A question on how the transistor works

Now I am not getting how the second equation is made. Because how can you measure VBC? Because then it will be like measuring the potential difference between two split wires... Something like this...

So is that possible?

Answer

You can measure the voltage between any two points. For example, I could measure \$V_{kitchensink,tongue}\$ like this:

1. place positive lead of voltmeter on kitchen sink


2. place negative lead of voltmeter on tongue


3. read measurement from voltmeter

While this is probably not a meaningful measurement, it is defined anyway, because my tongue exists at some electric potential, and my kitchen sink exists at some other electric potential, and the difference between them is the "voltage", which is more accurately called the electric potential difference.