A current flowing through you of more than 5 ma is considered dangerous. Why do
ID: 1454384 • Letter: A
Question
A current flowing through you of more than 5 ma is considered dangerous. Why do we see warnings about high voltage, rather than high current? What voltages are dangerous even if the skin is dry? (Dry skin has a resistance of 100 kohms while wet skin can be much less.) An electroencephalogram (EEG) is a recording of differences in voltage observed on the scalp by attaching electrical contacts. If voltage differences that are observed are about 0.5 mv, and the resistance of the skin is about 10 kohms, what size currents are involved?
Explanation / Answer
The reason is that, voltage is the force that will cause current to flow if it can. Voltage can exist without current flow. It is difference of potential. Just like at the outlet of a wall plug with nothing plugged in. Current is the actual movement of electrons in a conductor. Voltage will cause current to flow it a complete circuit path exists. There can be no current without voltage and a place for current to flow. Let's look at the last element of a basic circuit - resistance. The resistance is just that - the inability of a circuit to perfectly conduct current. It's the opposition to current flow. The last piece of the puzzle is that voltage (in volts) equals current (in amps) times resistance (in ohms). This isn't hard. Just follow along. If we have a 1 ohm resistance, and we have 1 amp of current flowing through it, we have 1 volt pushing that current. It has to be like that. We have to have the 1 volt to get the 1 amp to flow through the 1 ohm. Simple and easy. Now keeping it simple, but without the numbers, if we double voltage, we double current (through the same circuit). Triple the voltage means triple the current. Again, simple and easy. The higher the voltage in a circuit, the more current that will flow through a given resistance. If we don't have a circuit, we can't have current flow. So what good is a warning of high current? It does have some applications, but they are narrow ones. It is thehigh voltage that has a potential to cause high current if a current path is provided. And it is the high voltage that we must warn anyone around the equipment about. The high voltage is the thing that will reach out and touch us if we do something improperly. It has the potential (because it is a difference of potential) to do some real damage if we give it a chance.
Since current flowing in our body more than 5 mA is dangerous.
So, calculate V = IR = 5x10^-3x100x10^3 = 500 V
So, in the case of dry skin voltage more than 500V is dangerous for us.
Part Three:
The requisite current, I = (0.5x10^-3)/(10x10^3) = 0.05x10^-6 A = 5.0x10^-8 A.