Last week I gave a very brief introduction to the fundamentals of electricity. I mainly focused on what an amp was and explained the dangers of even small currents. What I didn’t explain was how we get to amps from the other pieces of Ohm’s law: volts and resistance.
Have you ever plugged a flash drive or other USB device into your Apple USB keyboard and had your machine tell you there wasn’t enough power to run the device? It’s an annoying problem, but why does it happen? USB is primarily a data transmission bus, but it can also supply power. How much power? Well, the tiny wall adapter that charges your iPhone will output about 1A at 5VDC (5 watts). The iPad chargers are a bit beefier and output 2A at 5VDC (10 watts). Most USB ports on computers however are limited to 500mA. This is why plugging your iPad into your computer won’t necessarily charge it. It’s also why you get that warning about being unable to power the device. If you daisy-chain another device into a USB device like a keyboard, that single port has to supply power to both of those devices, and that may exceed the 500mA limit.
As I said last week though, between 0.1 and 0.2 can be lethal. How are people not constantly dying from USB-related electrocution? In order to have current, you need to have voltage. Voltage is the pressure that makes the charges move. Higher voltage equals higher pressure. Zero volts means zero pressure, and that means zero amps: I = V / R. 5VDC like from a USB port is not a lot of pressure at all. If Ohm’s law was I = V though, 5VDC would mean 5 amps! Yikes! But there’s one more piece: resistance. Every medium that electricity flows through has a resistance. Based on the equation, holding the voltage constant, a bigger resistance means a smaller current. We use copper wires because copper has a very low resistance. Therefore we can move the most charge with the least volts. But if you had enough voltage, you could make a circuit out of anything…even a human body, which is what happens during electrocution.
It’s difficult to measure the resistance of the human body. It depends on several factors including distance between the connections, and whether or not the skin is wet, or broken. Wet or broken skin will lower the resistance substantially. Common numbers given range from 100,000 Ohms on dry, normal skin, to just 1,000 Ohms on wet or broken skin. It’s all very complex and depends on many, many factors. However, these numbers alone are enough to show us why no one is dying from USB-related shocks. If we take the worst case scenario and say that we touched the positive and negative terminals of a powered USB connection on wet skin, how much current would flow? 5VDC / 1000 Ohms = 0.005 amps. You’d barely feel that. If it was done on dry skin, with a 100,000 Ohm resistance, you wouldn’t feel anything.
This is why high voltage is dangerous. The high resistance of your body is not enough to resist the higher pressure created by high voltage. The 120VAC from common household electrical outlets is orders of magnitude greater than the measly 5VDC coming from your computer. This is why you can get a pretty good shock if you touch live wires in a house. 120VAC / 1000 Ohms = 0.12 amps = death. Again though, the exact resistance provided by your body can vary widely. Whether or not the voltage is AC or DC affects the risk characteristics as well.
When you get into especially high voltages, like power transmission lines, microwaves, or other types of voltage step-up transformers, you simply don’t have enough resistance to protect you. The voltage pressure will overwhelm the resistance and create a deadly current. But even this isn’t the whole story. Did you know that your barbecue lighter lights the gas in its chamber using a spark gap created by a voltage potential of thousands of volts? Look out for next week’s article where I’ll explain some of the nuances of electrical systems and why no one is dying from barbecue-lighter-related electric shocks.