Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 05/01/2019 12:15:47
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Assuming no smoothing, is an incandescent lamp, powered by 50Hz AC mains, effectively flashing on and off 100 times per second?
My rationale for this deduction is that the supply voltage - and hence energy delivery - is following a sine wave function. Per cycle, the PD rises to a maximum, then returns to the baseline, and then progresses to a minimum before returning to the baseline.
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So, I would expect the energy delivery - and hence brightness - to rise to a maximum, then drop to zero, then rise to a maximum again (albeit with the current flowing in the opposite direction) and then drop back to zero.
Hence, fifty times per second we get: on -> off -> on -> off - i.e. two lots of "on" 50 times per second. Hence 100 "ons" or flashes per second.
Am I right?
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Yes, in principle.
In practice the thermal inertia of an incandescent lamp suppresses the ripple so it never completely "goes out", and modern lamps such as LEDs and CFLs use a high frequency inverter to produce almost a constant DC voltage.
You can see the ripple on the output of a simple dental x-ray machine (at 50 Hz, with alternate half-waves missing) and some legacy "full wave" medical x-ray units (100 Hz), but modern kit all works on HF inverters at 25 - 250 kHz to save the weight and costof 50 Hz transformers.
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Good - glad I'm on the right wavelength ;)
What informed this question was an enquiry from a listener about seeing a car going past a "palisade fence". To her eye, the wheels of the passing car "looked like they were turning backwards".
I've certainly seen this on an accelerating vehicle passing along a road under street lamps at night where, presumably, the flickering lights strobe the wheels and make them appear to turn slowly or even reverse. The same illusion is also present in some films, where the wheels of carts appear to reverse direction as they speed up and drive away.
So what's going on with this lady's fence? Are the gaps in the fence effectively strobing the wheels?
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I believe chris it is to do with the refresh rate of the human eye.
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Due to the variation of resistance of the of the tungsten filament with temperature the flow of current does not follow the applied voltage giving the lamp some of the properties of an inductor.
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Empirically pulsing at 100Hz, (in Europe) ...
https://youtu.be/eUprJS9sXYU?t=41s (https://youtu.be/eUprJS9sXYU?t=41s)
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Are the gaps in the fence effectively strobing the wheels?
Yes.
And street lighting is seldom incandescent.
Gas discharge lamps and LEDs do "flash" at twice the mains frequency.
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I believe chris it is to do with the refresh rate of the human eye.
The eye does not have a refresh rate - it doesn't work by frame raster scanning. In fact, visual persistence has pretty much the opposite effect, smoothing out the saccadic wobbles and integrating the image of a TV or movie to produce the illusion of continuous movement.
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The same illusion is also present in some films, where the wheels of carts appear to reverse direction as they speed up and drive away.
The classic case is the film or TV shot of a propellor aircraft starting up - it looks as though the prop spins backwards intermittently, and many first-time passengers are surprised that it doesn't!
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is an incandescent lamp, powered by 50Hz AC mains, effectively flashing on and off 100 times per second?
I recall doing this experiment with an electronics kit as a child:
- Build a circuit with a solar cell and a transistor amplifier feeding an earpiece
- Put the solar cell close to an incandescent lamp, and you hear a buzz in the earpiece.
As Alan says, the effect is more pronounced with fluorescent, gas-discharge and LED lights, as these don't have the same thermal inertia as incandescent lamps (although the fluorescent lamps do have a phosphor emission half-life).
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I do not know if any small aircraft use two stroke engines that will rotate in either direction but have there been any recorded instances of pilots opening the throttle to take of and finding the aircraft going backwards ?.
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The light emitted by an incandescent light bulb is directly proportional to the filament temperature. In order to flicker, there would have to be a measurable drop in temperature at the instant of current reversal. This drop would be equal to the amount of energy lost as light during that instant not replenished by the interaction of the current and the filament resistance. An infinitesimal amount I'm sure.
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I do not know if any small aircraft use two stroke engines that will rotate in either direction but have there been any recorded instances of pilots opening the throttle to take of and finding the aircraft going backwards ?.
Propellors work by altering the angle they meet the air at, like a wing on its side. The difference in the profile along the length of the propellor is to do with the difference of the rotational speed at different radiuses from the centre. Some planes only have one gear.
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The light emitted by an incandescent light bulb is directly proportional to the filament temperature.
No, it isn't.
The total radiation emitted varies as the 4th power of the filament temperature, and the fraction of that which is visible also increases with temperature. So the light emitted varies with temperature even more than the total radiation.
In order to flicker, there would have to be a measurable drop in temperature at the instant of current reversal.
No, it's not an "instant".
There's the whole part of the cycle where the voltage (and thus the temperature) are falling.
Essentially, that's very nearly half the time.
An infinitesimal amount I'm sure.
It's puzzling that you think it's infinitesimal, even though others have pointed out that it's quite easy to observe.
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I do not know if any small aircraft use two stroke engines that will rotate in either direction but have there been any recorded instances of pilots opening the throttle to take of and finding the aircraft going backwards ?.
It's conceivable but more likely in future rather than the past. There's a growing interest in electric motors for small planes, particularly because you can make coaxial contrarotating propellors. This increases the efficiency of the prop, nullifies torque, and allows you to quickly engage reverse thrust, even more smoothly than with a jet engine.
There have been instances of variable-pitch props spontaneously reversing, with dire consequences, but this has even less to do with mains flicker.
Back to the subject: the lamp fitted to a lathe is always (or certainly used to be) a low voltage or DC powered unit. The thicker filament of a 50 volt 1 amp lamp compared with a 250 V 200 mA unit meant more thermal inertia and less strobing of the spinning chuck.
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My apologies. I had intended to phrase my post as a question. Thanks for the clarifications.