Negative current?

1. Apr 24, 2005

infamous_Q

Is it possible to have a negative current in a circuit? (i'm aware this is probably stupid..and so is the next thing). and if so...if coupled with a negative voltage does it produce power going the other way? (because - * - = + )

...plz..however answers this..be kind if this is as dumb as i think.

2. Apr 24, 2005

shyboy

well, current is a vector quantity, so it can have any projection on any axis. Even if we are dealing with 1D system, as in the case of a single wire, the current still can have positive or negative direction with respect to the wire direction. So, if you get a negative current, that means that it flows in the direction opposite you assumed.
The power dissipated on the ohmic load is proprtional to the square of the current. So the power always flows from the source (a battery, for example) to a load. The power does not flow from the negative to the positive terminal or otherwise. I was speaking about usual circuits where the Ohm's law is valid.

3. Apr 24, 2005

futb0l

You probably wired your multimeter the wrong way round...
Power should be taken as magnitude, if I'm not mistaken.

4. Apr 24, 2005

James R

Actually, current is a scalar, although it can be positive or negative. Perhaps you're thinking of current density?

5. Apr 24, 2005

shyboy

you are right, my mistake.

6. Apr 25, 2005

rbj

Power does have direction (at least on a pair or wires) as well as magnitude. there are these things called wattmeters that will tell you which direction power is flowing and can take on negative values. if you define (positive) current as the current going out of the "+" terminal of a battery or power source (an equal current is going in the "-" terminal), the power delivered from battery to load (whatever the battery is connected to) is the product of the current and the voltage. however, if your "load" ends up being an even tougher power source than the battery (more voltage), current will flow into the "+" terminal of the battery (or "negative current" is flowing out) and the power delivered from battery to load is a negative number (which means the power delivered from "load" to battery is positive). then, if your battery is rechargable, it would be charging up.

r b-j

7. Apr 25, 2005

infamous_Q

ok. thanks everyone! but again...if i have a negative voltage (say flowing to the left) and a negative current (also flowing left) is the total power going left or right?

EDIT:

also...in addition to this, what if the voltage is + and teh current -, or the othr way around? as in.... is -50V and 10A the same as 50V and -10A?

Last edited: Apr 25, 2005
8. Apr 25, 2005

shyboy

1)the voltage cannot flow, voltage is a potential difference
2)the power in your example will flow neither left nor right. The sign of the power is positive that means that the power dissipates here (i presume we are talking about usual circuits). Actually it will flow towards the load.
3) (-50V)x(10A)=(-10A)*50V

9. Apr 25, 2005

infamous_Q

1) ok. so a current carrying a negative voltage, flowing left...so the power will still reach the load as if the current and voltage were positive?
2) what if the sign of the power is negative?

10. Apr 27, 2005

Cliff_J

1) like I said in the EE forum, the load doesn't care! Think of AC, its switching how many times a second yet the power is still delivered

2) this is more or less a point of reference question - a load (light bulb) getting power delivered to it has a postive sign and the source (a battery for example) could be said to have a negative power since it is performing work on the load and is using up its chemical stores to do this. This is almost a play on words since the stardard for power is work performed so therefore adding work is a negative act. Its like an inverse frame of reference - if we were standing face to face am I not holding out my right hand on your left?!

11. Apr 30, 2005

SGT

In an ohmic resistor, the current flows from the positive to the negative terminal, so current and voltage have the same sign. The dissipated power is positive.
In a battery, the current flows from the negative to the positive terminal, so the dissipated power is negative. Negative dissipated power means that the battery is supplying power to the rest of the circuit, instead of dissipating it.

12. Apr 30, 2005

topside

Whoa guys - lets not confuse the issue.

Current does not "flow" left or right per se.

If I have a non-metalic conductor, and it's movable charged particles are H- ions (protons). The protons will flow towards the (-) side of the battery. However, say I had a metal conductor with movable charge being electrons, then it connects with your tongue (which uses POSITIVE IONS as its movable charge) then connects back to metal conductor with electrons.

Consider with potential differance here causing a NEGATIVE charge to flow from right to left.
+ --- metal wire(- electron) ---> your tongue(+ ion) --> metal wire(- electron) --> -

The charged particles will move in the WIRE from right to left due to the movable charge being negative. This will then meat up with your tongue, which uses POSITIVE IONS to facilitate charge flow. This will cause the ions in your tongue to flow the OPPOSITE way as the electrons. This will cause a negative charge on the left side of your tongue (in respect to diagram) and will cqause electrons on the left wire to flow to the left.

Anyway, my point of this whole post is that negative charge is not the only thing that can "flow" in current.

A proton moving left has the *same exact* current (same exact magnetic field too) as electron moving right.

13. Apr 30, 2005

topside

Be careful when you say current flows from negative to positive. When you say current flow direction - do yo umean the direction of a positive charge or a negative charge?

Code (Text):

Electrons (-) move counter-clockwise in the metal wire.

,----------------------
|                      |
|                      |
|                      |
---|(+) Battery (-)|--'

Inside the battery, positive ions move CLOCKWISE.

Last edited: Apr 30, 2005
14. Apr 30, 2005

rbj

you're right about that.

i would say that there *is* a measurable difference between positive charges moving to the left and negative charges moving to the right. this difference is demonstrated in an experiment called the "Hall effect". maybe look it up in wikipedia. this was one way physicists could demonstrate that "holes" in a semiconductor behaved as positively charged particles of positive mass.

r b-j

15. May 1, 2005

SGT

When we talk about current flow, we mean the conventional current flow. Positive charges going from higher to lower potential. It has nothing to do with the movement of real charges: eletrons or ions.
The conventional reference can even be contrary to the actual flow. For instance, if a charged capacitor is connected to a resistor, the real (positive) current will flow from the lower to the higher potential in the capacitor and from the higher to lower in the resistor. The power in the resistor is positive, meaning that energy is being dissipated and in the capacitor power is negative, meaning that the capacitor is delivering energy to the resistor.