# About calculus and resistors [closed]

how can you know the voltage of a circuit, after passing through a resistor? for example there is a 5 volt supply and a resistor of 120 ohms, what will be the voltage after passing through this resistor?

• Welcome to EE.SE! Unfortunately, your question is very unclear. Can you edit your question to clarify it? – user2943160 Jul 18 '16 at 2:22
• Sorry, i am from Brasil and i dont have a very good english, what i want to know is if there is a way to calculate the tension of a circuit after passing by a resistor – jfinizolas Jul 18 '16 at 2:24
• 1. "tension" is hardly ever used (except in the phrase "high tension") to describe circuits in English. We say voltage or potential difference. 2. I think I know what you mean, but it's not 100% clear what you mean by "passing by" a resistor. If I walk past a resistor that doesn't give me any special way to tell what voltage is across it. But if you mean current passes through a linear resistor, then you can use Ohm's Law to know exactly what the voltage is across it. – The Photon Jul 18 '16 at 2:27
• RE: your edit. You need to know what the other end of the resistor connected to or what's the current through the resistor. – The Photon Jul 18 '16 at 2:46
• I think closure is somewhat harsh here, the meta question is obviously 'how do resistors behave?' If the OP doesn't realise he has that wrong, the question can't be 'made more clear'. – Neil_UK Jul 18 '16 at 9:38

If you measure the voltage with a real DMM, having an input impedance of 10M or so, then the voltage will drop very slightly to 4.999... something (not the right number of 9s, I haven't done the exact sum, but it's very close to 5v).

If you put another 120 ohms load to ground, then the voltage drops to 2.5v, with 2.5v across your original resistor.

If you connect it directly to ground, the voltage will drop to zero, with all 5v appearing across the resistor.

A resistor only reduces voltage as a ratio with the load it's driving. No load, no voltage reduction.

I had exactly this incorrect take on what resistors did when I was very young, so I have sympathy with the OP, and feel that closure of the question is somewhat harsh.

Given a complete description of any circuit made up of linear resistors, independent DC current and voltage sources, and linear dependent current and voltage sources, you can exactly calculate the voltage at every node and the current through every branch in the circuit. Two of the best known ways of doing this are called "node analysis" and "mesh analysis".

These methods can also be extended to circuits with AC voltage and current sources and linear inductors and capacitors.

More complex methods are needed to solve circuits containing nonlinear elements. And it may not be a unique solution if the nonlinear elements are sufficiently badly behaved. But that is the realm of circuits with transistors or deliberately chosen nonlinear devices, not simple resistors and power sources.

for example there is a 5 volt supply and a resistor of 120 ohms, what will be the voltage after passing through this resistor?

It depends on what the other end is tied to. If you're just tying a resistor across a battery, then expect it to heat up.

If you really want to figure out a voltage based on resistors, then make a voltage divider network by connecting two resistors to the battery all in series (as a loop). The voltage where the two resistors connect will be as follows:

- take value of resistor connected to ground.
- Divide it over the total value of both resistors.
- multiply the result by power supply voltage.