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Re: [Phys-l] Electrocution in Bathtub - Hollywood style



A demo can be set up to help feel this on a small scale.

Probably catch some negative comments, but during my internship I was taught a demo that I now call the electric fish tank of death. Dramatic and effective.

Set up 10-20 gallon tank filled halfway with fresh water.

Stand metal plates at each end and attach the hot and neutral of 120V AC line voltage to the plates through a GFI. Alligator clips work fine.

Place left hand on top of head and leave hand there while tank is plugged in.

Use an AC field sensor to show that the water is 'live' - no Hollywood sparking blue fire steaming water.

Standing on a non-grounded floor with rubber shoes, insert two fingers (of the right hand!) into the water keeping the fingers together. The AC sensor will show that you are now 'live'. Do not lean on the tank, counter or contact anything else. Students are stunned at your continued existence.

Separate the fingers into a inverted peace sign configuration with the tips of the fingers wet and parallel to the metal plates. (left hand still on head - or in pocket). No tingle since no potential difference.

Repeat with the fingers perp. to plate. Tingle grows with separation. Painful if hangnails or cuts.

Other demos:
Connect a test wire to ground and drop in tank - GFI trips.
Connect two wires to a bulb and mount on a compass-like (the drawing kind) rig to show when the bulb will light when it will not. Mine is made from two rulers fastened on one end with the wires running down the rulers. Use in place of fingers for the faint of heart.
Discuss how putting a finger from each hand into the water will NOT trip the GFI and WILL place a current across the heart.
unplug the plates and drop a incan. bulb in. Plug the bulb in. Try it with a sacrificial hair dryer - it runs underwater just fine.


It's not called the fish tank of death for nothing. Extreme care must be taken, but no more relative danger than if one were using a tablesaw. I let students volunteer to do this with direct supervision. I think they take more of a hit from the trick shocking pens they keep bringing to school - those really hurt.

I have always wanted to see if a fish would learn (quickly) to swim parallel to the plates, but that seems too cruel.

As to the bathtub, I would not like be in a tub with a grounded drain and my body between an appliance.

Scott



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Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
603-942-5531e218
sgoelzer@coebrownacademy.com
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