Can't turn on my pc

[cyberfish]

My electronics teacher said a friend of his touched a screw driver to a big capacitor in a large speaker/amplifier one time... The guy was thrown across the room and the end of the screw driver was melted.

Was it a capacitor the size of a water tower?

I would imagine it takes a lot of energy to melt the end of the screw driver and throw a guy across the room (heat makes gas rapidly expand?). Capacitors don't store a lot of energy. They just can discharge very quickly. Their energy density is a few orders of magnitude smaller than that of batteries.

[cpjust]

The current they store is pretty small, but the voltage they can discharge is really high.
Feel free to prove my teacher wrong, but I'll just take his word for it.

[Akkernight]

Reminds me of when I touched a open wire from a freezer, I just stood there shaking for a little while, like in movies and such xP But I have to say it was refreshing, most likely 'cause it gave me adrenaline

[cyberfish]

Well, current would depend on the equivalent impedance (resistance) of the load, and is really not up to the capacitor .

It will just discharge faster, and the voltage will drop faster, if the current is higher (if you have infinite resistance, aka open circuit, and an ideal capacitor, it won't discharge at all), since for a capacitor, q = CV (where C is the capacitance, and q is charge stored), and delta_q = I*t. The total delivered energy will be the same (that's the conservation of energy).

[cpjust]

I haven't done anything with electronics since high school, so whatever you want to call it... The amount of electrons stored in the capacitor, or whatever.

[laserlight]

Originally Posted by cyberfish

current would depend on the equivalent impedance (resistance) of the load, and is really not up to the capacitor

Apparently, one of the teachers in a school of mine once said to a class of both boys and girls: "when the charged capacitor is all alone with the resistor, it cannot resist the temptation to discharge into it".

[abachler]

Originally Posted by cpjust

The current they store is pretty small, but the voltage they can discharge is really high.
Feel free to prove my teacher wrong, but I'll just take his word for it.

Assuming a 75 Kg individual, beign thrown 3 meters in lets say 3 seconds, nto realy thrown, mroe liek pushed, but for arguments sake. That woudl require 75,000 Joules of energy. Since a Coulomb can store more or less 1 Joule, that means you would need 75k coulombs of charge. Lets say the PSU was in Europe and was charged to full line peak voltage of 312 V. That means you would need a 240 Farad capacitor. Since the capacitors in a power supply are electrolytic, and rarely even approach 1 farad, that means your hypothetical capacitor would be at least 240 times the size of a normal power supply unit. Also remember that this is assuming 100% efficient transfer and conversion of the stored energy into kinetic momentum in the individual being thrown.

Your teacher exaggerated his facts, but is correct in telling you a story that will scare you into not doing something stupid like sticking a screw driver into a PSU to see what would happen.

[cpjust]

Originally Posted by abachler

Assuming a 75 Kg individual, beign thrown 3 meters in lets say 3 seconds, nto realy thrown, mroe liek pushed, but for arguments sake. That woudl require 75,000 Joules of energy. Since a Coulomb can store more or less 1 Joule, that means you would need 75k coulombs of charge. Lets say the PSU was in Europe and was charged to full line peak voltage of 312 V. That means you would need a 240 Farad capacitor. Since the capacitors in a power supply are electrolytic, and rarely even approach 1 farad, that means your hypothetical capacitor would be at least 240 times the size of a normal power supply unit. Also remember that this is assuming 100% efficient transfer and conversion of the stored energy into kinetic momentum in the individual being thrown.

Your teacher exaggerated his facts, but is correct in telling you a story that will scare you into not doing something stupid like sticking a screw driver into a PSU to see what would happen.

When people say they were thrown by electricity, it isn't the electricity itself that throws you, it's your own muscles that react to the electrical shock that throws you.

[Akkernight]

Originally Posted by cpjust

When people say they were thrown by electricity, it isn't the electricity itself that throws you, it's your own muscles that react to the electrical shock that throws you.

No no, it means you stand shaking for awhile and smoke comes from your hair, then you get sent across the room and into a bar or something that would 'cause extra damage... Maybe also light a fire that interacts with some gas or fuel and then have a massive explosion... That's what Hollywood told me.

[cyberfish]

The screw driver is a lower resistance path. Why would the current want to go through you?

Even if it did, assuming the hand is 2 cm away from the conducting core of the screw driver (shortest air path), and air has a dielectric strength of ~2MV/m, the capacitor will need to be charged to about 40000V to cause a dielectric breakdown (air partially ionizes and starts conducting, like in lightning).

And to melt, let's say, 1ml (8g) of steel, which has a heat capacity of ~500 J/kgK, heat of fusion of 247J/g, and melting point of 1500 degrees C. One would need

(1500-20)*0.5*8 + 247*8 = 7896J. That is a lot of energy (the maximum power your outlet can put out without blowing the fuse, for 5 seconds).

Originally Posted by wikipedia

Conventional electrostatic capacitors provide less than 360 joules per kilogram of energy density, while capacitors using developing technologies can provide more than 2.52 kilojoules per kilogram

[cpjust]

This sounds like a job for Myth Busters.