[TriEmbed] Rocket derby help

[Brian]

Hi Craig,
 > Think this is telling me to expect around 12 Amps on a fresh battery?

..not exactly.  :-) The Wikipedia article is listing "amp-hours", which 
is a measure of charge, or the ability of the battery to continue 
supplying current for a period of time.  In (over-) simplified terms, if 
a circuit drew 12 amps from the battery, it would be depleted in one hour.

If you're bored, read on!  Here's some Electrical Theory 101.

There are several measurements one can make to analyze a DC electrical 
circuit, and they're all proportional to each other.  You've probably 
heard of Current, Voltage (more correctly called "potential 
difference"), and Resistance.

You may have heard of the primary way those three measurements are 
related, known as Ohm's Law: Potential Difference (E) is equal to 
Current (I) times Resistance (R) -- E = IR.

So what /is/ potential difference and current?  Well, what /is/ electricity?

Electricity is the set of phenomena that can occur when electrons (tiny 
subatomic particles) move from place to place.  All of the measurements 
associated with electricity are measuring (in essence) numbers of electrons:

Potential Difference (Volts): How many more electrons are at point A 
than at point B?

Current (Amps): How many electrons are moving from point A to point B in 
a period of time?

Resistance (Ohms): How many electrons CAN get from point A to point B in 
a period of time?

So, what causes electrons to move?  To understand that, we need to 
introduce the concept of Charge.

Every electron in the universe has exactly the same charge, measured in 
Coulombs, and that charge is defined as 1.602e-19 coulombs.  The number 
itself isn't important to our discussion, but the fact that every 
electron has /exactly/ the same charge is.

Have you heard "like charges repel, opposites attract?"  Since electrons 
all have the same charge, they all repel each other.  Every electron in 
the universe wants to get as far away from every other one as possible 
(fortunately there are a lot of other forces keeping some of them close 
together!).  Because of that, electrons always want to be as evenly 
distributed along a circuit as possible.  Any time there are more 
electrons in one part of a circuit than another (a "potential 
difference"), the electrons will try to flow from the area of many to 
the area of few.  When they flow, we can measure how many of them pass a 
specific point in a period of time; this is called "current."  Since 
every individual electron has exactly the same charge, we can express 
"current" as an amount of charge moving per period of time, and in fact 
we do: One Ampere is equivalent to one Coulomb of charge moving through 
a space every second.

So what can we extract from this so far?

- Electrons move when there's more of them in one place than another 
(and there's a path for them to follow)
- Conversely, electrons /don't/ move if they're already evenly distributed

Now we can actually talk about what a battery does.  Your typical 
non-rechargeable battery uses a chemical reaction to push electrons 
inside the battery toward the negative terminal.  The chemical reaction 
itself provides the "force" to keep more electrons at the negative 
terminal than at the positive terminal.  When you connect a circuit, the 
electrons now have a path to flow in an attempt to evenly distribute 
themselves again.

But what happens when they show up at the positive terminal?  That same 
chemical reaction grabs them and shoves them back to the negative 
terminal, inside the battery.

When the battery is fresh, the chemical reaction can move a whole lot of 
electrons from the positive terminal back to the negative terminal per 
unit time.  In other words, it can keep a large number at the negative 
terminal even if the outside circuit allows them to flow to the positive 
terminal in large numbers (low resistance).

As the battery is depleted, the chemical reaction slows down, and can't 
keep pushing the electrons inside the battery from positive to negative 
as efficiently.

If there's no circuit outside the weak battery, eventually it can still 
pile up the usual number of electrons at the negative terminal (the 
battery's rated voltage).  A good voltmeter does not allow much current 
to flow, so it will read the full rated voltage from the weak battery, 
because the electrons still don't have a good path to get back to the 
positive terminal.  However, when there's a circuit connected, the 
electrons have a path back to the positive terminal again, and suddenly 
the chemistry, in essence, can't keep moving the electrons from positive 
to negative as quickly, so the voltage across the battery (difference in 
number of electrons!) drops.  Since current only flows when there is a 
potential difference, less current flows through the external circuit. 
Eventually the battery's chemistry comes to a complete halt, and no 
current flows at all--without the chemical reaction, the electrons 
succeed in evenly distributing themselves throughout the circuit and 
stop moving.

So, what is the Wikipedia article telling you with a figure such as "12 
Amp-hours?"

It's telling you the energy available in the battery due to the chemical 
reaction.  Recall, amps is coulombs per second, so amps times seconds 
equals coulombs.  A 12-Ah battery has a nominal ability to move (12 * 60 
* 60) 43,200 coulombs of charge internally from its positive terminal to 
its negative terminal.  How long the battery lasts depends on how 
quickly the electrons are able to get back to the positive terminal: if 
the external circuit has low resistance, and 12 amps of current can 
flow, the battery will be depleted in one hour; if the external circuit 
has high resistance, and only 0.1 amp can flow, the battery will be 
depleted in 120 hours.

In conclusion, your rocket ignition battery is dead, Jim.  XD

I hope this has been educational!

Cheers,
-Brian