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Old Feb 26th 2009, 09:24 PM   #1
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A 12 V battery...

A 12 V battery sends a charge of 84 Ah through a circuit.

a) How many coulombs of charge does this represent
b) If the charge undergoes a change in electric potential of 12V, how much energy is involved?

... please explain, I missed class and am having trouble understanding...
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Old Feb 26th 2009, 10:29 PM   #2
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"Ah" = Amp hours? Then you can answer 1) by using the definition of a an Amp. 1 Amp is a "rate of flow" of 1 coulomb per second. You can pretend that 84 Amp hours means (84 Amp)(1 hour) To make progress, you must convert Amps to coulombs per second and convert 1 hr to seconds.

(84 Amp hours could also mean 42 Amps flow in 2 hours, but you would get the same answer if you decrease the Amps and increase the seconds properly.)

You can measure energy in Joules or Watt-hours or kilowatt-hours etc.

Let's try Joules.

A Watt is the the unit of power equal to 1 joule of energy per 1 second.
So
1 watt = (1 joule)/ (1 second) and
1 joule = (1 watt)(1 second)

1 Watt is also equal to (1 volt)(1 amp), though this may not be obvious.

So 1 joule = (1 Watt)( 1 second) = (1 volt)(1 amp)/ (1 second)

Replace 1 volt by 12 volts, replace 1 amp by 84 amps and replace 1 second by the number of seconds in an hour to get the number of watts.
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