Electricity Q&A: About Electrification

[bentzy]

Hi, coleagues.

I'd like to hear others' opinions regarding these:


a. why we don't get electrified when just touching an exposed conducting wire, despite the fact that current does flow thru us ? (while the touching body is kept isolated and unearthed).

b. when an exposed wire is being touched by a bare hand (in principle), can the hand lit a bulb while unearthed ?

c. the danger in touching a wire while earthed is due the immense current due to the voltage falling on the body, according to [[Ohm's law]] (since the voltage can reach hundreds to thousands of Volts). Does this explanation describe the situation, correctly & fully ?

Thank you.

 

[willem2]

a. If a body that is isolated from the Earth touches an exposed live wire there won't be current flowing through it, so that's why it doesn't get a shock.

b. no,see a.

c. voltage of residential installations is somewhere between 110 and 240 volts, depending on what country you're in. If your skin is dry, and the contact area isn't very large, the current is likely below the 100 milliampere that will kill you. Even with wet hands touching a large surface area, I wouldn't call the current immense, but a current that can kill you isn't all that big.

 

[CWatters]

a) The human body has a capacitance to Earth so even when insulated (eg by shoes) some AC current can flow through the body to ground. However the capacitance is quite small. I think it's of order 10-100pF.

At 50 or 60Hz I believe you need >30mA to kill you (which is why RCD breakers are typically 30mA).

I'll let you work out the impedance of a 50pf Capacitor at 60hz and then the voltage required to push 30mA through it. You will find that the voltage needed is quite high.

b) YES. However it won't work with an ordinary light bulb. Electricians sometimes use a screwdriver with a neon bulb in it...

http://www.intaheat.co.uk/product/regq600_neon_mains_test_screwdriver

I don't recommend you play with one of these. Too dangerous.

c) No. You should explain that the body uses electricity to control the muscles including those of the heart. The electric shock dissrupts that mechanisim causing the heart to beat irregularly or stop totally.