Thread: Voltage/Frequency/Current Question

Im curious about varied power consumption for different devices - say household stuff - a cell phone charger and a toaster oven for example.

all household devices run on the same voltage/frequency (120v/60hz where I am)

however different devices "draw" different amounts of current (the cell phone charger less then the toaster).
ive read that the current draw is related to the resistance of the device. more resistance draws more current.

im confused about this dynamic though.
if the voltage and frequency are static 120v/60hz - and the speed of propagation is constant (is it?) then shouldnt a constant amount of current flow?

the amplitude(voltage) of the wave is constant, and the frequency/wavelength is constant - so how does more or less energy get transferred?

... i have been reading about wave propagation and i dont know if im mixing up different fields of physics btw...

-two ducks

You have it just the opposite...the LESS resistance, the more current. Think of a garden hose....if it's filled with crap, it provides more resistance to the flow of water. (current) So get let less liters per second of water out of the end of hose (amps) Clear the crap out and you have less resistance to the flow, more current...more liters per second of water coming out of the end. Voltage is sorta like the "pressure" that pushes the water (electricity) down the hose. The greater the pressure the greater the flow in the same diameter hose.

Start with the basic formula for electricity: Ohm's Law

Voltage (volts)= Current(amps) X Resistance (Ohms)

and

Power (watts) = Current X Voltage.

Originally Posted by MacGyver1968

You have it just the opposite...the LESS resistance, the more current.

roger that!

although i still dont see how different devices would "use" more or less power if the frequency/voltage is constant - thats the main part of my inquiry =)

example: a big electric fan has to do a lot more "work" than a small electric fan.
so the big fan draws more current - and doesnt it also have more resistance? - you have to "push" it harder to spin in? hence it "resists" more? which would also be why it required more energy to spin?

No, it has less resistance. Electrical resistance is a different property than friction. Resistance restricts the flow of electricity. The symbol for current in an equation is "I" So... I=V/R or current=voltage/resistance.

Hopefully one of the smarter chaps will be along soon to explain it better.

Originally Posted by c0sm

although i still dont see how different devices would "use" more or less power if the frequency/voltage is constant - thats the main part of my inquiry

A potential difference (a voltage difference) just means a lot of electrons in one terminal (the negative terminal) compared to the other terminal (the positive terminal.) They could sit there forever without transferring any energy. It is not until you connect the two via a conductor that current can begin to flow, and energy be released. The better the conductor (i.e. the lower the resistance) the more the current flow.

example: a big electric fan has to do a lot more "work" than a small electric fan.
so the big fan draws more current - and doesnt it also have more resistance?

No, it has less resistance.

- you have to "push" it harder to spin in? hence it "resists" more? which would also be why it required more energy to spin?

Don't confuse electrical resistance with mechanical resistance - they are very different things!

thank you both! interesting!

i am still confused though about this difference in current used.
i have a (tenuous)grasp of voltage potential.

so it sounds to me that one device can "use" more electrons than another. yet both devices are powered by an energy wave which is the same amplitude and frequency.

that seems like a conflict to my sense of reason... doesnt a wave with static amplitude and frequency have a static energy density?

Originally Posted by c0sm

thank you both! interesting!

i am still confused though about this difference in current used.
i have a (tenuous)grasp of voltage potential.

so it sounds to me that one device can "use" more electrons than another. yet both devices are powered by an energy wave which is the same amplitude and frequency.

that seems like a conflict to my sense of reason... doesnt a wave with static amplitude and frequency have a static energy density?

You are indeed mixing various concepts, so you are making things unnecessarily complicated.

Forget about "waves." They are totally irrelevant here.

Let's start with good ol' Ohm's law for purely resistive circuits: Voltage = Current x Resistance. Also: Power = Voltage x Current. Notice that frequency doesn't enter into this at all. Just voltage, current and resistance. If we allow for components that store energy (inductors and capacitors), frequency enters the picture indirectly. But frequency is not characteristic of an energy density, static or otherwise.

If a device "consumes" more power, that will be reflected in the voltage-current product. If the voltage is fixed, then a higher power will be associated with a higher current.

When we talk about power consumption, do not conflate that with "electron consumption." Ain't no such thing. No electrons are killed off. They simply yield some of their energy, that's all.

You should also know that most "transformers" are actually complex beasts these days. They convert AC into high-voltage DC, and then regulate that downward to the final voltage. This complexity is suffered in exchange for a significant reduction in size and weight, compared to a classical transformer (which consists of wire wound on an iron core).

Whether the transformer is a classical one or a modern one, energy is conserved, so whatever power it delivers will be reflected as a power taken from the wall. Efficiencies are always less than 100%, of course, so you will take more than you give. The balance is given off as heat.

OK. The classic way to explain electricity is to compare it with water:

Think of Voltage as water pressure.

Think of Current as water flow.

Imagine a garden hose pipe connected to the garden outside tap at one end and with a valve on the other end. You turn the garden tap on and the hose pipe gets pressurised, but no water flows because you haven't opened the valve at the end yet. The pressure inside the hosepipe is like voltage in a battery - it has potential, but it is not doing anything and no work is being done by the voltage, so no power is dissipated.

OK, now you open the valve at the end and water flows out. If you point the water jet onto a small water wheel, it will turn. This is like current flowing and doing work, so energy is expended.

The hosepipe restricts the flow of water, so only a certain amount of water can flow through it in a given time. If you changed to a hosepipe twice the internal diameter, more water could flow in the same time, (and your water wheel would turn faster). This illustrates another electrical phenomenon: resistance. Resistance restricts current flow and causes a voltage drop - in the same way that our hose pipe restricts water flow and causes a pressure drop.

If you pointed the original hose at a water wheel ten times bigger, it would not turn as fast as the small water wheel - it would need more water flow. To achieve this you could use a larger, less restrictive hose, or ask the water company to increase the mains pressure, to force more water down the original sized hose.

As others have said, Ohm's law is:
V = IR.
Voltage = Current (I) multiplied by resistance.
The voltage drop across an electrical item is equal to the current running through it in Amps* multiplied by the resistance of the item in Ohms.

The power of an electrical device is found by IV = W. Current in Amps multiplied by Voltage in Volts equals the power in Watts.

Both these equations can be rearranged to find any quantity you need. For example if you measured 10 Volts across a resistor of 10,000 Ohms, then a current of 0.001 Amps (1 milliamp) is flowing through the resistor. The power being 'used' by the resistor will be IV, or 0.001 times 10, which is 0.01 Watts (10 milliwatts).

That's the basic idea. There are complications, for example with alternating current and reactive loads, but which are beyond the scope of this simple explanation, and will confuse the uninitiated. Forget about "energy waves" etc., they are a red herring at this stage.


Hope that helps !


OB


*PS: Very confusingly, Current measured in Amps is designated 'I' in equations but 'A' at all other times, I don't know why this is, but there must have been a conflict somewhere along the line.

Originally Posted by tk421

You are indeed mixing various concepts, so you are making things unnecessarily complicated.

Forget about "waves." They are totally irrelevant here.

Let's start with good ol' Ohm's law for purely resistive circuits: Voltage = Current x Resistance. Also: Power = Voltage x Current. Notice that frequency doesn't enter into this at all. Just voltage, current and resistance. If we allow for components that store energy (inductors and capacitors), frequency enters the picture indirectly. But frequency is not characteristic of an energy density, static or otherwise.

this is where i apologize for asking silly questions =)
im (relatively)familiar with ohms law, and as i understand current, 1Amp is defined as a measure of charge passing a point in a period of time. i gotta say though, that the definitions for this get a little hazy for me. (1coulomb/s =1 amp?) 1 coulomb = 6.24x10^18th electrons?

is this definition of measurement reasonable for AC - since the electrons move back and forth? wouldnt you just be counting the same electron each time it jiggled back an forth in span of a second? so more jiggles per/s more amps?
if this is totally stupid i hope you at least get a laugh out of it =)

my impression of frequency implores me to consider it a factor of energy value. if i wave my hand slowly it is less energetic than if i wave it fast. i realize im conflating different fields of physics with that analogy.... /scratching head

Originally Posted by tk421

If a device "consumes" more power, that will be reflected in the voltage-current product. If the voltage is fixed, then a higher power will be associated with a higher current.

i have a tenuous grasp on the math... ive built simple LED circuits. but the concepts dont make sense entirely... i know the "if this then this" formula - i think im just trying to work out conceptual mechanisms for the how and why...

Originally Posted by tk421

When we talk about power consumption, do not conflate that with "electron consumption." Ain't no such thing. No electrons are killed off. They simply yield some of their energy, that's all.

You should also know that most "transformers" are actually complex beasts these days. They convert AC into high-voltage DC, and then regulate that downward to the final voltage. This complexity is suffered in exchange for a significant reduction in size and weight, compared to a classical transformer (which consists of wire wound on an iron core).

Whether the transformer is a classical one or a modern one, energy is conserved, so whatever power it delivers will be reflected as a power taken from the wall. Efficiencies are always less than 100%, of course, so you will take more than you give. The balance is given off as heat.

=) reading about rectifiers right now. stoked for your input!

Originally Posted by One beer

. Resistance restricts current flow and causes a voltage drop - in the same way that our hose pipe restricts water flow and causes a pressure drop.

ive heard the water analogy and its good in a lot of ways... on this point about resistance though..

resistance in the circuit would cause voltage in the circuit to drop, but it would cause the voltage in the supply to spike wouldnt it?

so if we're talking water; if u restrict the hose, the pressure backs up in the pipes supplying the hose, while the pressure hitting the water wheel (for example) would decrease.

so i assume there is a similar phenomenon of "back pressure" with voltage in power supplies/circuits?

Originally Posted by One beer

If you pointed the original hose at a water wheel ten times bigger, it would not turn as fast as the small water wheel - it would need more water flow. To achieve this you could use a larger, less restrictive hose, or ask the water company to increase the mains pressure, to force more water down the original sized hose.

pressure is analogous to voltage in this metaphor? as i understand voltage and current are not entirely equivocal correct? its possible to generate very high voltage that does not have much current?
so more pressure/voltage wouldnt necessarily mean more water correct? especially if you increase resistance by directing the current @ a bigger water wheel?
high R = low I ?

Originally Posted by One beer

As others have said, Ohm's law is:
V = IR.
Voltage = Current (I) multiplied by resistance.
The voltage drop across an electrical item is equal to the current running through it in Amps multiplied by the resistance of the item in Ohms.

The power of an electrical device is found by IV = W. Current in Amps multiplied by Voltage in Volts equals the power in Watts.

and backward right? so V/W=I ? (120V/60W=2A ? ) <-like a light bulb?

Originally Posted by One beer

Both these equations can be rearranged to find any quantity you need. For example if you measured 10 Volts across a resistor of 10,000 Ohms, then a current of 0.001 Amps (1 milliamp) is flowing through the resistor. The power being 'used' by the resistor will be IV, or 0.001 times 10, which is 0.01 Watts (10 milliwatts).

That's the basic idea. There are complications, for example with alternating current and reactive loads, but which are beyond the scope of this simple explanation.


Hope that helps !


OB

thanks! =D
that sounds interesting....
(looks up reactive loads)

Originally Posted by c0sm

this is where i apologize for asking silly questions =)

No need to apologize for asking questions! Education is one of the chief aims of this forum.

im (relatively)familiar with ohms law, and as i understand current, 1Amp is defined as a measure of charge passing a point in a period of time. i gotta say though, that the definitions for this get a little hazy for me. (1coulomb/s =1 amp?) 1 coulomb = 6.24x10^18th electrons?

It's subtly incomplete, but for now, we can go with that definition. [The subtlety is that we can define a current even if there are no electrons -- more on that later, if you wish.]

is this definition of measurement reasonable for AC - since the electrons move back and forth? wouldnt you just be counting the same electron each time it jiggled back an forth in span of a second? so more jiggles per/s more amps?
if this is totally stupid i hope you at least get a laugh out of it =)

You're not as far off as you think, but we can still close the gap some. If you were to make a graph of current vs. time for an AC waveform, it might look like a sinusoid, for example. That means that the current rises to a peak, then descends from that peak, passes through zero, and then reverses sign. That's not the same thing as seeing one electron dance.

my impression of frequency implores me to consider it a factor of energy value. if i wave my hand slowly it is less energetic than if i wave it fast. i realize im conflating different fields of physics with that analogy.... /scratching head

Here you've gone off the beam. You've equated the waving of a hand at some frequency, with the energy of all things waving at some frequency, including things that may not be waving at all (but nonetheless having a frequency). Energy is not uniquely tied to frequency. For example, I think you'd agree that waving your hand at a 10 cycle per second rate is very different from waving a one tonne lead weight at that same rate. So clearly frequency does not equal energy.

i have a tenuous grasp on the math... ive built simple LED circuits. but the concepts dont make sense entirely... i know the "if this then this" formula - i think im just trying to work out conceptual mechanisms for the how and why...

The circuits may be simple, but an LED actually isn't -- at least, it is not a simple resistor. It is an example of an element that disobeys Ohm's law (which isn't a very good law, evidently!).

=) reading about rectifiers right now. stoked for your input!

Mmmm...rectifiers.

Originally Posted by tk421

It's subtly incomplete, but for now, we can go with that definition. [The subtlety is that we can define a current even if there are no electrons -- more on that later, if you wish.]

a current without electrons?! that does sound interesting =)

Originally Posted by tk421

You're not as far off as you think, but we can still close the gap some. If you were to make a graph of current vs. time for an AC waveform, it might look like a sinusoid, for example. That means that the current rises to a peak, then descends from that peak, passes through zero, and then reverses sign. That's not the same thing as seeing one electron dance.

in an AC sine wave, isnt it the voltage and and frequency which are graphed? or are you saying you could also/instead map the current and the frequency?
at the moment im reading about leading/lagging current vs voltage - which blew my mind when i first came upon the concept....

and i realize its not really "one" electron - i was kinda just trying to simplify. the concept which i was asserting is that if Amperage is equal to # of electrons passing a point, then you could get the same effect from passing many electrons past a point or from 1 electron passing a point many times. correct?-ish? =)

Originally Posted by tk421

Here you've gone off the beam. You've equated the waving of a hand at some frequency, with the energy of all things waving at some frequency, including things that may not be waving at all (but nonetheless having a frequency). Energy is not uniquely tied to frequency. For example, I think you'd agree that waving your hand at a 10 cycle per second rate is very different from waving a one tonne lead weight at that same rate. So clearly frequency does not equal energy.

yes definitely agree on the hand vs the 1 tonne weight. i didnt intend to conflate hands with all things in that way.. i intended to compare same things with other same things moving more vigorously...
sticking with the electrical theme, an electron moving back and forth at 1hz should be less energetic than an electron moving back and forth at 10hz - presuming the same amplitude of the wave. - ?

Originally Posted by tk421

The circuits may be simple, but an LED actually isn't -- at least, it is not a simple resistor. It is an example of an element that disobeys Ohm's law (which isn't a very good law, evidently!).

DISOBEYS OHMS LAW?! i didnt realize! i love breaking laws =)
gonna have to play with LEDs more!

Your enthusiasm for learning about electronics makes my heart happy as hell. I'll help when I can...guys like TK and Bill really know their stuff. Don't be afraid to ask questions...sometimes people (me) learn things by looking up the answer....and think of the lurkers that aren't posting...you may be asking questions for them too.

If you would like to learn more...we can come up with some experiments you can perform to better your knowledge of electricity and it's components. We get E-karma points for making new techs.

Plus...once you have mastered the basics of electricity...you get to laugh at movies and television when they get it oh so wrong.

How old are you C?

If you are interested in learning more...one of the first projects you can do is build yourself an adjustable DC voltage source. This will allow you to experiment with DC circuits, which is what they first teach you in electronics school. Many of the components can be salvaged from old electronics....a skill you should master to save money. If you are interested, I'll post a schematic and a parts list.

Here's the one I built myself some 20 years ago with nothing but savenged parts... the voltage regulators we're the only part that wasn't "re-purposed" including the case.

Originally Posted by MacGyver1968

How old are you C?

If you are interested in learning more...one of the first projects you can do is build yourself an adjustable DC voltage source. This will allow you to experiment with DC circuits, which is what they first teach you in electronics school. Many of the components can be salvaged from old electronics....a skill you should master to save money. If you are interested, I'll post a schematic and a parts list.

28 =)
and i am interested in more learning - more-so than actually building anything in particular at the moment. i dont even have a soldering iron right now :P but theyre cheap and most other things can be scavenged as you noted...

the only thing ive built in the past (circuit wise) was a piezo triggered LED light system for my drumset - hit the drum and lights flash =)

it was my first and only electronics project thus far... i just copied a circuit diagram someone else had designed. it was something like a piezo trigger that opened a gate on a transistor. there was also a 555 timer and a potentiometer used to tune the length of the flash i believe. 9v battery and some LEDs =)

something like than anyway, it was a few years ago.

i like building stuff but i think im gonna wait till i stumble onto something that looks interesting/useful - which i imagine will happen as i learn about more things - maybe i can figure out how to build something better than the way its currently done. =)

Reading between the lines, whole thread, can't help thinking someone is being "had" here. (u)cosm has technical terms down pat I have never even heard of, but maybe it's just me? jocular

Perhaps, but I don't think so. Piezo will be a familiar term to someone in a band - acoustic guitar pickups use piezo elements. And someone can build an electronic circuit by following instructions, without understanding how it works.

resistance in the circuit would cause voltage in the circuit to drop, but it would cause the voltage in the supply to spike wouldnt it?

so if we're talking water; if u restrict the hose, the pressure backs up in the pipes supplying the hose, while the pressure hitting the water wheel (for example) would decrease.

so i assume there is a similar phenomenon of "back pressure" with voltage in power supplies/circuits?

Sort of, but the reason the pressure/voltage is changing is because the flow/current demand is changing. The two are related through Ohms law - more current drawn leads to more voltage drop.


Originally Posted by One beer
If you pointed the original hose at a water wheel ten times bigger, it would not turn as fast as the small water wheel - it would need more water flow. To achieve this you could use a larger, less restrictive hose, or ask the water company to increase the mains pressure, to force more water down the original sized hose.

pressure is analogous to voltage in this metaphor? as i understand voltage and current are not entirely equivocal correct? its possible to generate very high voltage that does not have much current?
so more pressure/voltage wouldnt necessarily mean more water correct? especially if you increase resistance by directing the current @ a bigger water wheel?
high R = low I ?

Yes it would; this is why it can be helpful to think of voltage as pressure. If you increased the pressure supplying the hose, more water would flow through it. If you increased the pressure beyond a certain level, the hose would fail. Ditto in electrical circuits.

Think of a 120 volt battery. If nothing is connected to it, it's voltage is 120V but the current flow is zero. With the hose pipe, if no water flows out of it, then the pressure inside is the same as the water company supply pressure to the tap. When you let water flow out, the pressure coming out of the hose pipe will not be the same as the pressure at the tap - the 'resistance' of the pipe has caused the pressure to reduce. The resistor in an electronic circuit behaves exactly like the hosepipe.

In the water wheel analogy the water is pouring into the buckets and filling them. If the wheel is bigger with larger buckets, you need more water flow per second to fill them. So to make it turn it needs more water per second (analogous to current).



Voltage and current are linked with resistance through Ohms law as I say. So a given amount of voltage put across a given resistance will cause a certain amount of current to flow. Increase the voltage or decrease the resistance and more current will flow. The voltage is the force of the electricity, the current is the electrons actually moving through the circuit*.

Originally Posted by One beer
As others have said, Ohm's law is:
V = IR.
Voltage = Current (I) multiplied by resistance.
The voltage drop across an electrical item is equal to the current running through it in Amps multiplied by the resistance of the item in Ohms.

The power of an electrical device is found by IV = W. Current in Amps multiplied by Voltage in Volts equals the power in Watts.



and backward right? so V/W=I ? (120V/60W=2A ? ) <-like a light bulb?

Almost. You rearranged the equation wrong. You meant W/V = I, so your light bulb will use 0.5A. (60W/120V = 0.5A)

You can rearrange the equation to find whatever you need. (Note this is true for DC voltage and/or purely resistive loads such as heaters and filament light bulbs. With Alternating Current, and reactive loads such as motors or capacitive light fittings (e.g. fluorescent), different effects occur and different equations and considerations are required.


OB


* Let's not get into holes and current flow direction here, let's keep it simple.

Also,

however different devices "draw" different amounts of current (the cell phone charger less then the toaster).
ive read that the current draw is related to the resistance of the device. more resistance draws more current.

...........is the wrong way round.

Devices that draw more power have LESS electrical resistance*. They need more current to provide more power, so from Ohms law and IV = W, they need less resistance. The power taken by your toaster will be about 2000W. The power taken by your cellphone charger will probably be 0.25W. So if my maths is correct, the resistance of your toaster will be about 7 Ohms, the resistance of your cellphone charger will be about 57,600 Ohms.



(*As somebody said, don't confuse electrical resistance with mechanical resistance.)


OB

lol @ jocular - i know a little bit about a lot of things =)

all good input from everyone thanks!

as ive continued to ponder this, it occurred to me that i may have missed a particular factor.

ive been thinking the the utility company "sends" 120v AC, but really they send much higher voltages, and the higher voltages are transformed before they get to me.

so even though i "get" a steady 120v out of the wall, they may actually generate varying (much higher) voltages to counteract the changing loads in the total system. correct?

ive read about power grid issues relating to large numbers of people all turning things on at the same time for example. they power company would need to increase the voltage they generate in order to deal with load and resistance changes then? but that voltage increase isnt "seen" by me since it happens pre-transformer?

i thought of an interesting analogy relation to soundwave propagation/speaker technology which may(?) be pertinent here..

a speaker does a push-pull motion relative to the +/- voltage applied to it. to produce a pitch of 1000hz the speaker is push/pulled 1000 times/s.

now to increase the volume of the sound, and maintain the pitch, you have to increase the speed of the push/pull motion, without increasing the frequency - the only way to do that is by increasing the distance it travels.

so if a speaker travels 1mm forward and 1mm backward(1000times/s), and you increase the amplitude, the speaker will move 2mm forward and 2mm backward, in the same same amount of time(1000times/s). it moves double the distance in the same time frame - increasing the amount of energy in the soundwave while maintaining the frequency.

(the things i think about when trying to fall asleep....)

Originally Posted by c0sm

ive been thinking the the utility company "sends" 120v AC, but really they send much higher voltages, and the higher voltages are transformed before they get to me.

so even though i "get" a steady 120v out of the wall, they may actually generate varying (much higher) voltages to counteract the changing loads in the total system. correct? The voltage supplied by utilities is held as nearly constant as possible, this requiring that they (the utilities) constantly increase and decrease the quantity of energy they are "shipping out". Another serious consideration is maintaining the frequency of alternating current as constant, otherwise devices dependent on the 60 HZ frequency, namely electric clocks, would not keep time correctly. To achieve this, utilities monitor frequency very closely, and vary it above and below 60 HZ as needed so that the average is always 60 (in the U.S.) I think some places used 50 years ago, today, I am not sure.

ive read about power grid issues relating to large numbers of people all turning things on at the same time for example. they power company would need to increase the voltage they generate in order to deal with load and resistance changes then? but that voltage increase isnt "seen" by me since it happens pre-transformer? Electric power carried long distances always presents the difficulty of maintaining constant voltage at the users' locations. However, history of usage provides fairly accurate information making needs predictable with respect to time of day, day of week, month of year, etc. The greatest load-producing user devices are in general the most predictable: air conditioning and heating requirements, this being from a residential standpoint. Commercial users such as manufacturing plants, factories, pose yet another kind of challenge.


(the things i think about when trying to fall asleep....)

It should be mentioned that high-voltage used for long-distance power transmission lowers the need for large conductors, which are heavy, thus which increase cost of supporting structures. Current intensity needed is lower (amperage), to transmit a given amount of power (wattage) if voltage is raised. Years ago, several hundred-thousand volts was about the highest used, today lines are in use carrying 1/2 million volts. A new line was proposed (and probably built) when I still lived in Phoenix, Arizona, to transmit power from the Hoover Dam area to Phoenix, likely about 250 miles as the lines run using DIRECT CURRENT. Perhaps someone will research that fact?

Power is generally "moved" in multiples of the familiar 120 volt number we all are familiar with, though that is not necessary, this keeping transformer ratios reasonably straight-forward. Residential "alley" line potentials used are often on the order of about 7,200 volts, locally dropped down to 240/120 for the homes. Some areas use around 13,000 volts for residential distribution. Cost of supporting structures, as well as maintenance costs, increase quickly as higher voltage is used. jocular

Here is a video showing high-voltage line maintenance work. Rather hair-raising, and amazing! Having done some crazy things in my time, almost as perilous as this work is, I don't believe I would undertake it. jocular

I think the hydraulic analog to electrical circuits works well here. It's been a while for me, but ...

Electrical → Hydraulic
Voltage → Pressure
Current → Flow
Resistance → Restriction

Basically, the electric power company keeps the voltage (pressure) the same when the load varies, and it's the current (flow) that changes with variations in the load. True, if the load increases too quickly for the power company to compensate, the voltage can drop, causing a "brown out".

The electric power companies never output 120VAC, it's always higher — much higher — voltages (maybe 100,000's of volts) to reduce the loss in those huge overhead high-voltage lines held above by towers. Power loss in transmission lines can be computed as I²R, so you would want to minimize the current and the resistance to minimize the power loss. Then, at electric sub-stations, transformers step down the voltage toward a more usable form, probably in the 1,000's of volts range. Even then, it's the relatively-small distribution transformers on utility poles that we see in neighborhoods that steps down the voltage even more to the 120VAC that America uses.

And the term "step down" is misleading. Transformers have a completely isolated input and output, and I suppose it could be said that there's no electrons flowing from the input to the output. It's not the same current. Transformers convert electrical power to magnetic power and back to electrical power. It's the ratio of turns in the input's coil to the turns of the output's coil that determine the input-output voltage ratio. If the ratio of turns is 25:1, then a 3,000VAC input will produce a 120VAC output, a 30VAC input will produce a 1.2VAC output, etc.

The size of a transformer indicates the amount of power that it can handle because of the size of the conductors inside (remember the I²R loss). Your tiny cell phone charger has small wires that might have a ratio of, for example, 17:1 which steps down the 120VAC input to a 5VAC output, which when converted to direct current and smoothed produces the, for example, 5VDC output that your phone needs to recharge. True, you could build a cell phone charger the size of a house, but what you pay for in materials wouldn't justify the savings in the smaller power loss.

More awesome high-voltage displays: the first is a 3-phase line being open-circuited while under load. The moving "contacts" are cross-sectionally massive enough to not become excessively hot at their ends before the arc extinguishes.






The second shows explosive "splicing" of high-tension lines, below.




Below, the sudden explosion of a large gas turbine engine, as might be used to generate electric power.

Originally Posted by jrmonroe

Basically, the electric power company keeps the voltage (pressure) the same when the load varies, and it's the current (flow) that changes with variations in the load. True, if the load increases too quickly for the power company to compensate, the voltage can drop, causing a "brown out".

Can you tell us anything about the method used to convert today's D.C. transmitted high-tension power back to A.C.? jocular

Sorry, HVDC is new to me.

Originally Posted by jrmonroe

Sorry, HVDC is new to me.

Me, too! rotary mechanical converters are cumbersome, and surely not applicable directly to voltage levels in the hundreds of kilovolts. Solid state by some sort of cascading? The current levels must be as awesome as the voltage!

Perhaps a Salt River Project Engineer may by chance see this exchange, and enlighten us. SRP is the major player in high voltage DC transmission to Phoenix, I believe. jocular

I found this:

How HVDC Works | High Voltage DC Transmission (HVDC)

which describes rudimentarily the process of HVDC transmission. It seems thyristors are used in both AC-DC and DC-AC conversion. The company whose website is the above is building operating a 700KV transmission line, using DIRECT CURRENT!

Harold, I wonder if perhaps you might be privy to some info on this? Forgive my childish way of seeking your attention. joc

Another serious consideration is maintaining the frequency of alternating current as constant, otherwise devices dependent on the 60 HZ frequency, namely electric clocks, would not keep time correctly. To achieve this, utilities monitor frequency very closely, and vary it above and below 60 HZ as needed so that the average is always 60 (in the U.S.) I think some places used 50 years ago, today, I am not sure.

Yes; Here in the UK, our line supply is 240V @ 50 Hz. Double the voltage, so half the current for the same power, therefore either less transmission loss or smaller conductors needed.

cOsm, you are basically correct; To make your speaker louder, you need to increase the voltage of the alternating waveform fed to it. This will increase the current in the speaker coil which will increase the strength of the magnetic field which moves the cone. As you say, the frequency remains the same, but the amplitude of that frequency increases, and this increases the movement of the cone.

You are obviously curious about alternating current. Without going into detail; it is basically used by electricity supply companies because it is easy to generate with a rotating machine, and easy to change it's voltage with a transformer.*

Why do they want to change voltage? - so they can step the voltage up to hundreds of thousands of volts, (to between 132,000V and 400,000V here in the UK), to reduce transmission losses - by reducing the current in the wires - and then step it back down to line voltage to feed houses and businesses.


OB


*(A transformer is just two coils wound round a metal core - no electronics, and no moving parts)

Originally Posted by jocular

Harold, I wonder if perhaps you might be privy to some info on this? Forgive my childish way of seeking your attention. joc

I don't know anything about it first hand. Can't add much to what's in the Wikipedia article:
High-voltage direct current - Wikipedia, the free encyclopedia

Or you can look in another direction...pop the hood of your car and look at the cables that connect to the battery...the conductors are as big around as your finger. Why?...because the car battery is only 12 volts. To get the amount of power required to turn over the engine from only 12 volts requires 100's of amps. That much current requires super big wires.

Originally Posted by MacGyver1968

Or you can look in another direction...pop the hood of your car and look at the cables that connect to the battery...the conductors are as big around as your finger. Why?...because the car battery is only 12 volts. To get the amount of power required to turn over the engine from only 12 volts requires 100's of amps. That much current requires super big wires.

NOW we finally know why "bigger is better"! joc