Why amps? - SailNet Community
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Why amps?

Why do we ever talk about amps? It seems to me that in most situations when we specify the amperage of something, it's really the wattage we're after. For example, I am of the understanding that...

- maximum current in a wire: a 10 amp current at 1 volt for 1 second heats up a wire just as much as a 1 amp current at 10 volt for 1 second.

- power consumption of a device: a 20-watt light bulb draws more amps from a battery at 12.0 volts than it does from a battery at 12.6 volts.

- charging: a 30 W panel recharges a battery faster at 13 volts than 15 volts (assuming the battery can accept the current in either case).

Maybe my understanding is wrong in these cases. But it seems to me that it's power applied or power consumed that we're generally interested in. The fact that circuits are "typically 12 volt" just confuses the matter because in fact they're rarely exactly 12 volts, either due to a discharged battery or to voltage drops in the circuit.

Of course I've been thinking in terms of DC the whole time. Maybe AC is different, but I don't see why. Even if you're considering AC safety, I doubt that a certain number of amps is enough to kill you regardless of voltage. Maybe there's no specific wattage of AC that will fibrilate your heart, but that hardly seems like enough of a reason to set a convention for all electrical discussions.

Okay, rant over.

s/v Laelia - 1978 Pearson 365 ketch

Old 04-06-2011
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I think you'd be well served by reviewing Ohm's Law, and all it's permutations.

Emoting isn't the same as calculating, and Ohm's Law is one of those few immutable things in electrical theory...you can't get around it, and you can't just give it your own interpretation.

Bill
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Old 04-06-2011
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Sometimes quoting Watts would be more appropriate, but in many cases such as wire, switch capacity etc current is the limiting factor.

Quote:
Originally Posted by AdamLein View Post
- maximum current in a wire: a 10 amp current at 1 volt for 1 second heats up a wire just as much as a 1 amp current at 10 volt for 1 second.

.
No if the wire is supplying power to something like a motor or bulb this is wrong.
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Quote:
Originally Posted by noelex77 View Post
No if the wire is supplying power to something like a motor or bulb this is wrong.
Er, let's say the wire is supplying power to a "resistive load". That was implied when I specified the current.

So you're saying the amount of energy dissipated by a wire as heat, per second, is dependent on something other than how much energy is passing through that wire per second?

My understanding is that you can specify any three of the following four variables: resistance, voltage, current, power; and the fourth is determined by the formulas. Resistance in a wire operates by converting electrical energy to heat, so if you know the resistance and the power, you know how much heat is being generated per second.

s/v Laelia - 1978 Pearson 365 ketch
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Quote:
Originally Posted by btrayfors View Post
I think you'd be well served by reviewing Ohm's Law, and all it's permutations.

Emoting isn't the same as calculating, and Ohm's Law is one of those few immutable things in electrical theory...you can't get around it, and you can't just give it your own interpretation.

Bill
Sorry, I'm not sure what you're trying to say here. Are you saying I am wrong? Could you explain?

I'm pretty sure I understand what Ohm's Law states. How do you perceive I am misinterpreting it?

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Quote:
Originally Posted by AdamLein View Post
- charging: a 30 W panel recharges a battery faster at 13 volts than 15 volts (assuming the battery can accept the current in either case).
Actually I will readdress this one, for clarification purposes: "A panel currently outputting at 30 watts recharges etc."

In all cases, in my examples, when I specified a power, I was referring to the actual operating power, not the rated power, of the device.

I think the issue is that energy is the ability of a system overcome a particular resistance, and power is how quickly that energy can be applied/delivered/expended/released/whatever. When you're talking about amps, you're talking about the rate at which energy-carrying parcels are being delivered, without any discussion of how much energy those parcels happen to be carrying. If you don't know how much energy is being delivered, you have no idea what sort of tasks the system can be performed, full stop.

It's not an major issue when the system is generally 12 volts, but so what? Why take the shortcut (i.e. why divide power by 12 volts)? Especially when the 12-volt assumption isn't exact, the shortcut glosses over an important issue.

s/v Laelia - 1978 Pearson 365 ketch
Old 04-06-2011
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What's the big deal? Sometimes when questions come in on such basic issues, they seem, well, contrived.

But this one's not too far off, it's simply a matter of measurements.

What, you say? Watt.

W = V times A

It's the same thing but you just use 12V, so amps = watts divided by 12. 12 is a constant, except when you're converting from 12V to 120V.

It really doesn't matter.

You like watts? Go for it. Everyone else, almost everyone else, will simply convert to amps.

Stu Jackson, Catalina 34, 1986, M25 engine, Rocna 10 (22#), Maple Bay, BC, Canada
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Old 04-06-2011
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The likely reason amps are used so often is because deep cycle batteries are usually sized in amp hours. 1 amp for 1 hour = 1 amp hour. Obviously we can use whatever measurement you want but most all the manufacturers of chargers, alternators, battery monitors etc. use amps.

Quote:
Originally Posted by AdamLein View Post

- charging: a 30 W panel recharges a battery faster at 13 volts than 15 volts (assuming the battery can accept the current in either case).

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Quote:
Originally Posted by Maine Sail View Post
With the clarification that the panel is operating at 30 W (maybe not realistic?) then at 13 volts, it's putting 2.3 amps, whereas at 15 volts it's putting out only 2 amps.

Isn't that how smart charge controllers work? They take the applied power of the panel, distributed between amps and volts one way, and redistribute it between amps and volts another way.

Maybe another way of asking my question is to ask why we don't just say that my batteries store 1800 watt-hours, so it'll take a 30 watt charger with a smart controller 50 hours to fully charge it from a fully discharged state (ignoring battery nuances and losses and such)? I mean, my electric bill is in watt-hours, so why not my boat's energy budget?

s/v Laelia - 1978 Pearson 365 ketch
Old 04-06-2011
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1)"- maximum current in a wire: a 10 amp current at 1 volt for 1 second heats up a wire just as much as a 1 amp current at 10 volt for 1 second."

It is true that in both of those examples you are producing 10 watts for 1 second, but that has nothing to do with the ampacity rating of a length of wire. If a piece of wire has an ampacity of 1 amp and you exceed that current, the wire will smoke irregardless of what your supply voltage is.

2)"- power consumption of a device: a 20-watt light bulb draws more amps from a battery at 12.0 volts than it does from a battery at 12.6 volts."

No. If a light bulb is rated for 20 watts at 12v, then it will only consume 20 watts at the voltage it is rated for. Provide a higher voltage and it will consume more power(and burn brighter and hotter and burn out quicker).

3)"- charging: a 30 W panel recharges a battery faster at 13 volts than 15 volts (assuming the battery can accept the current in either case)."

I doubt it. A 30W solar panel will probably only give you 30W at full sun, and will give the highest voltage it is capable of as well. As the battery you are charging is opposing the charge current, a higher charge voltage will force more current into the battery than a lower voltage will. As the voltage on a solar panel lowers, the current available also lowers. Using a good charge controller can increase the usefullness of a solar panel, but thats another discussion.

4)"Maybe my understanding is wrong in these cases. But it seems to me that it's power applied or power consumed that we're generally interested in. The fact that circuits are "typically 12 volt" just confuses the matter because in fact they're rarely exactly 12 volts, either due to a discharged battery or to voltage drops in the circuit."

I'm not how to answer that. You seem to have an understanding of the relationship of voltage/current and power, but there is obviously some confusion, which is understandable. Sometimes it is power consumed you are interested in, but sometimes its current you care about. It just depends.

5)"Of course I've been thinking in terms of DC the whole time. Maybe AC is different, but I don't see why. Even if you're considering AC safety, I doubt that a certain number of amps is enough to kill you regardless of voltage. Maybe there's no specific wattage of AC that will fibrilate your heart, but that hardly seems like enough of a reason to set a convention for all electrical discussions."

Without getting into a long discussion on AC, its the same in some ways but then you have to consider reactance as well as resistance. Make no mistake, it is amps that kill you. Somewhere around 0.1 amps (100 ma) through your heart would most likely kill you. Making contact from hand to hand is considered especially dangerous for that reason. GFCI's are made to trip at about 5 ma. There are many factors that would affect how much current would flow through you if you were to come in contact with a live circuit, but you could be killed by a standard 120v ac circuit without a doubt.
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