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Today in the lab work, we observed that when a DC voltmeter is connected to AC voltage source, it displays zero but I didn't understand why? Could you please help me and explains the reasons?
Try drawing an AC waveform, centred around zero volts.
Notice anything about how much of the time the wave is above 0V and how much of the time it's below 0V? What happens if you draw a straight line from left to right showing the average voltage?
If the DC voltmeter you were using had a needle, did the needle looking as though the needle was moving back and forth really quickly or shaking? If your DC voltmeter had a needle - as someone who isn’t an authority in electonics, and I hope that someone will correct me if I am wrong, my guess would be that your DC voltmeter translates volts into mechanical energy that moves the needle. So why would the DC voltmeter measure a nonzero and positive DC voltage? Well, would it measure positive? That might depend on how you hooked up the DC voltmeter to the circuit that you were testing. Let’s say that you had the probes hooked up to a DC circuit and the DC voltmeter happened to be measuring nonzero and positively. If you moved one probe to where the other one was, and exchanged the location of the other probe as well, you might find that the voltmeter measures nonzero and negatively. Why the change? The change, and again I have no authority in the field, has to do with the direction of flow of the electrons. Specifically, in DC current, the electrons flow in one direction. DC current is like a game of tug of war with all of the electrons on one side of the rope. The DC voltmeter is sensitive to the flow of the electrons, and therefore, if I am correct, will change from a positive reading to a negative reading when the probes are exchanged. It is like all of the electrons jumped to the other side of the rope and started pulling the needle in the other direction. If your DC voltmeter has a zero at the center of the scale, and positive and negative measurements to the left and right of this zero – as distinguished from a scale that has zero to the left and positive numbers to the right, then If you were to reverse the location of the probes, what might happened is that the needle may swung like a pendulum. In AC, or alternating current, on the other hand, and I may be wrong, the direction that the electrons flow changes directions. If you measure an AC voltage equivalent to the DC voltage, and the frequency that the electrons change directions is slow, then the needle might slowly swing or jump – depending on weather the current was sinusoidal or square, back and forth, to the places that the needle approached when DC was measured. Later you might try to predict how the needle would respond to an alternating current (AC) triangular wave being measured by looking this up. Now, what do you think would happen to the needle if the frequency that the electrons change directions was faster? You can read up on how the frequency that the electrons change directions is standardized in different parts of the world at https://en.wikipedia.org/wiki/Alternating_current. If you were to hook up your DC voltmeter to AC in which the frequency that the electrons changed directions was faster, then you might find that the needle moves back and forth faster. The needle might start moving back and forth so fast that the needle can’t keep up with the voltage that the needle is supposed to be measuring. As soon as the needle starts to swing in one direction, the voltage that the needle is trying measure starts to push the needle in the other direction, cancelling out the motion of the needle. Perhaps this can be compared to a game of tug of war in which the electrons are pulling equally on both sides of the rope, cancelling the movement of the rope out, ending in a stale mate. However, this might not be quite accurate because, really, the electrons would be jumping from one side of the rope to the other really fast. I don’t have my internet up now, writing this offline, so I can’t quote what a person before me wrote. However, are you in a better position to explain mathematically how the voltages cancel out in AC current when measured on a DC meter, on average, over time?
I may have misrepresented the way that pendulums move in my last post.
I was thinking about it, and this may be a more helpful way to address your question. Pretend that you are an electron crowded with other electrons in a wire. What gives wires, and all conductors, the ability to carry current, AC or DC, is the ability of electrons to move easily in the wire. Materials that electrons can’t move easily in are called insulators. From what I understand, the source of electricity, AC or DC, creates what is called a gradient in the wire. Basically, a gradient, in this situation, is a buildup of force directed toward one side of the wire. It would be a good idea for you to check what I am saying with an authority on you lab before accepting what I am saying as theory. In other words, accept what I am saying at your own risk. I don’t know if it would be correct to say that one end of the wire is more crowded with electrons than the other end, though. A more accurate description may be that it is magnetic repulsions that move the electrons along. I don’t know how much, if anything, electronic collisions have to do with electron motion. In the wire, the electrons, at all times, might be, for the most part, in DC current, direct current, directed by force to only one end of the wire, always travelling in the same direction. I think that the electrons have been observed to have a tendency to create what is called an equilibrium. The electrons create an equilibrium by adjusting their position in the wire – so that the electrons on one end of the wire move toward the other end. I’ve heard that many sources of DC are batteries - though I don’t know if this is always the case. So, in a DC current, the electron group you belong to is travelling down the wire mostly in one direction, perhaps – for some reason, toward a chemical in a battery that is relatively electron deficient. The DC volt meter measures how the crowd of electrons you belong to move in the wire.
In the case of AC, the forces directing the electrons alternate. The electrons in AC are at first directed toward one end of the wire, and then directed toward the other end. So, in AC, alternating current, the direction that the crowd of electrons is travelling in alters. I’ve heard that many sources of AC are generators - though I don’t know if this is always the case. So, imagine yourself in a wire travelling first in one direction, and then again in the other direction, and then again in the other direction, and then again in the other direction, perhaps being driven by magnetic forces created by a generator - such as a generator at a dam or on a windmill. After travelling back and forth so much you keep on passing the same place again and again and again. What the DC volt meter may be measuring by staying around zero is that you haven’t really gone anywhere - just passed the same place many times. Though, you’ve moved a lot - so this may be confusing. Sure, you don’t go anywhere when you’re pacing back and forth, but if you pace a little further in each direction, you might be considered to be going to and from different locations, like to and from school or your job, day after day after day. What defines the difference between pacing and commuting? Half a cycle of AC current, though very brief and short to us, might have an important physical effect on particles - even though the DC voltmeter doesn’t register. If you want to learn more about the way that needles on DC volt meters work, you might want to look up motors, because I think that some types of DC volt meters are essentially motors.
So, my question for you is, as an electron, how can you travel more efficiently - considering that some people believe that we are in an energy crisis?
Please remember to check what I wrote before accepting anything as theory. This may help explain the difference between a power meter and a volt meter.
Yeah, I was kind of leading into that some waveforms will destroy a meter even though the meter seems to be reading within range. Like if you have a 1kV spike riding on 9v with a very low duty cycle, the meter would read ~9v even while it's being ruined.
It's somewhat of a paradox; you need to have some idea of what waveform you're looking at in order to pick an appropriate measuring instrument.
With his meter reading zero he might have assumed that no voltage is present, which could be a dangerous assumption in some cases.
So, you are saying that modifying established experimental methods may damage equipment? That sounds like a good point. 1963’s post got me thinking about electron traffic. How many different types of gradients can you think of?
Another interesting thing to do is to measure a 5v, 50 percent duty cycle
SQUARE wave with a dc meter.
If the wave is centered at zero (+2.5v and -2.5v) again the meter reads zero.
If the wave is riding on ground the meter reads close to 2.5v.
The reason of course is that the meter tries to average out the waveform.
I've used this to do quick checks on uC output pins to see if they are roughly
the right duty cycle because the output voltage is closely related to the
duty cycle and the peak voltage of the rectangular wave.
Another interesting thing to do is to measure a 5v, 50 percent duty cycle
SQUARE wave with a dc meter.
If the wave is centered at zero (+2.5v and -2.5v) again the meter reads zero.
If the wave is riding on ground the meter reads close to 2.5v.
The reason of course is that the meter tries to average out the waveform.
I've used this to do quick checks on uC output pins to see if they are roughly
the right duty cycle because the output voltage is closely related to the
duty cycle and the peak voltage of the rectangular wave.
If I centered the wave around a voltage having a value equal to the maximum voltage of the AC triangular wave, am I correct in thinking that, given the condition that the frequency was normal, a DC voltmeter would display a voltage of 1/2 of the maximum voltage of the AC triangular wave? Or would this better describe a wave that looked like /|/|/|/|/|/|/|/|/|? Does a triangular wave usually look like /|/|/|/|/|/|/|/|/| or /\/\/\/\/\/\/\/\ or |\|\|\|\|\|\|\|\|\|? Using the method described, would measuring all three of these types of pictorially described waves look the same? Let's say that we were measuring AC at a very, very low frequency. If you could slow the frequency of an AC square wave down slow enough, would the needle on a DC volt meter jump back and forth? If you could slow the frequency of an AC sine wave down slow enough, would a needle on the same type of meter display a motion resembling motion of a pendulum? The motion of a pendulum is very specifically defined. I'm wondering if, assuming that I am thinking correctly, there is some type of special name or description of the motion of a needle on the same type of meter when measuring AC triangular waves. Just out of curiosity, why are these three types of waves so special? Are there other useful, but less represented, types of waves? If you know a lot about electricity, would you address my post about the range of voltage used to power headphones?
On a triangle-wave, the up-slope and the down-slope have equal durations.
A sawtooth-wave has one direction must faster than the other and it can be slowly rising and fast falling or the opposite.
The slopes are straight, not curved.
I think a sine-wave is like a pendulum. It starts and ends with its voltage changing slowly and changes the quickest near the center.
I looked up sine wave at wikipedia and couldn't find anything about sine waves. The two instances where I found sine waves mentioned at the Pendulum page at https://en.wikipedia.org/wiki/Pendulum, the mathematical formulas were not directly proportional to the sine or cosine of anything. I was wondering how a needle on a DC voltmeter might move if the the cycle of AC was slow enough. When do you think the electricity in AC is moving the fastest - when the current switches direction or between the times when the current switches direction? I wonder if magnetic repulsion or electron momentum have anything to do with it.
Pendulum motion is not a sinusoidal wave, it's roughly an approximation of one.
That means a true solution of a pendulum will not really be a sine, but that's no
problem here because we want to know how a sine wave acts, not how a
pendulum acts.
To find this out all we need is a hand or other scientific calculator that has the
function 'sin' on it. If we have the calculator set to 'degrees' we can then
enter the sine angle in degrees and hit the 'sin' button and get the amplitude.
The amplitude is the amount the dc voltmeter would show if we fed it a very
slowly varying sine wave with a peak of 1v.
Just to note, the entire cycle of one sine wave
is 360 degrees, where the first 180 is the first half cycle (above zero) and the
second half cycle (180 to 360) is below zero.
A few examples are:
If, and only if, a DC voltmeter is measuring the velocity of electrons, would it be correct to say that when measuring AC sine waves having a very slow cycle, the needle on the DC voltmeter would move approximately at a speed equal to the derivative of the sine function, and according to a cosine function? Why would the needle approach zero when measuring a sine wave centered around zero volts as the cycle of this sine wave was increased?
If, and only if, a DC voltmeter is measuring the velocity of electrons, would it be correct to say that when measuring AC sine waves having a very slow cycle, the needle on the DC voltmeter would move approximately at a speed equal to the derivative of the sine function, and according to a cosine function? Why would the needle approach zero when measuring a sine wave centered around zero volts as the cycle of this sine wave was increased?
Is the velocity of an electron in a conductor the current? If so, according to Ohm's law, it seems as though the current is directly proportional to voltage, and would be registered on a DC volt meter. Is it possible to make the needle on a DC voltmeter swing according to a cosine function by measuring AC sine waves having a very long cycle? If so, why would the needle on the DC voltmeter be swinging?
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