Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

The dangers of 'static'

Demonstration

Exploding a small film canister with a spark.

Apparatus and materials

Film canister

Piezo gas lighter

Ethanol

Eye protection for all 

Health & Safety and Technical notes


All students and staff to wear eye protection. Ensure that the students stand back with no-one in the line of fire. 
 
Although a high voltage is generated on the wires of the modified gas lighter, the electric shock hazard is minimal.

Ensure there is just fuel vapour in the canister. If the wires are wetted with alcohol they will not spark until they have dried. Visually check for a good spark by adjusting the wire spacing. 

Piezo gas lighter

Dismantle the gas lighter to expose the two connections inside. One will be joined to the metal outer casing, the other to the central electrode. 
 
Remove the central electrode and disconnect the metal casing. Solder about 6 cm of enamelled wire onto the central tip. Protect the soldered joint and insulate it with heat-shrink tubing. Replace the outer shielding. 
 
Solder another length of enamelled wire from the outer shielding forwards, the same length as the central wire. Strip the insulation from both wires at the tip. 
 
You may need to adjust the spacing of the wires to get a good spark.

Procedure


a First prepare the film canister by making two small holes in the lid. 

b Pass the wires of the gas lighter through the lid of the film canister. 
 
c Pour about 1 ml of ethanol into the canister and swirl it round to wet the sides. Pour out the ethanol, leaving just the liquid sticking to the surface, and replace the lid. Also check that the ethanol stock bottle is closed. 

d Hold the canister in your hand to warm it for about 30 seconds while you explain what is happening. Then hold the lighter at arm's length and ignite it. The ethanol should explode and the canister fly off. In a darkened room there is a visible flash as well as a loud bang.

Teaching notes


All sorts of things come out of this. It can be used as a way to emphasize the dangers of a static spark in a fuel-laden atmosphere, such as when refuelling planes. 

It can also be used to talk about energy transfers. Or as a start on kinetic theory - the hotter gas expanding and forcing the lid off as the pressure rises. 
 
The canister is noticeably very warm after the explosion.

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.