Calculate Electrical Energy Stored in 9V Battery (120 mAh)

[Mangastream]

Homework Statement

Calculate the electrical energy (in Joules units) which is stored in a battery with nominal voltage, 9v, and with a charge capacity of 120 mAh.

Homework Equations

J (Electrical Energy) = V (Voltage) x Ah (battery capacity)

The Attempt at a Solution

J = V x Ah
9 x 120 mAh
9 x 0.12 Ah

Answer: 1.08 Joules

Is this correct
Thank you

 

[gneill]

The value looks way too small. How many coulombs in an amp-hour? How about 120 mAh?

 

[Mangastream]

I was wondering if 9V should be multiplied by 120 mAh or 0.120 Ah, so the correct answer then should be

9 x 120 mAh?

 

[gneill]

"Calculate the electrical energy (in Joules units)".

While amp-hours are a perfectly valid energy unit (if multiplied by the operating voltage), the Problem Overlords seem to require Joules as the units in this instance.

The secret to all these energy conversions is to know what the underlying units are. So an amp-hour is simply an amp (coulomb per second) times an hour (3600 seconds). So an amp hour is 3600 coulombs.

A volt is a Joule per Coulomb. So volts x amp-hours yields Joules.

 

[Mangastream]

Then why do you say that 9V x 0.12Ah = 1.08 Joules is way too small?

 

[gneill]

Then why do you say that 9V x 0.12Ah = 1.08 Joules is way too small?

Because 0.12Ahr = 0.12A * 3600seconds

 

[Mangastream]

Got it. Thank you.