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Here are some examples for usage of this app. You can use it to calculate:

• the allowed power wattage of a power strip rated 240V / 3.5A
• the resistance and consumed current of a lamp rated 110V / 60W
• the maximum current supplied to a speaker rated e.g. 8Ω / 50W. The amperage is the determining factor in picking a cable of the right gauge.
• the power wattage of a power adapter rated 12V / 150mA.

NB: Beware of "milli" values specced as mV, mA and mW, such as in the last example. In those cases you'll have to divide the value by 1000 yourself, so 150mA should be entered as 0.150 A. Maybe the next version will support the scaling factor prefixes M (mega), k (kilo), m (milli) and μ (micro).

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First click on one of the left-most blue buttons to select a value known to you. The LCD display next to it will change from   ---- to      _ to indicate it is in data-entry mode. Now a displays is selected, you can use the numeric pad.

Use the ⌫ button (backspace) to delete the last digit and the C button to erase the whole value. The R button resets the whole calculator. Press the green ↵ button (enter) when done.

When two values are entered, the other two will be calculated.

When pressing a blue button several times, it will turn red. Then the associated value is fixed, which means it will always be regarded input. This is useful for example when using the mains voltage of 230V, which won't change.

Be careful not to click on a LCD display. They are quite cheap so you might break it.

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Your inputted values, which are the basis for the calculation, will have their blue button on the left lit up and the LCD display next to it is green.

The LCD display will show green-lit to indicate a computed value.

All values have a precision of 5 digits. This should be sufficient for most real-world applications. The next major version will feature scaling factor prefixes M (mega), k (kilo), m (milli) and μ (micro) to extend the range.

Values smaller than 0.00005 will be rounded down to zero. When a computed value is larger than 99999.5, it's display will show OVRFLW.

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The basis for this calculator are Ohm's law and Joule's first law. In these formulas the used symbols have the following meaning:

• V = Voltage or potential difference, measured in Volts (V)
• I = Amperage or electrical current, measured in Amperes (A)
• P = electrical power, measured in Watts (W)
• R = Resistance or impedance, measured in Ohms (Ω)

Ohm's law can be written in three equivalent formulas:

• R = V / I
• V = I * R
• I = V / R

Joule's first law states P = I² * R. Here R can be substituted by V / I from Ohm's law, which resolves in P = I² * V / I = I * V. This again can be written in three equivalent formulas:

• P = V * I
• V = P / I
• I = P / V

Since P = V * I and I = V / R
⇒ P = V * (V / A)
⇒ P = V² / R
⇒ V² = P * R
⇒ V = √ P * R 

And since P = V * I and V = I * R
⇒ P = (I * R) * I
⇒ I² = P / R
⇒ I = √ P / R 

That's basically all there is to it.

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The following tools were used for creation of this app:

• UltraEdit-32 v10 by IDM Computer Solutions Inc. for source code editing,
• www.w3schools.com for HTML5/CSS3/JavaScript syntax reference,
• Google Chrome v14 for HTML5/CSS3/WebKit/JavaScript testing and debugging,
• WebWorks SDK v2.2.0.5 by Research In Motion for compiling and signing.

A big thank you goes to the following people:

• Aad de Jong, my highschool science teacher, for teaching some basic electronics and getting me interested in computers,
• Tim van der Loo for beta-testing on Blackberry Playbook,
• Jaap Droogers for beta-testing on Android,
• Samuel Reynolds for providing the font LCDMono-Ultra for free,
• and last but not least: YOU for giving this app a try.

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