Ten Car Battery Presentation at Calgary Mini Maker Faire

I did a presentation at Calgary Mini Maker Faire using 10 car batteries to cover some points:

  • Which one hurts more, AC or DC?
  • Can we weld with car batteries?
  • Which one kills, current or voltage?

What the video to understand:

Well, the results are pretty clear, in my opinion maybe:

It is quite obvious that AC hurts much more than DC, again the reason being the capacitive property of the body. The impedance of a capacitor has a reverse relationship with frequency, meaning that it drops as the frequency increases. That is up to a certain point though as there are other properties like inductance. But for a capacitor, a higher frequency means a smaller impedance, and therefore a much higher current flow which hurts and damages the tissues easier.

Edison fought against Tesla and tried very hard to convince people against AC to sell his DC system. I’ve heard he killed many dogs to prove AC is more dangerous and everyone should use the safer DC electricity. But he lost the fight, mainly due to power transmission problems. It is not possible to transmit DC over long distances without the loss of much power, through conversions to high voltage and back. It is not possible to change DC level without first converting it to AC and every conversion means loss of power. But such problem doesn’t exist as much for AC because it is much easier to convert through transformers.

I showed that 120VDC can be touched. It can tickle or hurt a bit but definitely not as bad as AC. It also depends on where on the body you touch as different spots on human body show different resistances. The surface area of the contact matters a lot too. Putting that voltage on tong would be extremely painful and may permanently damage tong, considering a 9V on the tong already hurts very badly!

And again as I have always mentioned, it is voltage and current hand in hand that kill.

  • They are dependent and there is no current without voltage.
  • Voltage is the energy needed to move electrons and create current
  • 3 things contribute to death from electricity
    • high enough voltage to create high current
    • The frequency of the electricity (DC versus AC)
    • The duration of high voltage being present
  • You cannot have a high voltage source with limited current across the body. Say for 10kV you will have about 100mA through body because the body resistance is around 100kOhm. Because V = R x I.
  • The reason electrostatic discharge that is at high voltages such as 10kV doesn’t kill is because the pulse duration is very limited to 1 micro second or smaller. Other wise it creates the same lethal 100mA level.
  • The reason birds sitting on 100kV power lines don’t die is because they are sitting on one line only, and the voltage across their body is zero. If they could sit on two wires they would fry right away. And that’s one of the reasons they separate the power lines so far, so that it is not possible for birds to short them.

And welding of course, can be done much better at higher voltage. The current capacity of the source is not necessarily enough for welding as low voltage causes problems because:

  • The heat generated for welding is the electrical power that is lost over resistance. But which resistance?
  • The arc created between the electrode and the metal is the resistor you need
  • Power is equal to current times voltage. So if the resistance is zero due to shorting the metal and electrode, then no matter how much current you have, the power will be zero and there will be no welding.
  • At higher voltage it is possible to create a longer ark, which means a higher resistance which creates more heat and welding is done easier.
  • Too much of a voltage can cause too much power though, which will probably make a mess of the welding!