A light bulb using alternating current at 60Hz turns on and off 120 times per second but in a monitor 60Hz means 60 refreshes per second.
Why is that? Shouldn't 60Hz mean 120 refreshes?
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Hertz means '(something) per second'.
With the light bulb, that's cycles of the mains supply frequency. Being a balanced a.c. waveform, that crosses 0 V twice per cycle. That causes the 100 light bulb flashes per second from the 50 Hz supply, or 120 flashes from a 60 Hz supply.
With the monitor, a cycle is a top-to-bottom refresh of the display pixels on the phosphor or LCD screen. There's no a.c. aspect in play here. If anything, the term could be applied to the frequency of a monitor's VSYNC signal, if its got one, but that's not why the term arose.
The unit 'Hz', originally used for a.c. waveforms, was the one later used for refresh rates in television and thence in monitors. The use of Hz was to do with how early television sets derived timing from the mains frequency.
Light bulbs blink at twice the mains frequency because the AC waveform goes to negative side, and has two pulses per cycle:
Compare that to the brightness you would see with an old time CRT monitor, or nowadays with VR headsets:
The pulses = frequency x 2 behavior only applies to cases where the waveform goes to negative side and both half-cycles cause separate pulses.
Because you're referring to completely different cycles with different purposes.
AC lighting uses a sinewave, because that's what works best with power distribution equipment. That's really the end of that conversation.
Monitor sweeps use a sawtooth pattern, because it paints the picture from top to bottom, and then snaps back up to the top very quickly to do it again.
In both cases the thing repeats 60 times a second.
Note that TV monitor frequency is not inherently tied to AC power line frequency. 60Hz NTSC TVs could work on 50Hz. and in fact NTSC, PAL and SECAM refresh frequencies don't even match their regional power frequency. In actuality, NTSC was 59.94 FPS. You could see this on a mildly defective TV, as the AC power ripple crawls up the screen slowly. That may have been the point; so the ripple crawls rather than jumps spasmodically.
Consider what a 60Hz sine wave looks like and when a light bulb lights up.
The light bulb is basically a resistor so lights up the most when the sinusoid is at its peaks which happens twice per cycle (the positive peak and negative peak). Since there are two of these per cycle there are 120 of them per second in a 60Hz sine wave running through a light bulb makes it flash at 120Hz.
But a monitor refreshes at 60Hz because it was made that way.
Power is V^2/R (barring impedance effects). So if you have a voltage source of cos(60T), the power will be cos^2(60T)/R, which is equal to (1+cos(120T))/2R, which has a frequency of 120 Hz. In an AC circuit, the power cycles at a frequency twice that of the voltage.
Remember that the power is current times voltage, and current is proportional to the voltage. So when the voltage is at its most negative, the current is also at its most negative, and so the power is at its most positive. So for every peak in the voltage graph, there are two peaks for the power graph: one for when the voltage is most positive, and one for when it is most negative.
Whoa. Hertz does NOT mean cycles per second. Wherever did you get the idea that it was 'cycles' - only joking.
It's a real problem that when the SI system decided on the Hertz they didn't say what thing was being measured/counted over the period of time. This lead to various problems between random arrival things and regular or 'cyclic' events.
There was a note in the SI specification that all the ionising radiation units had to be created specially because of the issue.
Further, a default periodic unit for SI was the radian, so you (we) should be talking Angular Frequency, or Cyclic Frequency, (or event frequency) to try to clarify to readers just what is being counted, which lies at the heart of the 60 (cy) Hz mains frequency and the potential 120 (light pulse) Hz frequency.
In the older 8th Ed of the SI units [updated in 2014] it was the section "Units with special names and symbols; units that incorporate special names and symbols" http://www.bipm.org/en/publications/si-brochure/section2-2-2.html but it's now (new edition) all one pdf!