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Why can biting on aluminum foil be painful? | July 23, 2010

 
Interviewee

Welcome to BrainStuff from HowStuffWorks.com, where Smart happens.

Marshall

Hi, I’m Marshall Brain with today’s question. Why is it that biting on aluminum foil can be painful? Biting on aluminum foil can be painful, and it’s usually noticed if you have metal in your mouth from things like fillings or crowns. Basically, when you bite on foil, you set up a battery in your mouth, and the electrical current stimulates nerve endings in your tooth. That generates pain. Specifically, here’s what happens. First of all, the pressure from biting brings two different metals – in this case aluminum foil and mercury in your fillings or gold in your crowns – in contact with each other in a moist, salty environment. The salty environment is provided by your saliva. The two metals have an electrochemical potential difference between them – or a voltage – that gets generated because they’re coming into contact with each other. The electrons flow from the foil into the tooth, so you have this electrical current created by this little battery flowing into your tooth. The current gets conducted into the tooth’s root, usually by the filling or crown, and the current sets off nerve impulses in the root’s nerve. Those nerve impulses go to your brain, and then finally the brain interprets those impulses as pain.

The production of the electric current between two dissimilar metals like this is called the voltaic effect after Alesandro Volta who discovered it. Long ago, when he was creating early batteries, he made them by stacking dissimilar metal disks with blotter paper soaked in salt water between them. This was called a voltaic pile and it was an easy way to create a simple and very low-power battery.

If you have no metal dental work in your mouth, you’re not going to feel this effect. You’ve got to have these two dissimilar metals coming together for you to feel the pain that aluminum foil causes.

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