OFFICIAL46 What does the professor imply about the green patina that sometimes appears on copper?

[00:00.00]NARRATOR: Listen to part of a lecture in a materials science class.
[00:03.70]MALE PROFESSOR: So what's the first thing that comes to mind when we talk about uses for copper? Tammy?
[00:08.82]FEMALE STUDENT: The penny? It's made of copper…
[00:10.04]MALE PROFESSOR: Okay, good one… [00:11.69]but what’s a one-cent coin worth these days?[00:14.24] You might get back change, [00:16.31]like if you go to the store and give the cashier five dollars for something that costs four dollars ninety-eight cents, you'll get two cents back…but two cents doesn't buy much. [00:25.75]The value of the penny in terms of what it'll buy has gotten so low that there's actually a move afoot to eliminate the coin from U.S. currency.[00:34.21]But there's more to it.
[00:35.30]As Tammy implied, the penny looks like it's solid copper. [00:38.72]It's reddish orange, with a bright metallic luster when it's new; but that's just the copper plating.[00:44.23] The penny's not solid copper;[00:46.47] in actuality, it's almost 98 percent zinc.
[00:49.46]But, um, given the rising value of both these metals, each penny now costs about 1.7 cents to produce…[00:58.94]so it generates what’s called negative seigniorage.
[01:02.44]Negative seigniorage is when the cost of minting a coin is more than the coin’s face value. [01:08.08]Even though the penny generates quite a bit of negative seigniorage, there’s concern that if it is eliminated, we’ll need more nickels—because more merchants might start setting prices in five-cent increments… four dollars ninety-five cents, and so on. [01:22.18]So we need a trusty five-cent piece that can be minted economically. [01:26.61]But the nickel’s negative seigniorage is even worse than the penny’s…each nickel costs the U.S. Mint ten cents to produce!
[01:34.21]Also, some of us are pretty attached to pennies for whatever reason… nostalgia, and then there's collectors… [01:40.67]and people, if they see a penny on the sidewalk, they'll pick it up and think, “It's my lucky day.”
[01:45.57]Another scenario is that without pennies, merchants, instead of charging four-ninety-eight, might round up the price to an even five dollars. [01:54.25]So consumer goods would become slightly more expensive. [01:55.58]But, on the other hand, some cash transactions would be more convenient for consumers. [02:01.49]And, as I said, the government would save money if pennies were eliminated.
[02:05.17]FEMALE STUDENT: But wouldn't the copper industry suffer financially if the U.S. government stopped buying copper to make pennies?
[02:10.88]MALE PROFESSOR: But how much copper do pennies actually contain?
[02:13.88]FEMALE STUDENT: How much…Oh, got it…  right.
[02:17.84]MALE PROFESSOR: So, what else comes to mind when you think about copper?[02:21.23]What else is copper used for?
[02:23.74]FEMALE STUDENT: I know that copper can be shaped into all sorts of things: sheets... tubing.... My cousin’s house has a copper roof.
[02:30.42]MALE PROFESSOR: Yes, like gold and silver, copper’s extremely malleable, but it’s not a precious metal; it’s far less expensive than gold or silver. [02:39.54]It's also a superb conductor of electricity, so you can stretch it into wires, which go into appliances and even car motors.
[02:46.75]Copper also has superior alloying properties—it's, y'know, when it's combined with other metals.
[02:53.00]For instance, how many of you play a brass instrument, like a trumpet or trombone?
[02:57.77]Well, brass is an alloy of copper and zinc. [03:01.42]If your trombone was made of pure copper or pure zinc, it wouldn't sound nearly as beautiful as a brass trombone.
[03:07.37]Another alloy, a combination of copper and nickel, resists corrosion… [03:11.73]it doesn't rust, even with prolonged exposure to water.
[03:14.30]FEMALE STUDENT: But what about the Statue of Liberty, in New York Harbor?[03:17.18] It's made of pure copper, but it turned green. [03:19.95]Isn't that a sign of corrosion?
[03:21.74]MALE PROFESSOR: Indirectly. If copper's exposed to damp air, its color changes from reddish orange to reddish brown.
[03:28.90]But, in time, a green film called a “patina” forms, and the patina actually serves to halt any further corrosion. [03:37.63]It's one reason that ship hulls are made of copper-nickel alloys. [03:41.60]These alloys are also hard for barnacles to stick to. [03:44.45]If these little shellfish adhere to the hull of a ship, it produces drag, slowing the vessel down.
[03:50.67]Copper's also a key material used in solar-heating units and in water-desalination plants, which are playing increasingly important roles in society.
[03:58.74]Bottom line? If you're a copper miner, you won't lose any sleep should the penny get—[04:03.40]if you'll excuse the expression—pinched out of existence.