amperage. Stun guns can put out many 10’s of thousands of volts but with very little amperage. Amperage tells you how many electrons (or holes, depending on how you look at it) will be in a cross section of your conductor at any given time. To anthopomorphize, voltage tells you how badly the electrons want to get from one pole of the voltage source, through the circuit, to the other pole. The greater the attraction to the opposite pole, the higher the voltage.
Reading your question again, I may not have answered it to your satisfaction. I think you’re asking about the dangers of working with power sources.
As mentioned in my post above, most power supplies are constant voltage type, not constant current type. Generally speaking, there isn’t much danger when working with supplies that produce less than 40 V. And a high maximum current capability doesn’t mean a power supply is dangerous. A car battery, for example, is capable of producing hundreds of amps when connected to a load with very low resistance. You do not have very low resistance, hence hundreds of amps cannot flow through you from a car battery.
A few caveats:
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Be careful of constant current supplies. If you set a constant current supply to 20 mA, it will ramp up its voltage until 20 mA is achieved. However, this will only occur if the compliance voltage is high.
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A 10,000 VDC power supply may not be dangerous if it is current limited to 1 mA, for example. There are two ways to limit current: a) a series resistor (to increase its source resistance), and b) a sensing circuit that automatically decreases its voltage when the current reaches 1 mA. When a series resistor is used, the resistor and your body resistance form a voltage divider. None-the-less, you should still be careful, since dynamic current produced by output capacitance can be very high.
There is little danger of electrical shock/electrocution. That does not make low voltage/high current sources safe. Example: A wedding band or metal watch band placed across a car battery will heat to it’s melting point almost instantly, burning all the flesh under it in the process, requiring amputation of the finger/hand. It is also not too hard to start a car on fire by shorting an un-fused starter wire to ground. This sort of danger is why it is a good idea to disconnect a car battery when performing any automotive service, and why the negative terminal should be disconnected first, as it is not unlikely that a wrench will get between the terminal and chassis ground. This is a bad thing if that is the positive terminal and the negative terminal is still grounded.
Sorry for the hijack, but car batteries seem not to get the proper level of respect…it is usually either “it’s only 12 Volts” or “It’s the amps that’ll kill ya”. It’s only 12V, AND letting lots of amps loose can hurt you without electrocuting you.
Electrons are a chemical. A teeny weeny chemical, but thing is, they can take part in chemical reactions (chemical reactions involving electrons as either a reagent or a product are called electrochemical reactions; the branch of chemistry which uses electrons or produces them is called electrochemistry; we scientists are original and creative like that). We’ve already talked about how you can use chemical reactions to “make” electrons (well, the electrons already existed, but we can get them moving) in batteries, and how secondary batteries can have the same reaction go in two directions, one when they are being used and the opposite when they are being recharged.
In order to be able to take part in a specific chemical reaction, all its reagents need to have a minimum amount of potential energy; for an electron, this potential energy is indicated by the voltage. If you have a reaction which needs electrons to have a voltage of 1V:
having a single electron at 0.9V will not produce the reaction (not enough potential energy in that electron),
having a single electron at 1V will produce the reaction exactly once,
having a single electron at 10V, or at 1.1V, or at 10000000V, will still produce the reaction only once,
having 10 electrons at 1V will produce the reaction 10 times
having 1000000 electrons at 1V will produce the reaction 1000000 times.
So, which reactions can you get is defined by the voltage. How much of those reactions can you get, by the amperage (“electrons being pumped through the cable per minute”). Once the voltage you have is enough to Do Nasty Things to a human body (and just the voltage from a regular home battery is enough), then the higher the current (amperage), that is the more electrons are being packed through, the greater the amount of times Nasty Things can happen, the faster… well, the faster someone touching that wire gets fried, if you’ll excuse the image.
How chemical is it, when you spin a wire/conductor in magnetic field? Laymen see only movement.
I think it’s probably more accurate to say that all chemistry is electrons.
Electrons, in all of their myriad orbitals and excitation states is what makes up what we call “chemistry.” How different elements interact is all related to their electron configurations. Once you take electrons out of the equation (like dealing with just protons or neutrons), you are no longer talking abut chemistry, but subatomic physics.
Good stuff so far. Who wants to explain me reactive power / vars?
I think Nava is pointing out that current is matter. And moving a conductor through a magnetic field will cause a ‘chemical’ effect as electrons move between atoms. It’s not a bad way to describe the process, especially the part about what kills you. The Bill Beaty site has some good articles about this. But as Chronos warned, make sure you are in the educational section of the site, and not in the fringe science area.
Sure.
When dealing with Direct Current (DC) Watts = Volts x Amps.
With AC, Watts = Volts x Amps x power factor, where power factor is the difference in phase between the current and the voltage. VAR is Volts•Amps•Reactive. Basically, for a purely resistive load, the current and the voltage say in phase (the peak of the current occurs with the peak of the voltage). With an inductive or capacitive load (a reactive load) (which all loads are, to a certain extent), the current waveform is shifted with respect to the voltage. This is because inductors and capacitors store energy - inductors in a magnetic field, and capacitors in an electric field.
The reason that one needs to take this into consideration, is that from the load’s perspective, the power it is dissipating is equal to the instantaneous current times the instantaneous voltage. If the current is out-of-phase with the voltage, the power consumed by the load will be reduced.
Now, your local Power utility doesn’t like reactive loads, because you only pay for “real” power (that which is delivered to the load), but the power company still needs to generate all that current, and it gets wasted as heat in the transmission lines.
No i’s were injured in the writing of this post. I guess I have no imagination.
Strictly speaking, if you look at the mechanisms by which electricity kills, it’s not the amps or the volts which kill you; it’s the watts or the hertz. There are two ways that electricity can kill: The most straightforward is that it can just plain cook you. The damage here is of the same form as damage from any other heat source. And if you were also being heated by some other source, the power of that other source and of the electricity would just add together to determine how much damage is done.
The second way that electricity can kill is that the frequency of the alternating current in your wall is just right that it can disrupt the electrical signals that regulate the beating of the heart (the body’s natural pacemaker). Disrupt those, and different parts of the heart will contract at different times, resulting in blood just sort of inefficiently sloshing around, rather than pumping. If you get medical attention right away, they can get your heart back into synch, but if not, you’ll die of heart failure. It’s not entirely clear how we ended up basing our electrical infrastructure on such a dangerous frequency, but it appears to have something to do with the contests of ego between Edison, Tesla, and Westinghouse, more than sound engineering reasons.
I think you have this back to front.
Please ignore the above. I think I’m wrong.
Ha ha. But you mean j surely?
Energy flows only if there is a passage between electrodes.
That’s a lot, but why aren’t chargers build so that the circuit remains open until plugged to a phone?
I imagine the answer is “it’s cheaper and easier that way.”
Whenever a large company does anything that intuitively seems wrong, there’s a 99% chance it was done that way for cost and/or ease of manufacturing.
I don’t know a lot about it, but I do know that wall power is AC, and your phone is DC. The plug both converts from AC to DC, and brings the voltage down from 110V/220V to 5V (most likely 5V.) From what I understand, the way the voltage is brought down means that theye is always a little bit of electricity flowing in the “110V half” of the power supply. The 5V side doesn’t do anything because the phone isn’t there to complete the circuit, but the 110V side is still working…right? Maybe?
As a percentage, it seems like a lot of power, but in absolute terms it’s quite small. To meet Energy Star requirements, chargers must idle at less than 1 Watt. If the charger were left plugged in 24/7 it would consume 8.760 KWH in a year. At an average price of 10¢/KWH, that will cost you under a dollar per year. Compare that to an Air Conditioner, which might cost $200/month to run. I think it’s worthwhile to make these devices as efficient as possible, since there are so many of them in use, but it makes more sense to look at the bigger energy hogs first.
I thought of a magnetic lock - when DC circuit is completed by plugging the phone a magnet would switch AC on. It would require a little rechargeable battery inside the charger. Nothing more complicated…
Started writing this before reading beowulff’s enlightening comment - energy wasted is not worth the effort.
Pretty much, yes.
It’s worth pointing out that this way to die isn’t restricted to AC wall current. Any fairly small amount of current (~10-100 mA) across the heart can trigger fibrillation. This can occur under the right conditions if there is a current path through the heart, or during surgery, for example.
Larger shocks (shock = pulse of high current) to the heart are less dangerous as they tend to stop it, and usually it starts back up normally. In fact, this is how an electronic defibrillator works - it sends a short shock to stop the heart when it’s in fibrillation — the state which can be induced by a lower current. I imagine for some people a faulty pacemaker or weak heart could make this a third way to die from electricity, but it’s probably rather rare.
VAR:
Picture a kid on a swing with an adult pushing.
If the adult pushes only while the kid is swinging away, the adult is supplying work, and the kid swings higher. You can calculate the work the adult does by multiplying the force of the push by the distance it was applied.
If the adult instead pushes as the kid is swinging toward the adult, then the kid is supplying work, and the kid swings lower. You can calculate this work the same way. (taking the distance as negative)
The point is that in both cases the adult is pushing on the kid, but it is the timing between (phase) of the swinging motion and the force that determine the direction of energy flow.
Now consider the case where the adult times the push so that exactly half the push is the “wrong” way, and the second half is the “right” way. There is lots of pushing going on, but there is no net energy exchange…the adult first takes energy from the kid, then gives it all back. The kid swings neither higher or lower. If you tried to measure the work but ignored the sign of the distances, then you would get the wrong answer. To get the right answer you have to account for the “phase” of the pushing with regard to the swinging.
That is pretty much what happens when you have a purely reactive (rather than resistive) load. The reactance adsorbs (stores) power during two quarters of the sinusoidal waveform, and gives it back during the other two. If you try to use Watts=Volts x Amps and ignore the phasing, then you often get the wrong answer. Electrical engineers need to keep track of the used (real, resistive) power vs. the stored (reactive) power. To do this they use two dimensional values for the voltage, load impedance, and current. These usually take the form of either scale + phase angle (known as “phasor notation”, easier to multiply and divide) or sine component + cosine component (easier to add and subtract). (*)
If you use those two dimensional representations, then P=VxA still works, but you get a two dimensional value for the power that represents actual work + reactive “work” …Watts+VARs
Note that this only applies to a cyclical (AC) system. Take the kid off the swing and put him in a wagon. Now there is no way for the adult to continuously take and return energy to/from the kid by only pushing. This is why reactive power is mainly only a consideration in AC circuits. (DC circuits can have transients where it comes into play though)
(*)I have avoided the terms “complex numbers” and “imaginary numbers” as these tend to make newbs think that this stuff is harder than it really is, and/or that somebody just made it all up. EE’s that have to deal with biz school types learn never to say things like “imaginary power” even though that is how they do the math.