Re: Juice that shocks me!!!! Let me set my questions up. I go to the local

Date: Thu Jul 30 19:23:05 1998
Posted By: Moataz Attallah, Undergraduate, Mechanical Engineering, American University in Cairo
Area of science: Physics
ID: 901172011.Ph

Message:

Dear sir,

In a reply to your question, I would like to say first of all that your question includes a part of fun and a part of science. You say that when you wake up at night, and you to drink your juice from the can, you find a spark coming out of the can. To tackle the question directly, the answer to what you ask about is called electric potential, and electric field. Electric potential by definition is: " The work per unit charge that an external agent must perform to move a test charge from one point to another without a change in kinetic energy."(Serway:680)The electric force on the other hand is defined as: "The electric field at a point is the electric force acting on a positive test charge placed at that point divided by the magnitude of the charge." Thus what happens when the charge moves from one point to another that it is moving from high potential to low potential. Visualize it as an example of a rich gives to a poor. Deep through the concepts of electric potential there is a famous example that lead to a conclusion that usually conductors of small areas have higher electric fields, this means that electric field is inversely proportional to area.

That lead to the discovery of a phenomenon called "Corona Discharge." This phenomenon is always seen near to sharp edges of conductors. It appears as greenish or bluish speak visible to naked eye. What really happens is that these sharp edges acquire a high electric field, up to 3E+06 V/m. At that high field, air around the can gets ionized. Thus the ions existing in air are attracted towards others of opposite sign in a high speed. When charges of opposite signs collide, the produce that spark. This collision is accompanied by the spark you see.

Does it only happen in metal cans? No. I will give you right now examples from real life. Let me ask you a question, what is the highly damaged structures in thunderstorms? The answer is that it is usually the towers(top points), churches towers, minarets, and clocks. Did you watch Back to Future I? Do you remember the closing scene? The thunderbolt shot the clock of the tower clock, actually the clock arms. This is attributed to the fact that the arms have very small surface area hence they create a large electric field. The thunderbolt finds itself dragged towards that high electric field, and "boom" happens! Actually I wanted to go through more details to explain why electric field is higher in case of smaller area, but I think this is beyond the scope of your question.

I know you want to ask, but how do the charge accumulate on the can? It has been proven for that for any conductor, charges accumulate on its surface only. You can try it yourself at home. Get a metal spoon, and hold it using any insulator. Rub it in your fine carpet, and then put it close to the door knob, your friend’s face, or wherever. You will find the same spark coming out.

I hope the answer is satisfactory, waiting for more inquiries.

References: Serway, Raymond. Physics for Scientists and Engineers. Saunders Collage publishing, USA. Third updated edition 1990 Moataz Attallah The American University in Cairo-Egypt Mizoa@aucegypt.edu Undergraduate Mechanical Engineering Undergraduate Teaching Assistant-Physics Unit

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