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I am not from an engineering background, so this may be very simple and stupid but:

Voltage is the difference in electric potential between 2 points

Voltage drop describes the reduction in energy of the supply through a passive element, given by the amount of current through the resistance of the element

So for the 9V seen on the diagram, there is a difference in electric potential between the 2 terminals of the battery. the difference is 9V which means 9Joules per 6.242×10^18 electrons (unit charge) can be given to move from one terminal to the other terminal?

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    \$\begingroup\$ That's correct, except that a coulomb is 6.242×10^18 electrons. \$\endgroup\$ – Phil G Oct 16 '18 at 12:46
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    \$\begingroup\$ My god, thank you. I just searched on google how many electrons in 1 coulomb, the answer was 1018, however it meant exponential. Embarrassing! \$\endgroup\$ – zenarthra Oct 16 '18 at 16:10
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One point of confusion...the change in voltage is unrelated to the "amount of current". You can have a large change in voltage with a small current or a small change in voltage with a large current. Now, if you multiply the voltage across the resistor by the current through the resistor you will get the power, in watts, that is converted to heat by the resistor.

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