# Electrically common points?

#### fbchurch2009

Joined Jan 10, 2010
15
So I have been reading the chapters and I am on chapter 2 in dc circuits. Under the "circuit wiring" section, it says that between points 3 and 4 you will find 10 volts.... (when given a 10 volt battery). but in the previous chapter it says this: "because it takes energy to force electrons to flow against the opposition of a resistance, there will be voltage manifested (or "dropped") between any points in a circuit with resistance between them."

I don't get it?! any help?

.

#### Attachments

• 2.3 KB Views: 63

Joined Dec 26, 2010
2,148
So I have been reading the chapters and I am on chapter 2 in dc circuits. Under the "circuit wiring" section, it says that between points 3 and 4 you will find 10 volts.... (when given a 10 volt battery). but in the previous chapter it says this: "because it takes energy to force electrons to flow against the opposition of a resistance, there will be voltage manifested (or "dropped") between any points in a circuit with resistance between them."

I don't get it?! any help?

.
There is no contradiction whatever here, it is just that this circuit contains only one resistance, connected directly across a battery.

The current in this simple loop, multiplied by the resistance, will be equal to the battery voltage.

#### mrmount

Joined Dec 5, 2007
59
There is only one resistor in this whole circuit and the whole battery voltage (10V) will be manifested (or "dropped") between points 3 and 4. So what is your doubt ? Are you questioning whether there will be "drops" between 1-2, 2-3, 4-5 and 5-6 ? If your question is that, then no is the answer. There will be no voltage drop across them because there is no/negligible resistance between those points.

#### fbchurch2009

Joined Jan 10, 2010
15
I am not trying to prove any contradiction... I was just simply trying to understand it. I just expected a voltage drop between points 3 and 4 because there is resistance in between those points.... since the battery is ten volts, I expected less than ten volts. But it clearly says ten. as you can tell I am still confused.

#### SgtWookie

Joined Jul 17, 2007
22,230
In the "real world", you would be correct - as real wires have resistance, and batteries have internal resistance - the 5 Ohm load would cause voltage drops all through that little circuit.

However, the circuit is based on "ideal" components; the battery always puts out exactly 10v, and the wires have no resistance whatsoever.

Basically, what you have is 5 Ohms of resistance across a source that measures 10V.
There will be 2A current flowing through the resistor (and out of one side of the battery, and back in the other side) - and the entire 10V of the battery voltage will be dropped across the 5 Ohm resistor.

Did that help?

#### fbchurch2009

Joined Jan 10, 2010
15
You probably answered my question but I am still not totally understanding. Let me just rephrase.... I was told that current is the same throughout a circuit (up to this point, thats all I need to know) and "the amount of voltage (potential energy per unit charge) between different sets of points in a single circuit may vary considerably." Why isn't the voltage different between any of the points? Really sorry for all of the rudimentary questions! But I want to get a good grip on everything before i move on... thanks!

Last edited:

Joined Dec 26, 2010
2,148
This is an idealised circuit with only two nodes, so there can only be two voltage levels in it.

In reality, the wiring itself would have some resistance, so there would be progressive voltage drops along its length, and there would be a continuum of voltage along the length of the resistor. The battery would in reality contain cells connected at internal nodes, at different voltages, and each cell in turn would contain potential gradients set up in the resistive parts of which they were made.

All of this complexity is ignored in the idealised circuit. In practice things like wiring resistance are negligible in some cases, but not in others.

#### fbchurch2009

Joined Jan 10, 2010
15
ah, ok I think I got it! sorry about all of the confusion!

#### sbixby

Joined May 8, 2010
57
One way to visualize it will come later in the book, mathematically - Kirchoff's Law.

It's basically saying that in a circuit, everything adds up to zero. So with one voltage source and one resistor, you could think of it as "battery adds 10v, resistor drops 10v, so it all comes out the same".

This is what they mean by the resistor dropping the entire voltage.

If you add another resistor, then you'd have "battery adds 10 volts, resistor one drops some portion of 10V amount and resistor two drops the remaining portion of the 10v".

Note: I'm still wrapping my head around circuits, so I could be thinking of it wrong too.

#### baoneil

Joined Jun 24, 2016
5
I was having the same problem understanding this concept. After reading this I completely understand it now. I thought it might help others with the same question to hear an answer from another green horn learning this stuff for the first time. I also recommend you understand why voltage can be called potential difference instead of potential energy. Like others have mentioned, voltage is measured between 2 points. Therefore it's a potential energy difference. An electron gains or loses energy (in Joules) when it travels between 2 points in a circuit. This energy is equivalent to the voltage between said 2 points.

Last edited: