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*From*: John Denker <jsd@av8n.com>*Date*: Wed, 06 Aug 2014 08:38:15 -0700

On 08/06/2014 07:36 AM, Bob Sciamanda wrote:

Perhaps this is a correctly simulated instance of "Mandated Energy

Dissipation". EG., when a battery charges a capacitor, 1/2 of the

battery-produced energy must be dissipated.

More-or-less everybody assumes that, but it's not necessarily

true!

A whole lot of very smart people have gotten that wrong

over the years.

If we are clever, we can dissipate a whole lot less than

half of the energy. Proof by construction:

A battery, by definition, is a collection of /cells/.

Assume all N cells are in series. Assume we have access

to the individual cells. Let the cell voltage be u (small

u) while the battery voltage is V (big V) where V = N u.

Charge the capacitor step by step. Start by hooking it

up to just one cell. This moves a charge (C u) across an

average voltage drop of u/2, so it dissipates energy on

the order of (C u^2/2). This is very small, since u is

small. At the next step, hook the capacitor to two cells

in series. This moves an additional charge (C u) across

an average voltage drop of u/2. Again the dissipation

per step is on the order of (C u^2/2). After N steps

the capacitor has the full charge (N C u) i.e. (C V),

and the total dissipation is (N C u^2/2) i.e. (C V u/2)

which is N times smaller than the "conventional" but

unclever result (C V^2/2).

If you don't have access to the individual cells, you

can achieve the same result using an inductor and a

transistor to build a buck/boost DC-to-DC converter.

This has a number of real-world applications. For

starters, real-world CFL and LED light bulbs include

high-efficiency DC-to-DC converters.

http://www.pavouk.org/hw/lamp/en_index.html

This is a huge market.

Also note that modern computers, memories, and logic

chips are based on using switches to charge up capacitors.

Cutting down on energy requirements (including cooling

requirements) is always desirable, so there is interest

in "reversible computing" aka "adiabatic computing".

This provides yet another example showing the value of

/interdisciplinary/ work. If you're going to invent

adiabatic computing, you need to know a fair bit about

the application area (chip design), and also a fair bit

of fundamental physics.

**Follow-Ups**:**Re: [Phys-L] charging a capacitor reversibly (or nearly so)***From:*brian whatcott <betwys1@sbcglobal.net>

**Re: [Phys-L] charging a capacitor reversibly (or nearly so)***From:*Bernard Cleyet <bernard@cleyet.org>

**References**:**[Phys-L] LC circuit simulation***From:*Diego Saravia <dsa@unsa.edu.ar>

**Re: [Phys-L] LC circuit simulation***From:*"Bob Sciamanda" <treborsci@verizon.net>

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