MadSci Network: Engineering

Re: What is dangerous to a person, amps or volts?

Date: Sun Jul 5 00:39:28 1998
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Engineering
ID: 896992953.Eg

This is a great question. Electricity has been a part of everyday life for 100 years, but few really understand it. Before we get into biological effects, lets review what Volts and Amperes measure. Voltage measures potential difference, it's a lot like pressure in a water pipe. Current or Amperage is the rate of flow of electric charge, it's analagous to flow rate of water in a pipe. 1 Ampere is eqivalent to 6.28x10^18 electrons/second. The Volt is named after Count Alessandro Volta and the Ampere is named after Andre Marie Ampere.

The current flowing in a linear, time invariant circuit is given by Ohm's Law named after George Simon Ohm:

V (voltage) = I (current) x R (resistance)

Resistance is exactly that, the resistance to the flow of electricity. High resistance is like a narrow pipe, it takes more pressure (voltage) to push the same amount of water (current).

We live in a world of Voltage sources, often in the form of batteries or generators. The voltage from these sources is more or less fixed. The current delivered depends on the total resistance of a closed circuit which includes the internal resistance of the voltage source. A 12 volt lantern battery has a much higher internal resistance than your car battery, which means that your car battery can deliver significantly more current than a lantern battery. Even if I was to apply a dead short of zero Ohms across the terminals of a car battery, the current will typically not exceed 500 amps. The generators at the power station also have an internal resistance, although it is quite small.

So what does all this tell us about the dangers of electricity? There are a couple of ways electricity can hurt or even kill. The first is stopping the heart or causing fibrillation. Fibrillation is an erratic beat which does not pump blood effectively. Doctors often use an instrument called a de-fibrillator, which applies a calibrated electric pulse to the body, to hopefully restore a normal heart beat. The second danger of electricity is physical damage like burns or torn muscles and ligaments. Fibrillation does not require much power whereas burning and other physical damage does.

Now to the numbers. I've got a multimeter in front of me, so I'll measure the resistance between my hands. It reads about 1MOhm or 1 million ohms. It varies significantly depending on how hard I squeeze the probes and how moist my fingers are etc. If I was to put my hands on the terminals of a 12 volt car battery, the current flowing through my body would be about 12 microamps, not much. If I was to put my hands on the terminals of a 12 volt lantern battery, the current would still be about 12 microamps. The current is the same because the resistance of my hands is millions of times greater than the internal resistance of the batteries. It doesn't matter that the car battery can deliver 500 amps, my hands will only pull 12 microamps out of the battery.

A 120 volt wall outlet is a different story. If we were to apply Ohms law on a 120 volt circuit, about 120 microamps should flow through your body. That's still not enough to do much harm, but people are often killed by household circuits. The reason is that your skin resistance is not linear. At about 50 volts or so, the resistance of your skin drops dramatically. The current flowing through your body may not be limited by skin resistance, but by the ability of the voltage source to deliver current. In fact, you can feel a slight tingle at voltages as low as 24 volts.

Medical data shows that less than 100 milliamps is sufficient to push a human heart into fibrillation. If the resistance of human skin was linear, you would have to grab a 100,000 Volt circuit to get 100 milliamps. It turns out that 100 volts is enough. The product of voltage and current is power measured in Watts. 1000 watts will heat the elements in your hair dryer to hundreds of degress. If 1000 volts was to push one amp through your body, you wouldn't need a hair dryer. Of course, the 1000 volt source must be capable of delivering one amp.

On the other side of the spectrum, very high voltage is not necessarily dangerous. The ignition coil in your car or the flyback transformer in your TV will generate over 20,000 volts. If you were to touch either one, it won't be fun, but it probably won't do any serious damage. That's because the output voltage of either of these devices will drop significantly when you touch the terminals. This is because ignition coils and flybacks have high internal resistances, they can't deliver much power.

In my younger, more foolish years, I played around high voltage all the time. 20,000 volt shocks were routine. Once I accepted a 500,000 volt discharge from a home built Telsa coil. I'm here to tell you it was not enjoyable, but I didn't suffer any long term effects, I think ;-)

So what's the answer? Is it the volts or the amps? Basically, it's both. Low voltage (less than 50 volts) won't hurt you because of high skin resistance. High voltage can cause a heart attack if the source can deliver moderate current (100 milliamps). High voltage coupled with the availability of high current (like power lines) can result in lethal shock as well as serious burns and tissue damage. Electricity is an inanimate beast without compassion or remorse. It must be respected at all times. Take it from someone who has been educated the hard way.

If this wasn't confusing enough, drop me a line at

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