Science Buddies: "Ask an Expert"

We are doing Batteries Make Cents. Surprisingly, old pennies, made in 1978 and mostly copper, make much weaker batteries than new pennies that are copper plated. We re-ran this 3 times so we know it's accurate. We have a theory that old pennies, being an alloy all the way through, actually slow down the current because the zinc mixes with the copper, whereas new pennies, being plated in pure copper, allow better flow of current despite the zinc core, because the metals are not mixed. Any thoughts on this out there?

Hi dialupgirl,
Very interesting observations. I did a quick search on wikipedia to look at the composition of the penny and there's definitely a change around 1982. https://en.wikipedia.org/wiki/Penny_(Un ... ates_coin)

At any rate, if you look at the conductivity of brass and copper, I think you have your explanation for the observations.
One of our chemistry expert may be able to shed more lights on how the different composition of zinc and copper interact at the molecular level.

Good Luck and please let us know if you have any other questions!
Willey