Confusion about voltage in a circuit

[x86]

I don't misunderstand. I just make the point that people could read this thread and not realize what you and I know - because the concept of equipotential lines is totally misplaced here.
This (anonymised) post :
"do really voltmetres measure joule required to move a coulomb of charge?" shows the possible level of confusion that the thread can be generating.
But you and I agree, obviously, on the facts.

But isn't that the definition of voltage? For instance, if the potential difference between two points is 9V (A-B), doesn't that mean it takes 9J of energy to move a +1C charge from point A to point B by the electric field?

 

[davenn]

i am extremely sorry to both of you sir.it was me i think who led to mesh.i will not post here my comments again.sorry dave sir and Sophiecentaur sir.

You didn't do anything wrong

 

[sophiecentaur]

i am extremely sorry to both of you sir.it was me i think who led to mesh.i will not post here my comments again.sorry dave sir and Sophiecentaur sir.

Hey - don't apologise. One of the main reasons for the existence of PF (apart from letting some of us show off) is to straighten out misconceptions and help people see things in more productive ways. For heaven's sake don't stop posting (as long as you don't mind people disagreeing with you occasionally. ) It does everyone good to go over old ground and get things straight in their own minds.

 

[sophiecentaur]

But isn't that the definition of voltage?

Yes, that is a definition of the Voltage but that is not the same as actually measuring a voltage using those two quantities. In basic terms you could say that a voltmeter does determine the energy per coulomb but it really only does this by an indirect means - for instance by measuring the Current through High resistance. I think am making a genuine distinction here.

 

[jim hardy]

@CRT

Just another old timer here saying you needn't apologize at all

do really voltmetres measure joule required to move a coulomb of charge?

That question really interested me - i was trying for days to figure out an answer.

As Mr Sophie said, the plain vanilla everyday meter measures how much current an unknown voltage pushes through a known resistance.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html

Microamps flowing through the red coil create torque.
The needle deflects until the torque from current just balances the torque from the spring.
The scale is marked in volts rather than micro-amps.
You'd switch in various series resistor values to make a multi-range voltmeter.

There does exist a device called "Electrostatic Voltmeter" that measures potential more directly
http://en.wikipedia.org/wiki/Electrostatic_voltmeter

Electrostatic voltmeter utilizes the attraction force between two charged surfaces to create a deflection of a pointer directly calibrated in volts...
The pivoted sector NN is attracted to the fixed sector QQ. The moving sector indicating the voltage by the pointer P and is counterbalanced by the small weight w. In newer instruments the weight is replaced by a spring, thus allowing the meter to be used both in horizontal and vertical positions.

Hmmmm at least it uses Newtons...

An electroscope can measure charge...
http://physics.kenyon.edu/EarlyApparatus/Static_Electricity/Electroscope/Electroscope.html

The two electroscopes below are equipped with the bottom plate of Volta's condenser. The upper plate is of the same dimensions as the lower one, and has an insulating glass handle attached, and its lower surface is covered with varnish for insulation.
The condenser allows small quantities of charge to be detected. It is charged by touching the object under test to the lower plate, while grounding the top plate. If the lower plate has been positively charged, negative charge will be transferred to the upper plate. The ground connection is then broken, followed by disconnecting the test object. The capacity, C, is then decreased by lifting up the upper place. Since the potential across the capacitor, V, is related to the constant charge Q by Q = CV, the potential increased and the electroscope leaves diverged.

REF: Thomas B. Greenslade and Richard H. Howe, "A Modern Use of Volta's Electroscope", The Physics Teacher, 19, 614-615 (1981)

Now it strikes me that the act of separating the plates involves overcoming the attraction between them
so in principle one exerts some minute amount of Newton-meters in that experiment.

My meager brain isn't deep enough to figure out how one would make a joules-per-coulomb meter from an electroscope
but it looks like an interesting thought experiment.

So, Mr CRT, I thank you for your thought provoking question. It's given me something to think about while the TV is on..

old jim

 

[jim hardy]

@CRT Will you pardon an old guy a bit of nostalgia ?

This digression has a purpose, so please bear with me.

This is why kids should be shown how to work on old cars. It teaches them a lot.

I knew i'd encountered a simple, earthy "Joules per second " meter someplace in my distant past.
Just now came to me.

My 1953 Ford Stationwagon (wish i still had it)

had gages operated by "Heated Wire" technology of the 1930's. Rugged and reliable, if less sophisticated than today's computer dashboards.
I can't find a Ford shop manual explanation but this old Chrysler one will suffice to make the point:
http://www.allpar.com/history/mopar/electrical2.html
Basically a wire is heated by a current proportional to what it is you wish to measure.
The wire expands with temperature and that mechanical movement is transmitted to the indicating needle.

Now think about this --- the temperature of that wire is in proportion to the rate at which heat is put into it.
That'd be watts, or joules per second.

Deflection then is in proportion to Joules per second.

GIBRALTAR: the original Chrysler gauge design

Virtually all Mopar RWD passcars used thermal-type instrumentation (shown at right). Simply put, this means that the pointer of each gauge (excluding the ammeter, of course) is mechanically linked to a bimetallic strip. The strip is wrapped with resistance (heating) wire, just like what’s in old toasters, etc. As the current passed through this wire is increased, the wire gets hotter, transferring this heat to the bimetal strip, which bends more and more as the temperature is increased, deflecting the pointer. As you probably guessed, this gauge design is inherently very well damped and very slow to respond - which is probably a good thing. Nobody wants to see the gas gauge, for instance, swing wildly as the fuel in then tank sloshes around. And the designers clearly were just as happy that Vern can’t see the temp gauge fluctuate as the thermostat opens and closes, or the oil pressure drops down quickly to 20 PSI in traffic on a hot day.

One end of the Nichrome (resistance) wire is connected to the sending unit for that particular gauge: a thermistor (temperature-variable-resistance solid-state device, see above) in the case of the water-temperature gauge; a simple variable resistor linked to a float in the example of the gas gauge (fig. 2 in the photo section), and a variable resistor linked to a diaphragm in the case of the oil gauge (“A” in fig. 4, photo section.) (Cars without an oil gauge have a warning light, which is activated by means of a simple switch - “B” in fig 4).

The other end of all these gauges need a reference voltage supply. Something rock-steady and unwavering. If this voltage varies, so will the gauge readings. Take a look a the crude regulator in fig. 6, at right. This relic of the 1930s is all that’s there for your gauges to work off of. It’s junk!

That gets me one step closer to your direct indicating "Joules per coulomb" voltmeter.

Old brain hasn't clicked onto it just yet..
BUT
This beats watching TV commercials !

Those old gages were well suited to their task
and provoked thought - as one drove down the road, all gages would shift slightly when you hit a shady part of road and the dashboard cooled a degree or two.

As i said - an old car is good for a kid. But he needs the shop manual and some tools.

thanks mentors for indulging me...

old jim

 

[CRT]

hats off jim hardy sir your enormous experience can do anything impossible...

 

[CRT]

(joule per second)/ampere gives joule per Coulomb can the readings from that old car metre be callibrated to give j/c

 

[Orien Rigney]

Capacitors placed across a D.C. battery will allow no fluctuation other than a steady rise to the maximum pressure (potential) a battery has stored in it. But! While a battery does not pass D.C. current, it will pass a cyclical maximum of A.C. potential.

 

[CRT]

yes you are correct.its only due to frequency of the supply.a capacitor after full charged behaves as open circuit due to infinite capacitive reactance when frequency is 0 say in dc

 

[jim hardy]

(joule per second)/ampere gives joule per Coulomb can the readings from that old car metre be callibrated to give j/c

i don't think so. I can't make it work in my head, at least.

But keep up the 'thought experiments'. And try some out with real parts.

 

[zoki85]

With voltmeters/ampermeters it is possible to measure various things, including the value of gravitational acceleration (g) near Earth surface.

 

[Orien Rigney]

I'm familiar with gravitational potential energy but have never tried using a voltmete to measure this energy. Please explain how it is done.

 

[zoki85]

I'm familiar with gravitational potential energy but have never tried using a voltmete to measure this energy. Please explain how it is done.

Some methods how g can be measured:

1. Climb the building 50 meters high. Drop the instrument and measure with a stop-watch how long it takes to hit the ground level. Then calculate g≈2h/t²
2. Hang the instrument to the ceiling by rope of length l. Measure time of a period of small oscillations of the pendulum. Then calculate g≈4π²l/T²
3. Read the manual of the instrument if the mass is listed there. Hang the instrument by dynamometer hooked to the ceiling and measure force. Then g=G/m

I would prefer methods 2 and 3 over 1 which is destructive and more expensive methods. And there are certainly other, more rafined methods of measurement

 

[jim hardy]

Fun with physics !

That's the theoretical approaches.

A practical fellow would go to the nearest university, find the geophysics department , and inquire of the lab supervisor:
"I'll give you this fine voltmeter if you'll tell me what is the precise value of local gravity."

Where i worked local gravity was ~0.17% less than standard
and it made that U of Miami geophysicist's day when i asked
because he'd just recently measured it to be around 978._something gils at Dodge Island seaport using a pendulum ..
Disclaimer - no bribery was necessary. You know what a treat it is when somebody shares your interest...

@CRT everyday things are tremendously interesting. Indulge that natural curiosity.Sigh again I've digressed
But "...all work and no play..."
we learn from play, too.

old jim

 

[Orien Rigney]

I believe dropping a one kilo weight from a height of one meter onto a well calibrated strain gage or scale would also suffice?

 

[zoki85]

Read this anecdote: http://www.spiritsound.com/bohr.html
He,he

 

[donpacino]

Ah okay. Thank both of you for your help, and sorry about the confusion. It is actually pretty simple when I think about it. It takes 9 J to pump a 1 C charge from the (+) terminal to the (-) terminal in the above diagram, and if the wire has resistance, then we lose some of this energy to heat, and of course how much we lose depends on the length of the wire and the resistance of the wire.

It appears that others have answered your questions. I just want to offer some prospective

You're studying computer engineering right?
While looking at energy and charge are important in some aspects, for the most part you will likely not work with them in your career as a computer engineer. You will most likely only work with voltage, current, and power.
When working with those, for the most part, you do not need to concern yourself with factors such as energy and charge. Most of the time they won't be important when looking at circuit analysis and design from a higher level.