Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework-like questions.
A detailed explanation of what is and is not a homework (or homework-like) question, and how to ask one, is available on this meta thread ; what follows is a short summary.What is considered a homework question here?
A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved, rather than getting the answer itself. This includes not just questions from actual homework assignments, but also self-study problems, puzzles, etc.Can I ask a homework question here?
Yes, but there are a couple of things you need to make sure of first.
As a general rule, we do not discourage homework questions, as long as they are related to physics. But do keep in mind that Physics Stack Exchange is not primarily a homework help site ; it's a place to get specific conceptual physics questions answered. Thus, not all homework questions are on topic, and you should check the meta thread to see how to ask an on-topic one.
Additionally, if your question is part of a homework assignment or other such work under a honor code, you should check with your institution, and teacher whether it's OK to ask for help online and to post the problem on a public site.
Listen to Karl talk about Car Rust and How to Stop It
(You will need Real Audio which you can download for free )
Mark had rust on his car, and wanted to know if he could put some kind of sacrificial metal onto his car to stop it from rusting. After all, this "sacrificial metal" trick works on boats and ships and bridges. Are the car companies simply not doing this, so their cars rust sooner, and we have to buy more new cars.
The process is called "cathodic protection". You attach another metal which will corrode first, before the iron in your car, bridge or boat. The sacrificial metal that's usually used is zinc. As the iron turns into rust, it gives up electrons. If there's a lump of zinc nearby, the iron get the electrons back from the zinc and so stays protected, while the zinc begins to corrode away.
For this process to work, you need a complete electrical circuit to bring the electrons back. In the case of an outboard motor on a boat, the sea water completes the circuit. In the case of a bridge, the wet soil completes the circuit. But in your car, the only way to complete the circuit on all the metal in your car is to drive into sea water!
There are various products on the market claiming to provide cathodic electrochemical protection to your car, just by injecting electrons into your metal work - but they don't work. The FCC in the United States of America has actually got court orders to stop these products from being sold - simply because they don't work.
In your car, there are lots of little nooks and crannies where dirt and/or water can collect. The rust happens not where the metal is dry, nor where the metal is wet - but at the interface between the wet and dry metal. So if you screwed a lump of zinc right on the interface, it would protect it. But you'd have to have little lumps of zinc all over your car.
Modern car manufacturers often do a process called zinc electroplating on the entire chaisis of the car. So long as the zinc is complete, the car won't rust. Your best bet is to regularly clean out all the drain holes so that the water can't collect, scrape off any mud that has collected so that metal doesn't rust away underneath the mud, and remove the leaves and dirt. And of course, once you've washed the car, you should always take it for a drive so that any trapped water can slosh out.
© Karl S. Kruszelnicki Pty Ltd 2008.