# Measured voltage + current direction

Discussion in 'General Electronics Chat' started by jamjes, May 14, 2010.

1. ### jamjes Thread Starter Member

May 10, 2010
35
0
Lets see if I have this right:

A voltmeter has black and red leads.

Connecting it to a battery with black on - and red on + will read, say, +9v. This is because there is a net surplus of electrons with charge on the black lead and a net deficit of electrons on the red lead. The pd causes current to flow, charge moving from - to + on the battery.

My reasoning is the polarity on the voltmeter must indicate the direction of current flow.

But is this always the case?

Are there cases where a pd is +3v, but charge is flowing red to black?

Last edited: May 14, 2010

Mar 12, 2010
469
41
What is pd?

3. ### jamjes Thread Starter Member

May 10, 2010
35
0
potential difference

4. ### Bychon Member

Mar 12, 2010
469
41
Electrons always come out the negative end of the battery and go to the positive end, if the voltage is 9, or 3 or.03 volts. Electrons ALWAYS go the same way.

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
If you measure the pd across a battery, electrons flow out of the negative terminal.

If you measure the pd across a resistor, electrons flow into the negative side of the resistor.

This is true for all cases in DC circuits and resistive loads. If the load is s capacitor or an inductor then you have to know if at the moment of the measurement it is acting as a source or as a load.

6. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
No No No. Think about a half discharged battery, being charged. Current flows backwards "into" the battery. I do not want to get into the + to - direction of current flow issue.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Yes, that is right, you have to know if the component the voltage is measured across behaves like a load or like a source.

8. ### jamjes Thread Starter Member

May 10, 2010
35
0
So, during capacitor charging, its acting like a load, the voltage across it might be +3v and charge is moving towards the capacitor plates.

During discharge, its a source, and the voltage across might be +2.5 volts at some point in time, but current is moving away from the plates.

So... (particulary in AC circuits?), the sign of the voltmeter/oscilloscope cannot determine for certain the current direction ?

*starts wondering about NPN and PNP transistors*

9. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You cannot determine the direction of flow of current just by measuring only the voltage unless you know what is going on in the circuit.

10. ### Bychon Member

Mar 12, 2010
469
41
and the electrons are still flowing from the more negative place to the more positive place.