4 ms·
I know all analogies, especially those explaining complicated physics to 11th graders, have to oversimplify some aspects of a phenomenon, but this one always ru
by kmm 7y ago
I know all analogies, especially those explaining complicated physics to 11th graders, have to oversimplify some aspects of a phenomenon, but this one always rubbed me the wrong way. The hole moves with the electrons, if they move right, the hole also moves right.
The crucial point is that the electrons at the top of a valence band have effective negative mass, i.e. they move opposite to an applied force. Of course their true mass hasn't changed, this is simply a consequence of the fact that we're considering a system of uncountably many electrons moving in unison. This means a full band carries no current (which is the situation in isolators), but if a band has some electrons missing at the top, these holes will be transported in the opposite direction of the usual electron current, which happens to be the correct direction for positive charges. Hence, we can talk about positive charge carries.
A better analogy would be a bubble of air underwater in a river. It moves with the flow of the water, not against it. I know it doesn't really matter, but I feel an analogy must at least have a core of truth.
- techbio 7y agoBubbles in the shape of water. Loved reading your first line’s first phrase as the sentence “I know all analogies.”
- nobrains 7y agoThe technically correct sentence would have been "I know, all analogies..." But yeah, this one was fun and I read it like you did at first as well!
- specialist 7y ago"I know it doesn't really matter..." Au contraire. Analogies, mental models, metaphors help us make predictions. Better is better. "The difference between the almost right word and the right word is really a large matter. 'tis the difference between the lightning bug and the lightning." ― Mark Twain
- mkl 7y agoThe hole only moves with the electrons if the electrons are moving in unison. If they move one at a time, into the hole left by the previous electron, then the hole moves the opposite way. E.g. say a, b, c, ... are electrons: 1. abcde 2. abcd e 3. abc de 4. ab cde 5. a bcde 6. abcde (I don't know much chemistry or electronics, but that's how I've always understood the analogy.)
- kmm 7y agoThat is exactly the analogy I have a problem with, it is variously explained with cars in parking lots, people on seats, .... It is wrong however, the electrons do not move one at a time, they move in unison opposite to the normal flow of current. Apart from theoretical considerations, we know it has to be like this because the voltage induced by the Hall effect flips for these kinds of semiconductors, and since there are no positively charged free particles in solids, this has to mean the electrons are now moving in the other direction, giving a complete illusion of a positive charge carrier.