Basic electrical question- current and volts

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Old 05-11-14, 04:57 AM
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Basic electrical question- current and volts

So if there is current on the wire, that means there is voltage as well?

Say you measure 2A of current on a wire... cut it in half and measure voltage between the two cut parts and you measure 2VAC... does that mean there are 2 amps of 2VAC flowing thru that conductor?

On the other hand pretend you are talking about a wire connected to a 120V electrical system.... if you measure that same wire and get 2A and then measure between them as described and get 2VAC, then its only 2VAC flowing thru that conductor and not 120V?

That last part (or the first for that matter) probably doesn't make much sense....
 
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Old 05-11-14, 11:47 AM
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So if there is current on the wire, that means there is voltage as well?
That is correct.

I'm not quite sure what you are trying to determine.

Lets say you have a 120v circuit in operation. You have a 240 watt bulb in a ceiling light and you turn on the wall switch. You will measure 2A. (120v x 2a = 240w)

If you turn the switch off and measure the voltage across the switch you get 120vac.
That's the open circuit voltage. You could have measured 1A or 10A..... the open circuit voltage stays the same.

If you know the load on the circuit and the amperage draw.... you can compute what voltage is on the circuit.
 
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Old 05-11-14, 12:45 PM
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If you cut a wire like that and try to measure voltage...pretty sure you'll mess up your meter when it completes the circuit and current starts flowing. I might be thinking wrong.
 
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Old 05-11-14, 01:01 PM
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I might be thinking wrong.
Yup, your wrong. When using a meter you are almost always completing a circuit when you are measuring voltage. Whether it is from hot to ground, hot to hot, or hot to neutral, you are making a short circuit on purpose.
 
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Old 05-11-14, 01:47 PM
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Well, what the heck am I thinking of then? I don't mean with the meter set to ohms, that I'm sure of.

Something about cutting a single conductor and then measuring across the gap just doesn't seem right to me. It's probably the current measurement in the statement that's messing with my brain.
 
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Old 05-11-14, 03:29 PM
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Measuring across the gap (inline with the circuit) is how you directly measure current (although most would simply use a clamp-on for AC). Measuring across the circuit (shorting) is how you measure voltage.
 
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Old 05-11-14, 04:36 PM
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I was surprised when I returned to college that school leavers seemed to have no idea on how to use a multi-meter however when I look back I did blow my AVO Multi-minor as a kid so suppose I must have been the same.

Because it is so easy to get setting wrong I use a clamp on meter auto ranging so can't blow it up but a DC clamp on is expensive.

There is of course a difference between AC and DC with DC as stated volts x amps = watts but with AC volts x amps = VA and volts x amps x power factor correction = watts. Often VA and Watts are the same as we hope power factor is unity.

I was taught always leave meter on highest volt range but today meters have an off position. However it is easy to connect a meter then turn the switch which can mean going through a option which will cause a short circuit.

In the UK where I live our meter leads have to comply with regulations which means a very small tip around 1mm and a finger guard to stop fingers slipping towards the live part and a fuse. However it is only for working on mains GN38 is not for extra low voltage work (under 50vac) only for low voltage (50 - 1000 vac) work and since we use a higher voltage than US our chance of causing ionisation of the atmosphere is higher. This is main reason for fuse in the leads.

The correct leads are a real pain to use. The live parts of our sockets are not at the front so if your fingers were on the pins when you insert it nothing exposed will become live. All sockets have shutters so you have to use correct plug so to measure voltage we need a special plug which connects direct to meter no probes. We have gone heath and safety mad.

As a result it seems we no longer teach our children the dangers of electric and how to use a multi-meter.
 
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Old 05-11-14, 06:09 PM
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So if there is current on the wire, that means there is voltage as well?

Yes. In any complete circuit (that is "turned on"), there is a voltage difference between any two points in the circuit. However the voltage difference can sometimes be so small as to be hard to measure.

The following is a hypothetical although representative description of voltage differences. You may measure 120 volts between the two prong holes of a household power receptacle. You plug in a light fixture and switch it on and still measure 120 volts between the two prong holes. You measure 116 volts between the center contact and outer shell of the lamp socket. You measure two volts between the small prong hole (hot) and the center contact of the lamp socket meaning two volts were dropped in the wire between those points (assuming correct wiring). You measure one volt between the large prong hole (neutral) and the shell of the lamp socket. The sum of the voltage differences for the various parts of the circuit from large prong hole to small prong hole of the receptacle is 120.


Say you measure 2A of current on a wire... cut it in half and measure voltage between the two cut parts and you measure 2VAC... does that mean there are 2 amps of 2VAC flowing thru that conductor?

No. When you cut the wire, say it's the wire from the small prong hole in the previous paragraph, the current stops. The voltage across the lamp socket bottom contact and side shell drops to zero. The voltage between the wall receptacle prong holes is still 120. Now try measuring between the cut ends of the wire. The act of measuring between the cut ends (with your voltmeter) causes a very minute flow of current across the cut ends through the voltmeter. What now happens is the voltage from the small prong hole to the cut end is almost zero. The voltage from the other cut end to the lamp socket is almost zero. The voltage across the lamp socket (center contact to shell) is almost zero. The voltage between the cut ends is almost 120. Why this happens is due to Ohms Law; the voltage across any two points equal the current flowing times the resistance of the path (wire or lamp or other device) between the two measuring points. The piece of wire that sapped two volts (imposed 2 volts of voltage drop) when the amount of current needed to light the lamp (say 2 amps) flowed through it will now sap a tiny fraction of a volt from the tiny current that flowed through the voltmeter that re- completed the circuit that was broken when you cut the wire. Of all the items now in the circuit, the voltmeter has the highest resistance and by far.

On the other hand pretend you are talking about a wire connected to a 120V electrical system.... if you measure that same wire and get 2A and then measure between them as described and get 2VAC, then its only 2VAC flowing thru that conductor and not 120V?

It's not proper to say that there is "2 volts AC" flowing through a conductor rather you say that there could be 2 amperes flowing through the conductor and that there could a voltage difference of 2 volts between the ends of the wire . Regardless of the supply voltage, say, across the two prong holes of the receptacle you plugged your test circuit (wires, lamp, etc.) into, the number of volts sapped by the wire (between one end of the wire and the other end) depends only on the resistance of the piece of wire and the number of amperes flowing at that instant.

So if you had a 120 volt lamp drawing 2 amps of house current and one wire between the receptacle and the lamp socket subtracted 2 volts (as voltage drop), then if you had a 12 volt lamp that was actually drawing 2 amperes from a 12 volt car battery when connected using the same wires, one of the wires is still subtracting 2 volts. In this last example, the wire from battery to lamp is taking 2 volts and the wire from lamp back to the battery is taking 2 volts leaving 8 volts for the lamp.

In a simple circuit, say one wire from the power source to one thing (light, appliance, other load) and one wire back to the power source, the current (amperes) flowing in each part of the circuit is the same, say, 2 amps.

Turn off the power before doing resistance (ohms) or continuity measurements.

So long as the meter measuring volts is set to a voltage range greater or equal to any voltage that may be found in the circuit (including voltages coming out of transformers), it is safe to touch the meter probes to the cut ends of a wire (or to any two places in the circuit).

Open circuit voltage is the voltage supplied by a power source (solar cell, battery, etc.) with zero current being drawn i.e. there is no complete or closed circuit. But it is not possible to actually measure open circuit voltage because all voltmeters complete (close) a circuit and draw some current no matter how slight. The closest approximation of open circuit voltage is measured with a high impedance voltmeter.
 

Last edited by AllanJ; 05-11-14 at 07:09 PM.
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Old 05-12-14, 03:48 AM
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Edit needed above but the board won't let me.

"... You measure one volt between the large prong hole ... "

in the third paragraph should read:

"... You measure two volts between the large prong hole ... "
 
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