MadSci Network: Engineering

Re: What is the amperage of a typical Van de Graaf Generator?

Date: Tue Mar 5 15:15:26 2002
Posted By: Steve Taylor, Professional Engineer
Area of science: Engineering
ID: 1014586147.Eg

If you are doing a project with any electrical equipment it pays to
understand the concepts of VOLTAGE and CURRENT. A Van De Graaf generator
supplies enormous VOLTAGES, but very low CURRENT, unless extra (dangerous)
measures are taken.

Lets say at the outset, that, for all practical purposes CURRENT is what
kills. It takes voltage to drive current, so you can't have current without
some voltage, but current, particularly if directed through the heart can
kill. It takes as little as 0.001 amp (1mA) to kill a sick person, but
often more than 100mA to kill a fit person. In fact what matters more is
the ENERGY  transferred, which is related by current and voltage as energy
= voltage x current x time. This is why defibrillators, those miraculous
devices beloved of Hollywood for resuscitating the "dead", or certainly the
soon to be dead are calibrated in JOULES (units of energy), not voltage.  

Your mileage may vary. Don't try this at home boys and girls.....

Your question seriously confuses voltage and current. A water tank analogy
always helps here. Take a very large tank of water, so big that there is no
appreciable change in its level as water flows out of it. Make a hole in it
near the top. A trickle of water runs out. The PRESSURE of water behind the
hole causes a CURRENT of water to emerge.If you gather the water in a
bucket, you have stored CHARGE. The rate of flow of charge is the
definition of current. 1 Coulomb of charge flowing in 1 second is called 1

Knock another hole in the tank, lower down, the water jets out further and
more comes out. The energy in the emerging stream has increased. 

At the risk of pushing my analogy way too far, note that all real energy
sources allow current out of the tank through a hole, the available power
is a function of the size of the hole ! 
Your VDG generates lots of volts (pressure), but not much flow rate
(current). Changing the output circuits may increase the capacitance, and
an engineer should determine if the proposed change can create a hazard.

Now what REALLY causes problems with electrical safety is the storage of
electricity in Capacitors.A capacitor can deliver a virtually unlimited
current. A capacitor is like a bucket of water that is emptied quickly....

Your 30 Amp figure for a human  body discharge is actually pretty credible-
and it can be fed from 20,000 Volts or more, but the capacitance of a human
body is very small indeed, and not much energy is stored in it. When you
discharge the stored charge (at high voltage) to the door handle, a PEAK
current of 30 Amps or more can flow. The capacitance is so low that  the
energy transfer is only sufficient to make you yelp. Energy in a capacitor
is related by E=1/2 * C*V^2

I do not know the size of the spheres on your VDG, but the size determines
the capacitance of the spheres, and yours are unlikely to be more than 4 or
five inches across. Not knowing that means I can't estimate the
capacitance, I don't know the voltage, and I don't know what you are
discharging from, so I can't estimate current ! Old VDG could have
incredibly large spheres and have a very high energy storage capability.
These things would blow you to a plasma !

Basically don't worry. Though the volts are scarily high, the current kills
you, and by design there isn't enough energy to harm you. 

But it will make you jump if you aren't careful.....


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