What is the voltage at the negative terminal of a battery?

Thread Starter

Mooney117

Joined May 27, 2020
4
Hey everyone,

I'm just starting out doing electronics as a hobby in these troubled times and I'm having trouble understanding voltage:

I have a book on electronics "Electronics for dummies" and it says:

"Voltage can be considered positive or negative , but only when compared with some reference point. For example, the voltage at the positive terminal of a flashlight battery is +1.5V relative to the negative terminal. The voltage at the negative terminal is -1.5V relative to the positive terminal."

So if I have a 9V battery does that mean positive terminal is +9v and the negative terminal is -9V making the total voltage 18V?

Or is the negative terminal 0V and the positive is +9V making the total voltage 9V?
 

MaxHeadRoom

Joined Jul 18, 2013
20,700
You have to set a reference point, which is normally considered zero.
In the case of your 9v battery, if you use the Neg terminal as the zero reference, then WRT to this zero point, the +ve terminal you will therefore read +9v.
Max.
 

DickCappels

Joined Aug 21, 2008
6,386
Like they said, no. It means if you take a voltmeter and measure across the two battery terminals you will measure 9 volts

The voltmeter has a polarization if you put the negative probe on the battery's negative terminal and the positive probe on the battery's positive terminal you will measure 9 volts. If you reverse the meter's probes you will measure -9V.

More than you want to know: https://learn.sparkfun.com/tutorials/how-to-use-a-multimeter/all
 

MrChips

Joined Oct 2, 2009
21,126
So if I have a 9V battery does that mean positive terminal is +9v and the negative terminal is -9V making the total voltage 18V?

Or is the negative terminal 0V and the positive is +9V making the total voltage 9V?
A 9V battery has two terminals, one labeled + and the other labeled -.

Take a volt meter and connect the RED lead to the + terminal.
Where are you going to connect the BLACK lead of the voltmeter?

Pay attention: The + terminal is NOT at 9V.

Measuring voltage is like measuring distance.
What is the distance at London? That is a meaningless question.
What is the distance between London and Manchester? Now there is an answer to that question.
 

Alec_t

Joined Sep 17, 2013
11,295
It may help if you note that 'voltage' is the measure of potential difference. A difference requires two measurement points, either of which can be taken as a reference point.
 
You can place batteries in series too where they could add or subtract depending on the direction. You would have your multimeter attached to two "0-reference" points of a single battery. so, -1.5+(-9). Think of current as a loop. This will become apparent when you learn Kirkoff's Law and "Loop equations".

"Conventional current" has a negative to positive direction. Our battery has a (-) and (+) terminal, so if you enter the 1.5V battey at the (-) terminal and exit throught the +, that is +1.5V. Now you encounter a 9V battery at the +terminal. This is now -9V.
The voltage from end to end is +1.5-9, the way we defined our 0V reference.

You can place two 9V batteries together where the middle point is zero and get a +9 and -9V supply.

"conventional current" is NOT electrons. It's direction is negative to positive. Your DVM and current meters when you observe polarity will show this.

It matters in solid state physics, electro-chemistry and vacuum tubes and electron beams to some extent. The "water analogy" breaks very quickly: Voltage as pressure, Current as Flow and resistance the size of the pipe works for basic understanding.
It doesn't mean you put one electron in at one end of the wire and it comes out the other like water would.

I want to add something. Every component has parasitics. We "model" components as ideal devices. "Sometimes" three 1K resistors in series is not 3000 ohms. Sometimes a surface mount 1K resistor soldered on the thin edge is not the same component soldered on the wide edge. A wire moving on the earth is a generator, but don't tell your teacher that. Pushing on a teflon insulator generates a current too, but again don;t tell your teacher that. A piece of paper conducts current depending on it's water content, but for all practical purposes it's an insulator.

I need to point out that I challenged an answer to an exam and won. the reason I won: "Your not supposed to know that yet". Make sure you answer questions with the knowledge you were given which may not be the "real answer".

A wire 1mm apart would not conduct electricity, right? Your answer should be that it doesn't conduct electricity. My answer - it depends.
 
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dl324

Joined Mar 30, 2015
10,747
So if I have a 9V battery does that mean positive terminal is +9v and the negative terminal is -9V making the total voltage 18V?

Or is the negative terminal 0V and the positive is +9V making the total voltage 9V?
Neither statement is correct.

A battery has a certain nominal voltage across it's terminals. If one of them is connected to circuit ground, then the voltage at the other terminal will be relative ground.

The nominal voltage of a 9V battery is 9V, regardless of where you place the leads of a multimeter. One direction will read positive, the other direction will read negative.
 

sparky 1

Joined Nov 3, 2018
184
I enjoyed this explanation of the common (e-meter) emmiter
It is that once the lecture is over we might leave some details out. It is important to re-enforce what we are taught.
In simulation moving V and I probe on a slightly rearranged layout shown improves the deli very and clarity.
The concept amplifier diagram erased then which type of CE amplifier is given. In this example the first equation is (I sub e).
www.youtube.com/watch?v=KjSnufV2NT4&
 
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MrSoftware

Joined Oct 29, 2013
1,780
A volt meter is the rough equivalent of a measuring tape, for measuring distance. One end of the measuring tape reads 0, this is the black probe on your volt meter, the number on the other end is the rough equivalent of the red probe. So if I ask you, how high is the ceiling? Your next question is "relative to what?" So I say the floor. So now you put the 0 end of your measuring tape (the black probe) on the floor, and the other end (the red probe) on the ceiling, you measure 8 feet. The ceiling is 8 feet high RELATIVE the floor. So now I say, how high is the ceiling RELATIVE the shelf? So now you put the 0 end of your tape (the black probe) on the shelf, and the other end (the red probe) on the ceiling, and you measure 2 feet. So the ceiling is 2 feet high RELATIVE the shelf. So how do you get a negative number? Maybe I ask you, how high is the shelf relative the ceiling? So you would put the 0 end (black probe) on the ceiling and measure DOWN to the shelf, and you measure 2 feet. The answer is -2 because the shelf is 2 feet lower RELATIVE the ceiling. Voltage is the similar, it's measured as the potential difference between two points. So when we say what voltage is the battery, it is assumed that we mean what is the voltage of the + terminal relative the - terminal. We consider the + terminal to be the ceiling and the - terminal to be the floor. So for a 9v battery you would put your black probe (0v) on the - terminal, and the red probe on the + terminal, and you would read something near 9v. But if we asked what is the voltage of the - terminal relative the+ terminal. Now you put the black probe on the + and the red probe on the -, and just like measuring down from the ceiling, you would measure something near -9v.

Is it possible to have both positive and negative voltage in a circuit? Yes if your reference point is somewhere in the middle. Similar to picking the shelf as our reference point, then asking how high is the ceiling and how high is the floor relative the shelf. The ceiling is +2 and the floor is -6, relative the shelf. In a circuit sometimes your reference is in the middle somewhere, and that's how you end up with both positive and negative voltage in a circuit.
 
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BobaMosfet

Joined Jul 1, 2009
1,108
Hey everyone,

I'm just starting out doing electronics as a hobby in these troubled times and I'm having trouble understanding voltage:

I have a book on electronics "Electronics for dummies" and it says:

"Voltage can be considered positive or negative , but only when compared with some reference point. For example, the voltage at the positive terminal of a flashlight battery is +1.5V relative to the negative terminal. The voltage at the negative terminal is -1.5V relative to the positive terminal."

So if I have a 9V battery does that mean positive terminal is +9v and the negative terminal is -9V making the total voltage 18V?

Or is the negative terminal 0V and the positive is +9V making the total voltage 9V?
No. The actual value of voltage is always between a minimum of two points- one point relative to the other. If the positive is 9V, then we started at zero somewhere. Since the only other place we're comparing is the negative terminal, then it must be zero. Likewise, if we think of the positive terminal as zero, then the negative terminal must be -9V.

In electronics we always chose or create a reference point- this is the only way we have to make a consistent relationship between varying voltages throughout a circuit.

This will help you:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
 

sparky 1

Joined Nov 3, 2018
184
The teaching of current, it is introduced first. A simpler video, yep all you need is voltage.
What is the stuff at the negative terminal of a battery?


The direction of the current is the same the protons and the battery is the same no games is'nt that right ?
 
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