Fight my ignorance about volts and amps.

Factual Questions
1

I was watching an “Amazing Videos” type of show the other day. A bear had come into town, decided to climb a power pole, and got shocked pretty severely. The narrator stated that the bear experienced 50,000 volts.

I don’t understand volts and amps. I thought you could get thousands of volts from a static electricity shock and be fine, because the duration is short and hence the amperage is low. But I was watching Apollo 13 and when they checked the battery meters they said something like “0 volts, 15 amps.”

So what (in lay terms, PLEASE) are they and how are they related? Is it different for AC and DC? Is this why a AAA and AA and C and D batteries are all 1.5 volts—they’re different amperages?

2

In the simplest case, DC, volts and amps are related by resistance. Ohm’s Law states that E= I * R, where E is voltage, I is current and R is resistance. As an example, if you apply 10 volts across a 2 ohm resistor, a current of 5 amps will flow.

For AC, things get significantly more complicated.

3

[QUOTE=Q.E.D.]
In the simplest case, DC, volts and amps are related by resistance. Ohm’s Law states that E= I * R, where E is voltage, I is current and R is resistance. As an example, if you apply 10 volts across a 2 ohm resistor, a current of 5 amps will flow.

For AC, things get significantly more complicated.
[/QUOTE]

That makes no sense to me at all, and I have to sell electronics for a living. Perhaps you could give us the Primary School version- no mathematical equations or anything like that?

4

Electrical flow is analogous to fluid flow. Amperage is like the flow rate, and volts is like the pressure.

Confining ourselves to DC with no capacitors or inductors, duration doesn’t matter. As far as batteries are concerned, they all have the same amperage, but different storage capacities.

Here’s an explanation with some pretty neat animations:
http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html

5

Using the water analogy:

The amount of water flowing is the Amps.
The pressure of the water is the Volts.
The diameter of the pipe is the Resistance (Ohms).

For a given pressure (Voltage), more water will flow (Amps) through a bigger (lower resistance) pipe. If you keep the flow the same, but make the pipe smaller, you must increase the pressure.

At some point these analogies break down, but they’re a useful way to think about electricity.

Static electricity can have many thousands of volts, but there isn’t a continuous source of charge available, so all you get is a single, brief spark. If you had a 50,000 volt generator that was capable of supplying that voltage into a load resistance such as the human body, you would burst into flame (ouch!).

6

[QUOTE=beowulff]
Using the water analogy:

The amount of water flowing is the Amps.
The pressure of the water is the Volts.
The diameter of the pipe is the Resistance (Ohms).

For a given pressure (Voltage), more water will flow (Amps) through a bigger (lower resistance) pipe. If you keep the flow the same, but make the pipe smaller, you must increase the pressure.
[/QUOTE]

Ah, that makes sense. Thank you!

7

Voltage is the potential, or energy differential, between two (or more, sometimes) electrically charged poles. The polarity needn’t even be different. A pole with a -5 volt charge and another pole with a -10 VDC charge still has a 5 VDC difference.

A pole with a +10 VDC charge and another pole with a -5 VDC charge has a total voltage of 15 VDC.

Think of voltage as electrical “pressure.”

Amperage is the actual electrons flowing through the wires.

Amperage is “how much,” voltage is “how hard.”

That’s why a 50,000 volt static shock doesn’t do much more than make you jerk your fingers back in surprise; you’re talking miliamps (or even microamps).

VAC works a little different than VDC. VAC has all kinds of measurements: peak voltage, peak-to-peak voltage, average voltage, and rms voltage.

Since VAC switches polarity at regular intervals (~60 Hertz here in the U.S.A.), it is characteristically described as a sine wave.

Peak voltage (Vp) is the maximum voltage above the zero-reference line.

Peak-to-peak (Vp-p) is simply double that (the maximum voltage, from highest peak to lowest trough of the sine wave).

Average voltage (Vavg) is a mathematically derived formula that averages the voltage over one-half cycle of the sine wave (the average over the full sine wave would, of course, be zero), and is considered to be .637 Vp.

RMS Voltage stands for “root mean square,” and is used to approximate a DC equivalent of an AC voltage. An AC rms voltage will produce the same heating effect as a DC voltage through an equal resistance. For instance, a 120 VAC source has a:

120 VAC * .707 = 84.84 Vrms value.

8

Forgot to say: Vavg is what actually comes out of your wall outlet.

9

[QUOTE=ExTank]
Forgot to say: Vavg is what actually comes out of your wall outlet.
[/QUOTE]

No, that’s V[sub]RMS[/sub]. What comes into your house is actually about 170 V[sub]Peak[/sub].

10

You’re right. I always get that last bit confused.

11

Thanks, digesting all this (or trying).

I probably got the number wrong for the Apollo 13 example, but they must have been dealing with DC (batteries), right? So when they said 0V and 15A, that’s like saying “We have a bucket of water but no pressure to push it through the conduit”? IIRC, and I think I do, they said 0 volts…so I figured it couldn’t be a linear relationship.

I guess one problem with visualizing this would be how different appliances have different amp loads. For example, when I plug my clothes iron into the outlet, my lights dim. I know it’s drawing a lot of energy, but it’s the same 110V as a table lamp.

And why is it that a big appliance like a dryer is on a 220V circuit? I’m assuming that’s like saying, “One hose can’t handle this…you’re going to need two.”

Also I just bought a car battery today. What are those, 12V? It has 500+ cranking amps. Is there a different measuring system of the amps? I think my house’s circuit breakers are 200 (total) amps or something. But then, didn’t Edison go to great lengths showing that AC would electrocute you while DC would not? DC, more amps.

Strangely enough I caught a thing about Tesla the other day and for the first time, I kindasorta understand the difference between AC and DC.

ETA and back to that bear: she survived. But wouldn’t it make more sense to say how many amps she received than volts? Or is it a matter of knowing the volts but not being able to calculate the amps?

ETA2: or is it a matter of 50,000 volts sounding more impressive than 100 amps?

12

[QUOTE=lobotomyboy63]
Thanks, digesting all this (or trying).

I probably got the number wrong for the Apollo 13 example, but they must have been dealing with DC (batteries), right? So when they said 0V and 15A, that’s like saying “We have a bucket of water but no pressure to push it through the conduit”? IIRC, and I think I do, they said 0 volts…so I figured it couldn’t be a linear relationship.

I guess one problem with visualizing this would be how different appliances have different amp loads. For example, when I plug my clothes iron into the outlet, my lights dim. I know it’s drawing a lot of energy, but it’s the same 110V as a table lamp.

And why is it that a big appliance like a dryer is on a 220V circuit? I’m assuming that’s like saying, “One hose can’t handle this…you’re going to need two.”

Also I just bought a car battery today. What are those, 12V? It has 500+ cranking amps. Is there a different measuring system of the amps? I think my house’s circuit breakers are 200 (total) amps or something. But then, didn’t Edison go to great lengths showing that AC would electrocute you while DC would not? DC, more amps.

Strangely enough I caught a thing about Tesla the other day and for the first time, I kindasorta understand the difference between AC and DC.

ETA and back to that bear: she survived. But wouldn’t it make more sense to say how many amps she received than volts? Or is it a matter of knowing the volts but not being able to calculate the amps?

ETA2: or is it a matter of 50,000 volts sounding more impressive than 100 amps?
[/QUOTE]

You touch on a lot of different issues, some of which are non-trivial.
I don’t remember the scene in Apollo 13, but it’s not very realistic to have a reading of 0V at 15A. I suppose the voltage might be very low so that it was close to zero…

Comparing a table lamp to an iron: Yes, they both run off of 120v, but one has much less resistance than the other, so it draws more current. When you draw more current at a given voltage, you consume more power, or Watts. A Watt is a Volt times an Amp, so you can figure out how much current an appliance uses by dividing it’s Wattage rating by 120V. As has been mentioned before, AC voltage is more involved, but for power calculations, the AC voltage in RMS gives the correct figure (the RMS value is the value for an AC voltage that gives the same power as a DC voltage).
Your dryer or range uses 240v, not because 100v wouldn’t work, but because the current necessary to provided the required power would be so large that the wires would get too thick and expensive.

Car batteries are a whole other issue. The CCA is a rating which measures how much current the battery can provide to a load in cold weather. But, since the battery is only 12v, that load must be a very low resistance. So, a 500CCA 12v battery would need a load of no more than .024Ω to actually provide 500 Amps. This is why the cables running to the starter are so thick. This also explains why a 12v battery is safe to touch - your body resistance is much, much higher, so only a very small current will flow.

Edison was a genius, but he was also wrong on the AC/DC issue. At the time, AC was the only practical way to send power long distances, and he was too stubborn to admit it. The electrocution issue was pretty much a smokescreen. The reason AC can be sent over long distances has to do with Ohm’s law (see previous posts). AC can be stepped up in voltage easily with a transformer, and when the voltage is increased, the current is reduced at the same time. The lower current reduces resistive losses in the power lines. Today, there is technology to create very high-voltage DC to accomplish the same result, with some other advantages (reduced capactive /radiative losses), but this technology is quite recent.

As far as the bear goes - the power company knows how much voltage was on those lines, but figuring out how much current flowed through the bear would require a lot of assumptions. And as you noted, 50,000v sounds more impressive than 100A.

13

[QUOTE=lobotomyboy63]
Thanks, digesting all this (or trying).
[/QUOTE]

Here’s a link that is the exact opposite of what you want because it explains that it’s actually more complex and unintuitive than described (assuming the info on this web site is ok), but I found it interesting when I stumbled on it a few months ago:

http://amasci.com/elect/elefaq1.html#ae

14

[QUOTE=beowulff]
You touch on a lot of different issues, some of which are non-trivial.
I don’t remember the scene in Apollo 13, but it’s not very realistic to have a reading of 0V at 15A. I suppose the voltage might be very low so that it was close to zero…
[/QUOTE]

Right, I guess they might have had .01 volts or something and their instruments weren’t sensitive enough to show it. They were freezing up there of course, and cold raises hell with batteries. Many electrons in the battery but moving very slowly…low voltage but the amperage remains.

[QUOTE=beowulff]
Comparing a table lamp to an iron: Yes, they both run off of 120v, but one has much less resistance than the other, so it draws more current. When you draw more current at a given voltage, you consume more power, or Watts. A Watt is a Volt times an Amp, so you can figure out how much current an appliance uses by dividing it’s Wattage rating by 120V. As has been mentioned before, AC voltage is more involved, but for power calculations, the AC voltage in RMS gives the correct figure (the RMS value is the value for an AC voltage that gives the same power as a DC voltage).
[/QUOTE]

Thicker wire=less resistance, right, i.e. more paths for the electrons to follow? So something drawing a lot, like an iron, is going to have more/thicker copper (or other metal) wire for the electrons than a lamp?

Supposing you put a wimpy wire on an iron…because of the amperage it draws, would it heat, then melt the wire?

[QUOTE=beowulff]
Your dryer or range uses 240v, not because 100v wouldn’t work, but because the current necessary to provided the required power would be so large that the wires would get too thick and expensive.

[/QUOTE]

I always supposed it was really two outlets, i.e. double the wires to carry the electricity to it.

[QUOTE=beowulff]
Car batteries are a whole other issue. The CCA is a rating which measures how much current the battery can provide to a load in cold weather. But, since the battery is only 12v, that load must be a very low resistance. So, a 500CCA 12v battery would need a load of no more than .024Ω to actually provide 500 Amps. This is why the cables running to the starter are so thick. This also explains why a 12v battery is safe to touch - your body resistance is much, much higher, so only a very small current will flow.
[/QUOTE]

One little tricky bit I ran into while shopping for stereo equipment is that specs can be easily misinterpreted by the uninformed. For example, my car’s stereo claims 270W of power…but that’s at 4 ohms, which is typically what car speakers are rated at. IIRC that’s like 135W @ 8 ohms, which your home speakers are rated at, yes? Your description sounds similar.

[QUOTE=beowulff]
Edison was a genius, but he was also wrong on the AC/DC issue. At the time, AC was the only practical way to send power long distances, and he was too stubborn to admit it. The electrocution issue was pretty much a smokescreen. The reason AC can be sent over long distances has to do with Ohm’s law (see previous posts). AC can be stepped up in voltage easily with a transformer, and when the voltage is increased, the current is reduced at the same time. The lower current reduces resistive losses in the power lines. Today, there is technology to create very high-voltage DC to accomplish the same result, with some other advantages (reduced capactive /radiative losses), but this technology is quite recent.
[/QUOTE]

He never shoulda pissed off Tesla, according to the documentary.

[QUOTE=beowulff]
As far as the bear goes - the power company knows how much voltage was on those lines, but figuring out how much current flowed through the bear would require a lot of assumptions. And as you noted, 50,000v sounds more impressive than 100A.
[/QUOTE]

The electrocution issue has a lot to do with the path the electricity finds in the body, right? My bro said that one electrician’s trick is to use one hand—if electricity surges through your thumb and out your index finger, you can survive. But if the circuit is from hand to hand (i.e. through the body), it goes through the heart and the prognosis isn’t so good.

And on a related note, I got a demonstration recently of how much power batteries can have. I was taking some pictures and I had to change batteries.
They weren’t totally drained, but the flash recycle was taking too long. I use rechargeables so I put the two weak AA’s in my pocket, replaced, and continued.

A few seconds later, the heat on my thigh was UNBEARABLE. Short story long, as near as I can guess, the change in my pocket had created a short circuit between the batteries. I pulled them from my pocket but couldn’t even hold onto them—I dropped those suckas on the floor and it was five minutes before I could consider picking them up.

So I don’t know if you can really electrocute anybody with DC, but I’m pretty sure you could cook them.

15

[QUOTE=lobotomyboy63]
So I don’t know if you can really electrocute anybody with DC…
[/QUOTE]

Oh, you absolutely can. TVs and x-ray equipment contain DC voltages more than sufficient to kill you instantly; as much as 150 kV or more in the case of the x-ray power supply.

16

[QUOTE=ExTank]
That’s why a 50,000 volt static shock doesn’t do much more than make you jerk your fingers back in surprise; you’re talking miliamps (or even microamps).
[/QUOTE]
This comes up every time in a discussion like this, where someone eventually says, “It’s not the volts that kill you, it’s the amps.”

Amps are units of rate of flow. That is, if you want to continue with the flawed-but-instructive fluid analogy, it’s like “gallons per minute.” An amp is 1 coulomb per second (a coulomb is a unit of charge, like a gallon is a unit of volume).

So it doesn’t make sense (to me) to talk about 50,000 volts resulting in a flow of microamps through a person. For 50K volts to result in a flow of as much as 1 amp, you would need a load with 50K ohms. To get it down to microamps you’re talking 50M ohms.

Do you mean you’re talking millicoulombs, because the rate of flow is sustained for such a short time?

Also a 50K volt static shock is static, not current. Not sure how that comes into play.

(I am not a EE but took a circuit analysis course before I switched to Computer Science, so I know enough about electricity to be stupid.)

17

[QUOTE=ExTank]
Voltage is the potential, or energy differential, between two (or more, sometimes) electrically charged poles. The polarity needn’t even be different. A pole with a -5 volt charge and another pole with a -10 VDC charge still has a 5 VDC difference.
[/QUOTE]

This is a bit confusing. To keep it simple, it’s best to say that, at any instant of time, a single voltage exists between any two points.

[QUOTE=ExTank]
Amperage is the actual electrons flowing through the wires.
[/QUOTE]

This is true for wires. In general, “amperage” is the flow of charge.

[QUOTE=ExTank]
That’s why a 50,000 volt static shock doesn’t do much more than make you jerk your fingers back in surprise; you’re talking miliamps (or even microamps).
[/QUOTE]

50,000 “static” volts doesn’t shock you because the Thévenin Equivalent Source Impedance is extremely high.

[QUOTE=ExTank]
VAC works a little different than VDC. VAC has all kinds of measurements: peak voltage, peak-to-peak voltage, average voltage, and rms voltage.
[/QUOTE]
DC can also have those quantities (e.g. a pulse train where the voltage never dips below zero volts). “DC” simply means that the current never switches direction.

[QUOTE=ExTank]
Since VAC switches polarity at regular intervals (~60 Hertz here in the U.S.A.), it is characteristically described as a sine wave.
[/QUOTE]
AC simply means that the current switches direction at regular intervals. It need not be a sine wave.

[QUOTE=ExTank]
Average voltage (Vavg) is a mathematically derived formula that averages the voltage over one-half cycle of the sine wave (the average over the full sine wave would, of course, be zero), and is considered to be .637 Vp.
[/QUOTE]
That’s only true for a sine wave. Alternatively, you can define the average voltage as the average over one period. If you use this definition, the average voltage at your 120 VAC receptacle is 0 V. Interesting, huh? (But the half-cycle definition for average is usually more useful, since many loads don’t care about the direction of current. An incandescent light bulb, for example.)

[QUOTE=ExTank]
RMS Voltage stands for “root mean square,” and is used to approximate a DC equivalent of an AC voltage. An AC rms voltage will produce the same heating effect as a DC voltage through an equal resistance. For instance, a 120 VAC source has a:

120 VAC * .707 = 84.84 Vrms value.
[/QUOTE]
No, 120 is the rms voltage for your home receptacle. The peak voltage, as pointed out by Q.E.D., is around 170 V.

18

[QUOTE=Q.E.D.]
Oh, you absolutely can. TVs and x-ray equipment contain DC voltages more than sufficient to kill you instantly; as much as 150 kV or more in the case of the x-ray power supply.
[/QUOTE]

Here again, back to volts killing you instead of amps, which puzzles me.

But okay, I know capacitors can store energy, so if you had some old tube TV you better leave it unplugged for a couple days before attempting to service it yourself.

The post that follows QED’s kinda makes sense…the duration of static electricity is so quick that I figured the amperage would be nominal. I always (incorrectly, I guessed) figured voltage x duration = amps and divide all that by resistance or something. Divided by…Avogadro’s number and…multiplied by…the number of Ford Pintos still on the road. Subtract…something.

Another thing I have experienced: take a camera out on a cold day. The batteries give up. Take it back inside, warm up the batteries; they’re fine. So the voltage has gone down—not enough flow to make the camera work—but the amperage never changed. Right?

Seems like D batteries ought to have more amperage than AAA batteries, all other factors being equal. I say this b/c if not, why do we have different batteries? A big boombox needs more electrons than a pen flashlight…more amps.

19

[QUOTE=Crafter_Man]
That’s only true for a sine wave. Alternatively, you can define the average voltage as the average over one period. If you use this definition, the average voltage at your 120 VAC receptacle is 0 V. Interesting, huh? (But the half-cycle definition for average is usually more useful, since many loads don’t care about the direction of current. An incandescent light bulb, for example.)
[/QUOTE]

(Underlining mine). Some years ago, the powers that be decided to “polarize” outlets and plugs, right? Used to be you could plug either prong into either slot; not any more. Is this related to direction of current?

20

[QUOTE=lobotomyboy63]
Here again, back to volts killing you instead of amps, which puzzles me.
[/QUOTE]

Well, the thing here is that while the current does the “work” of killing you, in order to get that lethal level of current (which is generally considered to be approximately 100 mA through the heart–this, too, varies wildly depending, among other things, at which point in the cardiac cycle your heart is) you need a certain number of volts to push it. How many volts that is varies wildly depending on factors such as how dry your skin is, contact area and where on your body contact is made.