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Questions about AC & DC

  1. Sep 22, 2013 #1
    Hello everyone. I have just bought myself a multimeter just for fun. So I've been going around testing voltages and current of things... I have a few questions, and more to surely come later...

    1st. When they say Direct Current flows "in one direction" and Alternating Current flows in "both directions", what exactly does this mean? I heard that if you graph both currents, AC will look like a sine wave and DC will be a straight line, with a constant value? Is this what they mean?

    2nd. If AC is a sine wave whenever it reaches 0 there should be exactly 0 voltage and current right? Then what happens when it goes negative? How can electricity be negative?

    3rd. What causes AC to fluctuate like this? I know that current is the movement of electrons from atom to atom many millions of times, so what does this process look like under AC? Are the electrons literally moving in one direction from atom to atom, then reverse course?

    4th. If voltage is the 'pressure' of the current. Then something with a low voltage but very high current can still harm you correct? just like something with a high voltage but not so high current?

    5th. What's the voltage/current range you begin to feel something if you touch it and it's live? Obviously most batteries and the like, don't produce enough volts.

    Thanks guys, been wondering about this stuff all week!
  2. jcsd
  3. Sep 22, 2013 #2


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    The current flows in the opposite direction to what it was flowing before.

    Yes, the average motion of the electrons in AC is a back-and-forth oscillation. It turns out that such a motion is actually easier to generate than DC, when you use a generator as opposed to a chemical battery. If you take a loop of wire and rotate it continuously in a magnetic field (the basic method of operation of most generators) you naturally get AC out of it: one back-and-forth cycle of current per rotation of the loop of wire.
  4. Sep 22, 2013 #3
    The current can only harm you if it flows through you. Now, you cannot have high current without high voltage through your body, because they are related by Ohm's law, and your body's resistance is quite high. You can measure it, by the way.

    One caveat, though. Something with low resistance may have very high current flowing through it even at low voltage. That can generate a lot of heat and so be dangerous to your body.

    As to what voltage you can feel, your tongue can feel a few volts with ease. 9 V is pretty darn painful already.

    If you hold two terminals in one hand, then it takes a bit more to feel that. I remember that messing around telephone lines is distinctly unpleasant, and I think they use lower than 50 V. Note this sort of voltage can be fatal if applied in the vicinity of your heart.
  5. Sep 22, 2013 #4


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    This is true. But it is also a physical thing. The current, the flow of charges, literally goes in one direction through the circuit, stops, and then flows the other direction before stopping and doing the whole cycle over again.

    Careful here. The term electricity doesn't actually refer to anything. It's like a category. Anything having to do with electrical circuits is thrown into this category of "electricity".

    What happens in a circuit is that the voltage and current oscillate in magnitude and direction/polarity over time. When current is negative it just means it's flowing in the other way than it was before. When voltage is negative it just means that the voltage has changed polarity, IE the part of the circuit that was at a positive voltage is now at a negative.

    Without getting into the nitty gritty details, yes. The reality is, of course, far more complicated.

    The reality is, again, more complicated. If you don't know much about electricity, then just remember that ALL sources of electricity can be dangerous in the right circumstances. A low voltage source can kill you just as well as a high voltage source, while the opposite can happen as well. My dad's uncle survived 50,000 volts, yet I know of a person dying from 45 volts. It all depends on the circumstances.

    Stick a 9-volt battery to your tongue and tell me you don't feel it.

    The fact is that you feel current, not voltage. How much voltage is needed to cause enough current that you feel it depends on the situation, where it's touching you, and a whole mess of stuff.
  6. Sep 22, 2013 #5


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    Say you took a 9V battery and hooked it to switches so it could be reversed. Say you then connected it to a light bulb. As you flipped the switch over and back the current would flow one way and then the other. That would be square wave AC. If you could switch it over and back 60 times per second that would be a 60Hz square wave. The current flows one way, and then the other. The voltage is positive, then negative in an alternating pattern.

    AC is advantageous because it can be transformed easily and very efficiently to higher (or lower) voltages with transformers. Higher voltages mean lower currents for the same power and so lower losses in transmission lines.

    Very roughly speaking, as the AC frequency get higher, it becomes Radio Frequencies and energy is efficiently radiated with an antenna.

    As to shocks:
    Touching a 9V battery with you tongue is a good way to determine whether it is a good or bad battery. Toughing a 22V battery with your tongue is a good way to find youself on the floor (did it once).

    The amount of current is what determines the severity of a shock. Skin resistance must be overcome to force current through your body. Puncture the skin and a much lower voltage will produce dangerous currents. Static electricity is thousands of volts, bit very low current.

    From wikipedia:
    Death can occur from any shock that carries enough current to stop the heart. Low currents (70–700 mA) usually trigger fibrillation in the heart, which is reversible via defibrillator but can be fatal without help. Currents as low as 30 mA AC or 300-500 mA DC applied to the body surface can cause fibrillation. Large currents (> 1 A) cause permanent damage via burns, and cellular damage. The voltage necessary to create current of a given level through the body varies widely with the resistance of the skin; wet or sweaty skin or broken skin can allow a larger current to flow. Whether an electric current is fatal is also dependent on the path it takes through the body, which depends in turn on the points at which the current enters and leaves the body. The current path must usually include either the heart or the brain to be fatal.
    Last edited: Sep 22, 2013
  7. Sep 22, 2013 #6


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    It's not necessarily the volts that kills you, it's the amps.

    Tasers and stun guns use high voltage to stun and incapacitate an assailant. Both weapons use a small current, typicallay1-2 milliamps, but the voltage at applications is much different: a taser puts out about 50,000 V while stun guns can put out from 100,000 V up to 5,000,000 V. Both weapons will put most assailants on their ***, but the higher voltage of the stun gun will keep them down and disoriented longer after the shock has been applied.

    Household current is lethal because a typical circuit (in the US) may have 15-20 amps of current available, even though the typical voltage is 120 V.
  8. Sep 22, 2013 #7


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    You can feel even 4.5 V with your wet tongue...
    Usually voltage lower than 24 V is considered to give no or little harm.

    The household supply 120 V, 230 V is dangerous. The danger depends on your resistance and the path of the current. Touching the poles with two fingers, for a fraction of second is not lethal if you are isolated from the ground. The outer part of the dry skin has high resistance, so the current is small, but getting connected for a long time causes burns and when the current breaks the skin through, the body has very law resistance, so the current increases. It is high danger that the current causes convulsion : if you grab a live wire, you can not release it. The current flowing through your heart is very dangerous. Avoid touching live part with two hands, or standing on wet ground.
    It is from 1-2 mA that the current becomes dangerous, and it also depends on the time you are exposed to the current.

    AC is usually more dangerous than DC.

  9. Sep 22, 2013 #8


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    Out of sheer curiosity, what exactly was your dad's uncle trying to do? :eek:
  10. Sep 22, 2013 #9


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    Not sure. I just remember hearing he was standing on a ladder while working on something in a new building and got shocked, bad. So bad it burned holes in the soles of his feet where the current flowed from his feet to the metal nails that held his shoes together and into the ladder. This was sometime around the 1950's I think.

    The guy that died from 45 volts was shocked, hit his head on the equipment he was working on, knocked unconscious, and lay there with his head across the circuit for about 12 hours until someone found him dead the next day.
  11. Sep 22, 2013 #10
    Is there a Horrors section on the forum?
  12. Sep 22, 2013 #11


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    I asked because I thought it was somewhat hard to casually come across a 50,000 V line unless you are in an industrial or other exotic setting. Especially in the 1950's.
  13. Sep 22, 2013 #12


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    I'm near certain it was in an industrial building. I remember hearing several similar stories, such as the time he lit his butt on fire while welding a fuel storage tank at a Bell Helicopters facility or something. I just remember the story, "And then he felt a tap on this shoulder. He kicked his visor up and turned to see a firefighter standing right behind him. 'Hey, buddy. You're a** is on fire,' he said right before blasting Richard with a CO2 extinguisher."
  14. Sep 22, 2013 #13

    WOW, Maybe he was in the best circumstances ever (completely dry without sweat or something, current flowed on the right side bypassing the hearth, excellent insulating rubber soles...)

    And I got slightly socked by touching a 230V line trough a old tester screwdriver (those with a neon lamp) and a multimeter while a bit sweaty and barefoot... the multimeter measured 120V
  15. Sep 22, 2013 #14
    Thanks for the detailed responses guys! and the stories are interesting! I've heard the opposite though ehild, that DC is more dangerous, because it's "always on" your muscles will contract and stay contracted all the while you're touching it, making it harder to let go.

    Anyway I have 1 last question... In general houses here in the states have about 20 amps output. When a device such as a wall outlet or strip outlet that supplies amps, and when you plug a 2 amp devicce into it, does that amperage "Subtract" from the 20 alloted amps? so if you plug a 2 amp device in and a 4 amp device will your remaining amp output for that outlet be 14 amps?

    Does voltage work in the same way? will the voltage be decreased with the more devices you plug in?
  16. Sep 22, 2013 #15


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    The incoming current is 'shared' between the devices. If the supply can manage to produce all the current they want then the volts will remain the same. If you try to take too much, either your supply fuse will blow or the volts may 'sag' because of the resistance in the supply cables. You would normally assume that the supply volts remain the same, though.

    When you turn on more and more devices on your car (engine not running) the first few will have no effect on the brightness of the lights but eventually they will start to go dim when you add loads like the windscreen heater, which takes a lot of current. Remove the big loads and the lights come bright again - so it's not just running down the battery.
  17. Sep 22, 2013 #16
    Generally if you can supply enough amps for all your devices everything will work fine.
    If your devices require a lot more amps, as posted above either something will blow, or the voltage will be reduced.

    Let me give you an example of an experiment I had to realize in the lab:

    We had a certain electronic project to make, and we were using solderless breadboards, we were using a DC generator which provided +5Volts to our circuits.
    When the circuit grew large enough, and we used multiple ICs, we saw a drop in the voltage in the output of some chips (the values in the datasheets regarding the output currents and input currents were okay). We checked the DC generator and realised we had the current limited to 0.1 Amps, after we fixed this limit, everything went back to normal.

    If this generator didn't have a limit button (for the current), and we accidently made a short circuit somewhere, the generator would have been toast and the breadboard on fire.
  18. Sep 22, 2013 #17


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    Let's not get into this discussion. BOTH types are dangerous and can kill you if you are not careful.

    The rated "output" is the maximum that it can handle before a breaker or fuse blows. Yes, if you suck 10 amps up in one device, you only have 10 more amps to go before before blowing your breaker/fuse. Note that this 20 amps is only the maximum because of the breaker/fuse. If you just had a straight wire instead of a breaker, you could pull as much current as can come through the lines, and would eventually heat up the lines so much that you would burn down your house.

    Your household outlets are in parallel with each other, meaning that voltage is the same for everything plugged into them. If it was in series then you would have reduced voltage as you plug in more and more devices.

    If you really want to learn electronics you're going to need to take a class or pick up a book on it. Nothing here will make much sense otherwise.
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