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Negative Voltage & Amps?

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dumbass

New Member
Hi Everybody,

Just wondering if someone can explain negative volts & amps, ok i understand positive voltage & amps but negative?

Has negative voltage & amps got the same grunt as positive volts & amps??

Just say there is a negative going square wave of -10v & -1A going into an inductor, is this the same as charging the inductor with +10v & +1A.?

If so then by increasing the amperage to say -2 amps will this will increase the field strength around the inductor?.

Just seems all upside down to a newby??
 

dknguyen

Well-Known Member
Most Helpful Member
THey are just math conventions. The important thing is voltage can be one of two polarities , and current can flow in one of two directions. In math, we only have positive and negative so we just assign one polarity positive and one negative. To try and make things easier to work with everyone tries to define it the same way. Look at electrons and protons. Scientists weren't sure what direction electrons flowed so the assumed that electricity flowed "this way" and called that positive, so the opposite direction was negative. Well, they guessed wrong. That's why electrons have a negative charge even though it is electrons that are flowing when electricity flows.

So negative current and voltage just mean "the opposite" of whatever way you defined was positive. So if you defined current flowing out of the battery as positive then current flowing into the battery is negative. If you defined the voltage across a resistor as + one way, flowing the other way is negative. THis especially comes up when you are doing circuit analysis and you don't know which polarity the voltage is going to be or what direction the current flows...you just randomly pick one and call it positive, then the other direction is negative. WHen you get the answer, if it is positive then it means it's what you guess. If it's negative it goes the opposite direction or polarity of what you guessed.

Has negative voltage & amps got the same grunt as positive volts & amps??
Grunt? It's all relative- increasing in one polarity is the same as decreasing in the other polarity. ie. if I keep increasing in the negative direction, this is the same as decreasing in the positive direction.

Just say there is a negative going square wave of -10v & -1A going into an inductor, is this the same as charging the inductor with +10v & +1A.?

If so then by increasing the amperage to say -2 amps will this will increase the field strength around the inductor?
Because of the math convention stuff I just talked about. Saying +10 and -10 volts or amps is meaningless without an actual picture to show how you defined the +/- for the inductor. Also, the question in general doesn't make sense for a couple reasons.

1. For ANYTHING, you can't control the voltage AND the current without changing the component itself. Like in a resistor- I can't say I want 10V and 10A to go through it unless I change the value of the resistor itself.
2. Another reason your example is confusing to read and think about is because you are defining +/- for voltage and current independently. This is directly related to #1 where you cannot control voltage and current independently. You have to define the direction of current relative to voltage or vice versa for things to make sense since they are dependent on each other. Of course, you can do it any way you want as long as it's consistent. But to make it easier for you to communicate your ideas and for other people to understand your ideas, we use a standard:

We use the "passive sign convention" which is:
-Voltage positive/negative is whatever you choose it to be for the component
-If current flows into the end labelled as positive(out the end labelled as negative) then current is positive
-If current flows into the end labelled as negative(out the end labelled as positive) then current is positive

This convention causes the following things to happen:
-if current is positive, a component is using current (resistor, a capacitor/inductor charging), if current is negative then the component is supplying current (a battery or an inductor/capacitor releasing energy). I know it's strange that "negative current" means something is supplying power. But instead of thinking negative as decreasing and positive as increasing, think about it in terms of a voltage drop.
-This convention makes it so that if you follows the direction of current flow around a circuit, a resistor will have a positive voltage drop (the same as a negative rise, but we don't call it that), and batteries will have a negative voltage drop (same as a positive rise, but we don't call it that either).
-You also have to use some common sense...math does't take into account real life things. FOr example, a resistor NEVER supplies power- just because you guessed the direction wrong doesn't mean it's suddenly supplying power. Resistors consumes power in both directions. Inductors and capacitors on the other hand...in one direction they supply power, in the other direction they consume power (charging). Which way you pick to be +/- is up to you, but if you use the passive sign convention, then + current means it is charging/consuming and - current means it is supplying.
 
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Berserk87

New Member
a mod approved your post with that name? lol.


the negative voltages have kind of bugged me to. i dont remember much about em. i know how to set them up though :p
 

dknguyen

Well-Known Member
Most Helpful Member
Passive sign convention - Wikiversity

It takes some time to sink in, but what it boils down to is that all directions and polarities are relative to each other and you can choose whatever you want as long as you are consistent (math requires you to be consistent, don't you know).
 
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dumbass

New Member
Thanx heaps for the replies & the images it will help me.

I didn't word the original question to well & it led to some confusion.

I meant to say if we had an Inductor that was being induced by a square wave of -10v Amplitude & -1A of current, limited by a resistor only--assuming the inductor can take more voltage & current.

Would an Inductors magnetic field be stronger if we applied say -15v & -2A, basically what i am asking is by increasing the negative voltage & Amps does this increase the magnetic field around the inductor like it does when increasing the positive voltage & amps?.

0-volts being earth, ground or common depending on where your from.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanx heaps for the replies & the images it will help me.

I didn't word the original question to well & it led to some confusion.

I meant to say if we had an Inductor that was being induced by a square wave of -10v Amplitude & -1A of current, limited by a resistor only--assuming the inductor can take more voltage & current.

Would an Inductors magnetic field be stronger if we applied say -15v & -2A, basically what i am asking is by increasing the negative voltage & Amps does this increase the magnetic field around the inductor like it does when increasing the positive voltage & amps?.

0-volts being earth, ground or common depending on where your from.
As already mentioned, it makes no difference - it's only the convention of measuring from a specific reference that makes it negative or positive.

For your example above - 10Vx1A is 10W, 15Vx2A is 30W - which do you think is more powerful?.
 

dumbass

New Member
Thanx, now i understand, it was just the way i was looking at it.

Seems pretty obvious when talking total watts.

Thanx heaps.
 

Sceadwian

Banned
It's not directly related to passive components or any show that a negative voltage has any less 'grunt' as you say than positive voltage. But it is worthy of noting that semi conductor devices like transistors exist in two forms NPN and PNP. NPN transistors are control by positive currents into the base, where as PNP transistors are controlled by negative currents from the base. But P type semi conductor material doesn't conduct as well as N type does so a PNP resistor of the same physical construction (just the dopant regions switched) will not carry quiet the same current or have quiet the same gain. N or P channel mosfets are the same way, you'll generally see more positive voltage/current controlled devices than negative, although you'll still see plenty of P type devices because in many situations they're either required or it's just easier in general to use them.
 

codan

New Member
Hi All,

I was reading this thread & i am not sure that it was actaully answered correctly or maybe it was & i don't understand the replies?
I am only young & still wet behind the ears but there are some things that have a set reference -- datum point so we can all relate to & relay specific information.

Just like altitude or the height of mountains etc, these are not measured from any point we like but are measured from a datum point which is mean sea level.

In the case of electrical potentials the datum is taken to be the potential of the earth which is 0v so -20v is 20v below the 0v datum being the earth potential which is a set reference point or datum.
 

Sceadwian

Banned
In the case of electrical potential earth is only one option, ANY other point in the circuit can be defined as the reference point. Being that the real world is much more complicated than simple theory, GND is not always 0 volts. An individual's circuit's 'ground' might be actually elevated over the actual earth ground voltage from different supply circuits or real world circuit conditions. A simple example being that two wires of the same value between two points are used as GND and neutral reference. (such as in a home electrical system) But the neutral wire carries the bulk of the supply current not the GND connection, so because there is a finite resistance of the wire the voltages between neutral and earth wll not be 0 volts, it will form a voltage divider.
 
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codan

New Member
Ahh--Ha, thanks Sceadwian, i understand what your saying, i was told differently & have had trouble understanding some things, this makes it much clearer.
 
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