

    Simon Bridge

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    @Nullspace0: has that answered your question?
    @Abhinav R: welcome to PF; good to see someone contributing.
  8. Nov 10, 2012 #7


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    The frequency chosen for the mains supply was partly based upon the perception of flicker. It's right on the edge, actually, because the UK 50Hz is significantly worse than the US 60Hz.
    Flourescent tubes are significantly worse than 'laggy' filament lamps. Did you know that they tend to use low voltage AC lighting in machine shops (with rotating machinery). This is because low voltage filaments (for the same wattage) have more massive filaments and their temperature varies less over the AC cycle. This reduces flicker and means there is less chance of rotating machinery appearing to be stationary due to strobing. It helps with accident prevention in noisy environments.
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