EasyPhysicsHigh School (11–12)Electricity & Magnetism › Current ElectricityConceptualBoard ExamAPA-LevelIB
The electrons in a copper wire drift at less than a millimetre each second…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The electrons in a copper wire drift at less than a millimetre each second. A lamp at the end of a long wire still lights as soon as the switch closes. Explain why. Then say what happens to the drift speed if the same current is sent through a wire of half the cross section.
Need a reminder? The key ideaShow
A current is , and electrons cross a section, with C. The drift velocity is and the mobility is . The current is , so . The current density is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Why the lamp lights at once. The wire is full of free electrons everywhere, including the filament. When the switch closes, the field spreads along the wire at nearly the speed of light. Every electron starts to drift almost at once. The electrons in the filament do not wait for those from the battery.
Step 2. Half the cross section. For the same current, is in inverse proportion to . With half the area, the drift speed doubles.
Watch out. The signal and the electrons are not the same thing. The field travels fast. The electrons drift slowly.
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