# Dangerous Voltage

1. Jul 14, 2004

### FinalAvenger

According to this government page 50 volts can be considered "Life threatening." However, I read this article which states in the 4th paragraph that you can charge yourself with thousands of volts by rubbing on the carpet. If 50 is deadly, how come you don't die when you generate static electricity?

Also, is it safe to use the bottom prong of an outlet as grounding for any project or should it only be used with plugs?

Thanks.

2. Jul 15, 2004

### Integral

Staff Emeritus
It is current that is fatal, not Voltage. Something on the order of 50mA of Current through the heart is enough to cause fibrillation, which can be fatal. It does not matter what voltage is applied to achieve that current. I believe that skin resistance averages around 50k Ohms, so 50V through 50k Ohms would produce the fatal current...BUT... if your source is current limited to something LESS then 50mA, no amount of Voltage can be fatal. Thus the normal static shock which can be on the order of 10 - 40 kV is not fatal because it is very low current.

3. Jul 15, 2004

### FinalAvenger

Ohhh. I guess those "High Voltage" signs are misleading than because that's not the real cause of death?

They say that in the US each house gets 120 Volts of power. But is it possible to tell how big the current is?

4. Jul 15, 2004

### chroot

Staff Emeritus
Well, you can't have current without voltage, and current is proportional to voltage in resistive circuits. In other words, "high voltage" should be considered just as dangerous as "high current." Also, it makes sense to make a sign that says "high voltage," because there exists high voltages. There aren't necessarily any high currents until an unlucky person steps in and completes a circuit.

You are confusing your units. 120 volts is a unit of electromotive force, not power. You can find the current for a purely resistive device (like a heater, toaster, hair dryer, etc.) by Ohm's law:

V = I * R

where V is the voltage in volts, R is the resistance in ohms, and I is the current in amperes.

Power can be defined as:

P = V * I

in which P is in watts when V is in volts and I is in amperes. The amount of power being delivered to house depends on what's plugged in. If nothing is plugged in, the resistance is infinite, no current flows, and no power is delivered. As more and more things are plugged in, the total resistance drops, current increases, and power delivery increases too.

- Warren

5. Jul 15, 2004

### FinalAvenger

Ahhhh. I think I understand it better now. It just seems that the word "Volts" is used to measure electricity more than amps or ohms even though they are equally important. For example when talking about batteries it's common to hear "X volt" rather than "X amps".

6. Jul 15, 2004

### chroot

Staff Emeritus
Once again, the current depends on what you plug into your battery. Your battery always presents 12 volts of electromotive force. If you plug in a 1 ohm load, you'll be drawing 12 amperes of current, for a total of 144 watts. If you plug in a 12 ohm load, you'll be drawing only 1 ampere of current, for a total of 12 watts. You are probably already familiar with the concept that you can run a cell-phone for a very long time from a large battery, but you might not be able to run a bright light for very long at all. The cell-phone draws only a meager current, while the light bulb draws a very large one. Current depends upon load.

Now, with that basic concept explained, I should mention that sometimes you will see current ratings on batteries. One rating might be the maximum current the battery can provide. Real batteries are not perfect, and actually have some unavoidable resistance built into them. A battery with a low resistance can deliver a much higher current than a battery with a high resistance. This is often the reason why some battery-powered devices need D cells, while others get by fine on AAA cells, even though both types provide 1.5 volts. Sometimes the D cell device needs to operate longer, and D cells certainly contain more energy. Sometimes, however, the D cell device doesn't need to last any longer than an AAA cell device, but needs to consume more power while it's operating. D cells have a lower resistance than AAA cells, and thus can deliver more power.

You will also see specifications of amp-hours. Your car battery, for example, probably is rated at somewhere between 20 and 60 amp-hours. This is not a measure of current! An amp-hour is actually related to a measure of energy storage capacity of a battery. A battery rated at 1 amp-hour can deliver one ampere for an hour, or half an ampere for two hours, or a quarter of an ampere for four hours. If you know the battery's voltage, it's a simple matter to calculate the total power content. A 12 volt battery, for example, delivers 12 watts of power when one ampere is drawn from it. If the battery is rated at 10 amp-hours, then can deliver 12 watts for 10 ten hours, and thus contains 120 watt-hours of energy.

- Warren

Last edited: Jul 15, 2004
7. Jul 15, 2004

### FinalAvenger

AHhhh. That makes sense :D The only part I guess I don't really understand well is the resistance.

I think what kind of has me thrown of is the fact that I read this:

I think of the resistance as the wire size so I don't really see a difference between high and low resistance.

8. Jul 15, 2004

### chroot

Staff Emeritus
Wire size is not a good way to think of resistance. It's true that a small wire has a larger resistance, but not nearly as much as you'd think. For most purposes, wire is wire, and wire has no significant resistance.

The parts of an electronic system that provide resistance are the light bulb filaments, the motors, etc. Computer chips and so on also act as (complicated) resistances.

- Warren

9. Jul 15, 2004

### FinalAvenger

So basically the only way to get the ohms is to multiply the amperage by the voltage?

10. Jul 15, 2004

### chroot

Staff Emeritus
No. As I've already said, the relationship between volts, amps, and ohms is:

V = I * R,

where V is the voltage, I is the current, and R is the resistance.

Volts * amps = watts, a unit of power.

- Warren

11. Jul 15, 2004

### FinalAvenger

Ok got it. Is there any way to figure out say, how many volts or amps something is using (like this computer)?

12. Jul 15, 2004

### chroot

Staff Emeritus
Well, the computer's plugged into the wall socket, which provides 120 volts. The only way to find out how much current it is using is to use an ammeter, a device which measures current.

- Warren