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Re: [Phys-l] cars and physics



To be more specific, I feel compelled to assert that the statement:

“When I do the calculation, I get that the time to "charge
a capacitor" goes like RC, independent of voltage. This is
a famous and very useful result”

is vague enough to be true (and not useful). More usefully, it would read:

”When I do the calculation, I get that the time to "charge
a capacitor" TO A GIVEN FRACTION OF E goes like RC, independent of voltage.
This is
a famous and very useful result.



From: Bob Sciamanda
Sent: Monday, February 21, 2011 4:44 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] cars and physics
I also don't think this was a practical factor in the switch to 12 volt
batteries, but review the argument ,below:

In an E,R,L circuit, L/R is the time constant - independent of E - but E
limits the maximum current/voltage achievable.
So if it is the time to reach a given voltage/current that matters, then E
DOES also matter.

-----Original Message-----
From: John Denker
Sent: Monday, February 21, 2011 4:16 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] cars and physics

On 02/21/2011 01:46 PM, Quist, Oren wrote:
I was told this, but never verified it.

One reason cars switched from 6-volt to 12-volt (when I started
driving, most cars were 6-volt) was that the time constant to charge
up the condenser (what we call the capacitor) was too long to
effectively fire eight spark plugs at normal rpm speeds. As V-8's
became popular, the 12-volts became necessary. Of course the time
constant would not change as the voltage went up, but double the
voltage would lead to double the charge which was then enough to fire
the plugs.

True or false??? -- whatever, it makes for an interesting
calculation.

Can't be true.

When I do the calculation, I get that the time to "charge
a capacitor" goes like RC, independent of voltage. This is
a famous and very useful result.

Also that's not even the right calculation, since the circuit
used in those days didn't involve "charging a capacitor" at
all. To a better approximation, it was charging an inductor,
if you'll pardon the expression. That is, the capacitor was
part of an RLC circuit. Between sparks, energy was built up
in the inductor. The time constant for doing this goes like
L/R, independent of voltage. This is another famous and very
useful result.

You can design L/R to be anything you like, so that could never
drive the (re)design of the battery or anything else.

I reckon the real driving force was the fact that at higher
voltage you can transport more power using thinner wires. As
cars come to have more and more electrical stuff, this becomes
more and more of an issue. At constant power, as the voltage
goes up you need less copper but more insulation. Insulation
nowadays is very cheap, very light, and very good. Copper is
heavy and expensive.


Aircraft nowadays use 24 VDC for a lot of things, including
instrumentation and the main starting battery. And that's
not because of any spark-coil issues, because they never had
spark coils.
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