Found the PDF version of The Art of Electronics (Horowitz and Hill) recently, and, having nothing better to do... I thought I'd give it a read. It wasn't long before I hit a wall (courtesy: my inept high-school student self) when I reached a section on something called "Thevinin (Equivalent) Circuits".
Now I (think) I understand what Thevinin circuits are about: Any mess of resistors and cells/voltage-sources can be "resolved" into a simple circuit with a (Thevinin) cell in series with a (Thevinin) resistor.
The following example is provided in the book/PDF;
[Caution: My understanding of series and parallel arrangement of resistors is possibly imperfect]
How on earth was this analogy made?
I understand that the 10k ohm resistance at the top is supposed to be the internal resistance of the battery. This 10k ohm resistance is in parallel to the other 10k ohm resistance (located "below" the first in the schematic).
So the replacement of the two 10k ohm resistance with a single 5k ohm resistance in series with the battery makes sense.
But how come the 30V battery was replaced with an "equivalent" battery of 15V? I really don't get this part. Is it a typo? I don't see why we'd need an "equivalent" battery in this case, since we're only dealing with one battery (the same however, can't be said for the resistances: there are two of them).
I tried asking my teacher, but he says I should just focus on our textbook/syllabus and give The Art of Electronics a read once I get to College (i.e- He skillfully avoided my question) :(
Having no other place to ask this, I resorted to posting it here O:)