College Physics : Circuits
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Example Question #11 : Circuits
The picture below shows the electron energy levels of a hydrogen atom.
If an electron is excited to the 5th energy level, how many different transitions can it make to return to ground state?
Two
One
Ten
Three
Zero
Ten
Electrons don't always drop from the top to the bottom, sometimes they can make 2 or more transitions during the journey. For example, it could drop to the n=4 then to the n=1 levels. Or n=4 to n=2 then n=1. So when you count up all the possible transitions that can be made between n=5 and n=1, you get 10.
Example Question #37 : Electromagnetics, Waves, And Optics
Use this circuit diagram for the next question.
What is the current traveling through the R2 resistor?
So this problem has several steps to it. We need to think backwards. Ultimately we need to know the current going through R2. We will be using Ohms law so we need to know the voltage going through just that resistor.
Because it is in parallel with R3 and R4, all three must have the same voltage. However since it is in series with 2 other resistors, it will not be the voltage from the battery.
Lets reduce the parallel circuit to 1 resistor. Using the formula
That resistance is in series with R1 and R5 so to find the current going through each of them we need to add the resistances and use Ohm's law again.
Doing so yields a current of 
So all three of the resistors that are in parallel have a voltage of 
Now we can go all the way back to the R3 resistor knowing that it has a resistance of 
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