Having trouble intuitively understanding "voltage is measured across something"

[fuzzybabybunny]

I often use the water analogy to try and picture electricity. It seems to me that voltage should be a point measurement, since voltage is analogous to the pressure that the water would place on an object at any point in the loop.

For example, say that you took a faucet and placed a pressure gauge on the end and turned on the faucet. You'd get a water pressure reading. This would be analogous to voltage, wouldn't it? But this is a spot reading that only needs to be taken at one point. Likewise, say that you connected the faucet to a restrictive piece of hose and then placed the pressure gauge at the end of that hose. You would still get a pressure reading, which is again a spot reading. The pressure gauge never needs to be hooked up *across* two points of the restrictive tube (before the hose and after, as you would a resistor) - you just take the pressure reading at one point. But in a circuit, why is pressure (voltage) always measured across a certain object?

Another example: Say that you had a one gallon jug of water a foot above the ground. You have another gallon of water 100 feet off the ground, but with a 100ft hose dangling all the way to the ground. To get the pressure (voltage) of those gallons of water, you would still only need to install a pressure gauge at the end of the hose, never "across" anything.

Is there perhaps a better analogy that I can use to intuitively understand or picture why voltage (pressure of electrons wanting to flow in one direction) is measured across something rather than a point measurement?

 

[magomago]

FBB you are misunderstanding your water analogy - you aren't taking a 'point measurement', this is actually the pressure relative to air pressure outside, which is atmospheric pressure. Lets just go with PSI units since its used everywhere in the US.

If you take a "point pressure" measurement of the pressure against a hose, it is really "X psi above atmospheric pressure" where atmospheric pressure is simply the air pressure around us. You could take the pressure measurement between two points in your water system and say "oh point A is above X psi above point B" - we know nothing of the absolute pressure that is imparted, just the relative pressure in that instance (also known as "gauge pressure").

This leads to the differences between psig (psi gauge) vs psia (psi absolute). Does that help?

So for your 100 foot analogy: you are actually getting the pressure at the bottom relative to atmospheric pressure, which means you need two points to measure pressure. By calling it a "point measurement", you are conveniently saying that it is relative yo atmospheric pressure. This is largely because this intuitive to us growing up, whereas voltages are not intuitive so its more clear that it needs to be a voltage between two points.

If you are really interested in learning more, I recommend grabbing intro college textbooks (i.e. intro to network/mesh analysis for EE, or intro to mass transfer for flow)... that will get you going.


edit:

Actually many pressure gauges either (a) actually measure the '2nd point' which is usually open to atmospheric air (i.e. differential pressure gauge), or (b) assumes pressure relative to atmospheric pressure (and calibrate the dial to read "0" when exposed only to air pressure). In both instances its a 2 point measurement - the former is usually used for very sensitive applications when you need to be very precise.

 

[Red Squirrel]

The best way to think of voltage is a difference between two points. Often one of the points is ground. So when you see a high voltage line and it's 7,200 volts, that's really the difference between the line and ground. Ground is 0. But really that's just an arbitrary number we give it. A bird that lands on a 7,200v line is not going to have any current flowing as long as the bird is not touching something else as it does not complete the circuit. A device being powered (or electrocuted) is having current flow through it because there is a difference between two points.

To use the water analogy, consider a large pipe that say, has 10PSI of pressure, that pipe is just in open air outside. If you puncture a hole in it, it will be blowing water at you because the pressure difference between the open air and the pipe is 10psi. Now if that pipe keeps going and goes in the sea. If a scuba diver goes down and punctures a hole in it, it will actually have negative pressure because the sea water pressure is going to be much higher, so the sea water will get sucked in.

 

[Billb2]

The OP's "water analogy" is incorrect.

The correct analogy is:
Pressure = Amperage (amps)
Flow Rate - Voltage (volts)
Flow Resistance = Resistance (Ohms)

 

[deadlyapp]

Came in here to exactly post what Magomago posted. Pressure is a relative measurement and not a point measurement, that's why you always will see pressure readings in either gage or absolute (psig or psia if using US units). Gage is relative to atmospheric and absolute is the total pressure (only really useful in vapor pressure affected calculations).

 

[randay]

The OP's "water analogy" is incorrect.

The correct analogy is:
Pressure = Amperage (amps)
Flow Rate - Voltage (volts)
Flow Resistance = Resistance (Ohms)

nope,

pressure is voltage
flow rate = current = amps

ps. I like your sig

 

[Thebobo]

Another example: Say that you had a one gallon jug of water a foot above the ground. You have another gallon of water 100 feet off the ground, but with a 100ft hose dangling all the way to the ground. To get the pressure (voltage) of those gallons of water, you would still only need to install a pressure gauge at the end of the hose, never "across" anything.

Your "across" in this case is the normal pressure and the pressure in the hose.

 

[shortylickens]

I never liked plumbing as an analogy to electricity. Too many things dont match up and in the end you confuse more than inform.

 

[NutBucket]

I've used the water analogy with kids in school (middle school and high school) and it does work if presented properly.

 

[FeuerFrei]

Voltage is the difference in charge levels. How far does the charge drop when it spans a load? 4V drop? 10V drop? There may or may not be any current flowing, depends on if there is any continuity of the circuit between the two points of reference.

If you measure two points a few inches apart on the same wire (with no load betwixt those points), you will measure 0 volts. This is because wires are designed to conduct rather than retard electricity so the voltage drop between such close points is negligible and possibly undetectable by the instrument you use. As you spread measurement points out along the continuous wire eventually you will detect voltage drop because the the wire is not a superconductor and degradation of electrical signal is inevitable, especially in low-current circuits.

 

[NutBucket]

Charge drops because it is used up....like powering a motor or a light bulb.

Charge is potential energy so it's waiting to move from higher potential to lower potential. Without a path it just "waits".

 

[RLGL]

Maybe this will help clear thing a bit: https://en.wikipedia.org/wiki/Hydraulic_analogy

 

[Fenixgoon]

voltage (really, a voltage difference) is the driving force that makes electrons move, the same way that pressure (pressure difference) is the driving force that makes water (or any fluid) move.

in your faucet analogy, the only reason the water moves is because the pressure outside the faucet is less than the pressure inside the faucet (the pressure gauge is actually measuring the difference in pressure, not absolute pressure). if you could cap the other end of the faucet and apply equivalent pressure as the gauge reading, the water wouldn't go anywhere.

voltage is the strength of the driving force to conduct an electron, the same way pressure is driving fluid flow. current is the actual flow of electrons (1 ampere = 1 coulomb/s). likewise, the flow of fluid can be described as a mass flow (kg/s) or volume flow (m^3/s)

with enough voltage, you can make electrons move across very large gaps - lightning, for example, has a voltage of ~100,000,000 volts, which is why electricity can arc from thousands of feet above to the ground (your car battery has 12 volts). and with enough pressure, you can pump a fluid a large distance.

but neither voltage nor pressure tell you how fast the electrons (or fluid) are flowing. that has to do with the resistance of your conductor (or the diameter and restrictions of your pipe).

 

[Billb2]

 

[John Connor]

The OP's "water analogy" is incorrect.

The correct analogy is:
Pressure = Amperage (amps)
Flow Rate - Voltage (volts)
Flow Resistance = Resistance (Ohms)

I was thinking the same thing.

 

[John Connor]

nope,

pressure is voltage
flow rate = current = amps

ps. I like your sig

God dammit.

 

[PowerEngineer]

Pressure = Amperage (amps)
Flow Rate - Voltage (volts)

Nope!

pressure is voltage
flow rate = current = amps

Yep!

I never liked plumbing as an analogy to electricity. Too many things dont match up and in the end you confuse more than inform.

IMHO the problem is that (like all analogies) the water/plumbing analogy for electricity can only be taken so far. The water/plumbing analogy can help a newbie get an initial grasp on some of the concepts (e.g. voltage, current, resistance, and Kirchhoff's laws), but it's best to let the analogy go before getting into capacitance and reactance.

Put another way, a straight-line approximation to a curved function may be pretty helpful over some limited range of values, but completely useless and misleading outside that limited range. Water/plumbing is like a straight-line approximation to electricity.

 

[lakedude]

So I'm seeing a lot of mistakes.

I might help to use the term EMF for Voltage because EMF stands for ElectroMotive Force, which kinda describes pretty well what Voltage is. Voltage is a Force (like a push), Amperage is the rate of flow.

Going to the water analogy, the reason it seems like you can measure water pressure from only one spot is because you are comparing that pressure to atmospheric pressure. The electrical equivalent would be if one side of the meter was grounded, then you could take readings with the other lead and it might seem as if you are only needing one lead but the only reason it works is because you are using ground as a reference point.

In both cases you could choose a different reference point and the readings would be different. Unfortunately for the water analogy to work you must understand physics which isn't really any easier than understanding electricity.

 

[Red Squirrel]

I find it's easier to just try to understand electricity itself then to try to compare it with another system as you quickly learn even that other system can be just as advanced.

I find what really made it click for me is to understand that voltage is not "how many volts are coming out of a wire" but "what is the difference between two wires". A wire on it's own is not delivering any electricity, as it needs a return path, so when you measure voltage you are measuring between that wire and the return path, which is going to be ground, or another wire.

 

[Thebobo]

haha and now this

Researchers report reversal of current flow in a quantum system.

In a classical thermodynamic system, heat current flows from the hotter body to the colder one, or electricity from the higher voltage to the lower one. The same thing happens in quantum systems, but this state can be changed, and the flow of energy and particles can be reversed if a quantum observer is inserted into the system.

This is the main result obtained by the group led by Professor Ángel Rubio of the UPV/EHU and of the Max Planck Institute PMSD, together with collaborators at the BCCMS centre in Bremen. Their study has been published in npj Quantum Materials.

Read more at: https://phys.org/news/2017-10-reversal-current-quantum.html#jCp

 

[MtnMan]

The OP's "water analogy" is incorrect.

The correct analogy is:
Pressure = Amperage (amps)
Flow Rate - Voltage (volts)
Flow Resistance = Resistance (Ohms)

Not even close.

 

[CountZero]

If you want to torture the water analogy further imagine the following (which is likely physically impossible but bear with me):

You have a system that is moving water at 10 psi. Water comes in, a pump gets the water up to 5 psi and then another pump in the line gets it up to 10 psi and then the water goes out.

The pressure difference from one side of the input of the first pump to the output of the second pump is 10 psi. The difference between the input of the first pump and input of the second pump is 5 psi, same for the difference between the input of the second pump and output of the second pump. With this magical system you can now get a spot with 5 psi and a spot with 10psi.

In actual electrical terms think of batteries. Your AA provides 1.5V but connect them serially and you get 3.0V across both batteries even though across each battery you will still only get 1.5V.

For me voltage being across two points seemed easier when I thought of multiple power sources. Single source seems weird at first since we are describing it was 1.5V relative to ground but with two power sources I think you can start to see how relative voltage matters.

 

[VashHT]

I just think of voltage as a potential difference. The difference in potential between two points is the voltage that we measure, and current flows from higher potential to lower if there is a path.

 

[who?]

Water pressure is measured by seeing how far it will push against a spring.
http://jick.net/hr/skept/E_M/node13.html