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09-07-2011, 02:16 AM #1
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shintashi
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Why do Batteries Have Voltage?

I've been wracking my brain over this one for about a week now, trying to understand current, voltage, and resistance, and I'm still quite confused, despite all the LPF threads I read on the subject.

I understand current is amps, and voltage/resistance = current. I understand voltage to be similar to having a bunch of heavy weights pushing down on a balloon full of water, and the amount of water that comes out is the current. The size of the hole is the conductance, and that's the opposite of resistance or ohms, just like wavelength and frequency are inverse, the smaller the hole, the bigger the resistance.

A couple of things bother me though.

How can a battery have a measurement of current, or even voltage if the battery isn't hooked up to anything?

How do high tension power lines change voltage and go back to lower voltage to increase range? I thought you couldn't change voltage.

how does something "regulate" voltage?

If you know what the amps are, why does voltage even matter?

An finally, why didn't my little LED brightness change when I changed my potentiometer setting, but changing the battery size worked just fine? Did I line them up wrong or something?

edit: Let me explain my actual title question, since it seems silly:

if I = V/R and I is current, V is voltage, and R is resistance,

then I = V/0 inside a battery, so a 9 volt/0 = not applicable charge, yet the batteries have a listed charge.
So if I have 200mA = 9000mV/0, I feel like I broke the laws of math.

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Last edited by shintashi; 09-07-2011 at 02:54 AM.

09-07-2011, 03:30 AM #2
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Meatball
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Re: Why do Batteries Have Voltage?

A voltage actually exist without the presence of resistance or current flow.

All one needs, are two points and an uneven distribution of charge. So in the case of a battery, a voltage exists across the terminals that connect down into the depths of the cell where the chemical reactions take place.

"How do high tension power lines change voltage and go back to lower voltage to increase range? I thought you couldn't change voltage."

Power lines will have high voltages on them, since you can transfer power over some distance with less power losses due to the resistance in the long power lines. So its not really about extending the reach of usable power, its more about keeping the system efficient.

You can change voltage when you designed the system that distributes power, but most system do that automatically. In hot summer seasons, the power transmission system will "up" the voltage a bit to compensate for when power lines "sag" in the heat, adding to their resistivity.

Batteries are far different than transmitted power over the public grid. Batteries, by their chemical nature are "limited" to the potential that the ions allow. Most cells can only produce about 1.5V at best. Li-ions are currently at a miraculous 4.2V!

So batteries don't have to try very hard to regulate their voltage. Within a circuit, regulating voltage is a more complex topic all on its own.

"If you know what the amps are, why does voltage even matter?"

Depends on what you're doing. Voltage sources act differently than current sources. It can still come down to how much power (IV) you ultimately need.

"An finally, why didn't my little LED brightness change when I changed my potentiometer setting, but changing the battery size worked just fine? Did I line them up wrong or something?"

Post a schematic of what you put together, I'm sure there's a good reason why.

You didn't any laws, you simply didn't know all the components to sum up within the circuit. Even batteries have a series resistance called "internal resistance". Good batteries don't have very much, as internal resistance is not typically desirable.

And don't forget, there is no current flow inside the battery, until some resistance R is connected across its terminals. Even if you short a battery with a wire with a resistance of 0 ohms, the battery is still limited to some I (current flow) determined by the size of the cell, the chemical type of the cell, and the internal resistance of the cell.

Short a string of 8 AAA batteries in series together for 12V through a wire, and its not very exciting. Short a 12V car battery with a wire, and you'll be a bit more surprised!

I hope some of this makes sense..

Just clarify, and expand on your questions if you still don't get it.
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09-07-2011, 03:32 AM #3
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Cyparagon
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Re: Why do Batteries Have Voltage?

The current is a function of the load. You are correct in saying it has no current if it has no load.

Voltage is the potential - it is what causes (pushes, if you will) current. If there is no voltage, there is no current. The chemical reaction in the battery causes a surplus of charge on one end that wants to flow to the other.

Quote:
 Originally Posted by shintashi How do high tension power lines change voltage and go back to lower voltage to increase range?
The power lines don't change voltage, the transformers on either end change voltage. The reason that voltage is stepped up to go on the grid is related with power loss. P=IV and V=IR. Substituting, you get P=IČR. This means current has a lot more of an effect on lost power than resistance does, so it makes sense to keep current as low as possible. When voltage is stepped up, the current is stepped down to keep the P=IV equation equal. This minimizes losses on the grid.

Quote:
 Originally Posted by shintashi how does something "regulate" voltage?
To regulate just means to keep constant. There are many ways of regulating voltage and it is rather complicated.

Quote:
 Originally Posted by shintashi why didn't my little LED brightness change when I changed my potentiometer setting,
pic/schematic?

Quote:
 Originally Posted by shintashi then I = V/0
firstly, ohms law applies to circuits and their components when active. A battery is not active unless it is connected. Secondly, all batteries have internal resistance which ranges from a few mΩ to several Ω

Quote:
 Originally Posted by shintashi 200mA = 9000mV/0
200mA based on what load?

edit: foiled! (cbf to edit further )
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Last edited by Cyparagon; 09-07-2011 at 03:34 AM.

09-08-2011, 03:40 PM #4
 Class 4 Laser Join Date: Oct 2007 Location: Oregon, USA Posts: 5,478 Rep Power: 18104
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Re: Why do Batteries Have Voltage?

Batteries: the laws are absolute. Your meter has a very high resistance, but voltage can still be measured across it.

Power lines: power lines use alternating current, which means the voltage changes all the time. With such alternating current you can change voltage by using a transformer. Think of the current and wires in the transformer as a chain on a bike. You can make different gear turn at different speeds with different gearing, and likewise different coil ratios on a transformer.

Regulation: regulation is done with feedback. You measure something, compare against what you want, and adjust the output accordingly. Only you use a circuit to do this. Look up an op-amp circuit to understand one way it can work.

Why voltage matters: think about the flow rate out of a dam. You can consider that amperage. However, the amount of power that water flowing will depend on how much water is above it (the potential). A dam full of water pushing down a pipe at a certain rate has more power than the same rate of water moving along without such water above it.

LED brightness: LEDs need a minimum amount of voltage to turn on (forward voltage). After they're "on" current can pass, and the amount that passes determines the brightness, as long as the voltage is about the forward voltage.

Battery resistance: think of batteries as chemical "springs" that store energy. The resistance is the ability of that spring to release its energy without converting that energy to heat. For example, if the spring were in oil, it'd have higher resistance while releasing the energy. The same thing for resistance in a battery. It impedes the ability of the battery's chemical reactions from providing power to the output.

09-08-2011, 04:16 PM #5
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shintashi
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Re: Why do Batteries Have Voltage?

I tested out my other potentiometer and actually noticed some difference this time. I read up on pots and LED experiments and noticed my 25 Ohm pot is probably too little resistance to be noticeable.

I think that's because my understanding of electricity is way off.

I believed that ohms were the number you divide the power by, and the power had a direct correlation to the brightness of the light, after you attain a minimum charge to make it glow. So I thought 25 ohms would divide my brightness by 25. Clearly this isn't the case. My 100 Ohm pot changed the brightness by what looked like 10-20%.

I want to get a CA813 Lightmeter but they are \$175 and I still have a bunch of other tools to get first.

Basically, my whole understanding of Ohms is way, way off, and my book said I should think of it like kilo-Ohms X milamps = voltage. I'm not sure if that makes sense, but I know two things for certain:

1) 25 Ohms didn't put a dent in perceptable brightness of an LED powered by a 9V battery
2) for my 12X 405nm laser project, I need right around 800 mW of power

but mW aren't mA or Volts.
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09-08-2011, 04:28 PM #6
 Class 3B Laser Join Date: May 2009 Location: Sydney, Australia Posts: 3,256 Rep Power: 3264
bobhaha
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Re: Why do Batteries Have Voltage?

mW is your optical power output and mA is a current measurement.

You will pass x amount of voltage to the diode as a constant current (measured in mA).
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09-08-2011, 04:30 PM #7
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chipdouglas
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Re: Why do Batteries Have Voltage?

Quote:
 Originally Posted by Bionic-Badger Batteries: the laws are absolute. Your meter has a very high resistance, but voltage can still be measured across it. Power lines: power lines use alternating current, which means the voltage changes all the time. .

to be clear, ac doesnt mean the power changes all the time.... the longer the runs of wire, yes the voltage sags but the voltage isnt changing because it is ac.

at op... DC was the first type of power invented. but it didn't transmit well. AC transmits over long distances easily. with out AC we probably wouldn't have our power grid.
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09-08-2011, 04:58 PM #8
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bobhaha
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Re: Why do Batteries Have Voltage?

Quote:
 Originally Posted by chipdouglas to be clear, ac doesnt mean the power changes all the time.... the longer the runs of wire, yes the voltage sags but the voltage isnt changing because it is ac. at op... DC was the first type of power invented. but it didn't transmit well. AC transmits over long distances easily. with out AC we probably wouldn't have our power grid.
Umm actually AC does change all the time... It goes from 240V down to 0V then to -240V then back up again 50 - 60 times a second.
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09-08-2011, 05:08 PM #9
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chipdouglas
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Re: Why do Batteries Have Voltage?

^^^^thats the cycle (hz) the main 240 is stays in that cycle. but over distance will sag down to lets just say 200v. then a line conditioning step up transformer is used to get it back to 240.

michael.
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09-08-2011, 05:14 PM #10
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bobhaha
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Re: Why do Batteries Have Voltage?

Quote:
 How do high tension power lines change voltage and go back to lower voltage to increase range? I thought you couldn't change voltage.
Quote:
 Power lines: power lines use alternating current, which means the voltage changes all the time. With such alternating current you can change voltage by using a transformer. Think of the current and wires in the transformer as a chain on a bike. You can make different gear turn at different speeds with different gearing, and likewise different coil ratios on a transformer.
In this context he was talking about the cycle.
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532nm - 620mW+ lasersman Firedragon III* - 100mW PGL-III-M from JETLASERS* - 70mW Rayfoss Host*
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Total: 71,868.5 mW

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09-08-2011, 05:16 PM #11
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chipdouglas
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Re: Why do Batteries Have Voltage?

^^^ well in that case, a big to you!!

michael.
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09-08-2011, 05:40 PM #12
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shintashi
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Re: Why do Batteries Have Voltage?

man, I wish I wasn't in a summer session. (today's my last day), it prevents me from enrolling in an electronics class at a community college until next year, and I want to start making drivers so I can make a Christmas present.

I'm still not sure why 9v or 12v of 200mA fries a diode where 3.7v of 200mA doesn't. I thought all the electrons were moving at near light speed. How can they be moving faster? I think this is the part of pressure I don't get, because in fluid dynamics, if you increase the pressure through a pipe, you get more speed, but electrons are all transferring at the same speed, so I thought you were simply transferring MORE electrons, but since Amperage is how many electrons, I dont understand why Voltage matters as long as you already have your Amps calculated.
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Last edited by shintashi; 09-08-2011 at 05:46 PM.

09-08-2011, 06:00 PM #13
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bobhaha
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Re: Why do Batteries Have Voltage?

Think of it like this...

There are cars on a road parked randomly in any order or position on this straight piece of road. Now think of the electrons as trucks... The amount of trucks dodging and weaving along this road is called the current. Now think of voltage as the trucks speed. Resistance is measured by how many cars on on the road.

Now say there are three trucks coming down the road and only enough space for one truck to dodge the parked car. Well one truck will make it through, while the other two will smash.

Now increasing the voltage or speed in the scenario will cause those impacts to be more detrimental. This impact converts its kinetic (moving) energy into sound and heat. Well same as electrons, as they are metaphorically dodging and weaving through a copper wire, when they hit resistance, they create heat.

If you have too much heat it will burn, warp, melt and destroy things.

It also works the same with current... picture the truck scenario again.... but instead the voltage (speed) is low but current is high ( amount of trucks). Now a lot of trucks at hitting into the cars, plus hitting into other trucks! So again it causes heat.... So in a real world application it would again heat up the connections or a component to the point of destruction.

Hope that helped
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593.5nm - Regal 5
542.8nm - Green He-Ne laser ~2.5mW
532nm - 620mW+ lasersman Firedragon III* - 100mW PGL-III-M from JETLASERS* - 70mW Rayfoss Host*
~~~ - 175mW Rayfoss Host - <5mW WL Core - 55mW DinoDirect review laser*

488nm - On the hoist
473nm - 2x CNI lab lasers ~40mW
445nm - 3W Big Boyz Laser - >1.1W Cree C6 Host - 650mW WL Arctic
405nm - 65mW GLP-405 Aquarius host - 160mW 6x Blu-ray MXDL Host
Total: 71,868.5 mW

Goggles!
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Last edited by bobhaha; 09-08-2011 at 06:01 PM.

09-09-2011, 01:22 AM #14
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shintashi
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Re: Why do Batteries Have Voltage?

I totally get your analogy, I just have to "suspend my belief" that they aren't all moving at the same speed, or I could try imagining that goofy relativity equation where .999 light speed is way better than .99 light speed, even though to us its still basically the same thing. That or "Warp" from Star Trek. As long as I can pretend they aren't really the same speed, voltage makes sense I guess.

I've heard from Crackpots that Nikola Tesla was the only man who ever understood electricity. Whether or not that's true, I can see why people say it.
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405nm 600mW violet tactical
445nm Yobresal 1200mW
532nm Green Laser/Flashlight tactical 5mW
655nm Swiss Army Knife Mini-Laser 0.8mW
650nm my first" DIY failure" laser 100mW (?)

OEM Lasers OD 5+ 190-579nm Safety Glasses

09-09-2011, 05:45 AM #15
 Class 3B Laser Join Date: May 2009 Location: Sydney, Australia Posts: 3,256 Rep Power: 3264
bobhaha
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Re: Why do Batteries Have Voltage?

It's so true, Tesla was a bit of an idiot... a smart idiot though. He had so many inventions and ideas, but he would open his mouth and other people would come in and take his ideas. He actually invented AC electricity, but Westinghouse took his idea and ran with it... making a lot of money!
__________________
Lasers!
10.6um - >40W CO2 Laser - 25W RF excited CO2 Laser*
685nm - Being built!
660nm - 250mW LOC PowerLight B50 Host - 100mW WL pulsar
635nm - 430mW C6 Host*
632.8nm - Red He-Ne laser ~1mW
593.5nm - Regal 5
542.8nm - Green He-Ne laser ~2.5mW
532nm - 620mW+ lasersman Firedragon III* - 100mW PGL-III-M from JETLASERS* - 70mW Rayfoss Host*
~~~ - 175mW Rayfoss Host - <5mW WL Core - 55mW DinoDirect review laser*

488nm - On the hoist
473nm - 2x CNI lab lasers ~40mW
445nm - 3W Big Boyz Laser - >1.1W Cree C6 Host - 650mW WL Arctic
405nm - 65mW GLP-405 Aquarius host - 160mW 6x Blu-ray MXDL Host
Total: 71,868.5 mW

Goggles!
Laser glow LSG-532-Sport protective eye goggles OD 7+ 190-532nm HIGHLY RECOMMENDED!!
Jetlasers wrap around 190-540nm OD4+ goggles*
Eagle Pair Red OD4+ goggles
Scopeguy20 Blublockers for 405nm lasers!

* indicates a review has been done, on this item.

Check out my How to Build a Selectable Dummy Load Thread!

09-09-2011, 08:15 AM #16
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Seoul_lasers
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Re: Why do Batteries Have Voltage?

Quote:
 Originally Posted by chipdouglas to be clear, ac doesnt mean the power changes all the time.... the longer the runs of wire, yes the voltage sags but the voltage isnt changing because it is ac. at op... DC was the first type of power invented. but it didn't transmit well. AC transmits over long distances easily. with out AC we probably wouldn't have our power grid.
Right, Alternating current. Voltage and current are 120 degrees out of phase of each other.

Think like this, Voltage is potential, Current is flow.
I turn a tap on and water comes out. It doesn't have the power to blast the paint off my car. Low potential low current/pressure.
IF I try to wash the car with a fully pressurized fire hose, I blast the paint off my car and blow the windows out. High potential
High current (pressure). A garden hose with a pressure nozzle is like saying Low potential with High(er) current.

IF you already understand the concept ignore my teacher rant...

AC looks like ~ like this mark ~ on an oscilloscope.

DC is steady. ___________
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