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There are 2 base formulae which will help you to understand the relationship between current , voltage, resistance and power. If you have any two of the parameters, you can calculate the other two parameters.
Change of Resistance:
*Please note that the size of the 'opening' is is analogous to resistance. There is no physical restriction in a real piece of wire.
With the formula:
And for those who are more graphically inclined...
Change of Voltage:
With the formula:
Mathematical Example:
If you wanted to know how much current was flowing through the resistor, you would use the formula:
If you want more examples, the resistor page has more fun than a barrel of monkeys.
If you want to try a few for yourself, the calculators below will allow you to check your math. Find: Power dissipation and Current flow from Resistance and applied Voltage.
 Critically Important 
Adobe has deemed that the Flash content on web pages is too risky to be used by the general internet user. For virtually all modern browsers, support for Flash was eliminated on 112021. This means that those browsers will not display any of the interactive Flash demos/calculators/graphics on this (or any other) site.
The simplest (not the best) fix, for now, is to download the Ruffle extension for your browser. It will render the Flash files where they were previously blocked. In some browsers, you will have to click on the big 'play' button to make the Flash applets/graphics visible. An alternative to Ruffle for viewing Flash content is to use an alternative browser like the older, portable version of Chrome (chromium), an older version of Safari for Windows or one of several other browsers. More information on Flash capable browsers can be found HERE. It's not quite as simple as Ruffle but anyone even moderately familiar with the Windows Control Panel and installation of software can use Flash as it was intended. Note: In the following demo, you can adjust the voltage and the resistance of the circuit. Changing either one will change the current flow and power dissipation in the circuit. You should realize that an amplifier produces a voltage into a given load. The combination of the two will result in power dissipation (in the speaker's voice coil in the case of audio). Since an amplifier can produce a limited amount of voltage (limited by the internal power supply's design), the power output is limited when driving a given load (i.e. a 4 ohm load). To be able to produce more power, you can drive a lower impedance (resistance) load (within reason). This means that if you go from a 4 ohm load to a 2 ohm load, the power will double (assuming that the power supply is regulated). If we use the example of a 100 watt mono amplifier (100 watts into a 4 ohm load), we know it can produce no more than 20 volts across the speaker. If you set the voltage slider below to 20 volts, and the resistance slider to 4, you can see that the power is 100 watts. If you move the resistance slider to 2 ohms, you can see that the power is doubled (to 200 watts). Now, I know that I'm showing a battery as the voltage source (instead of an amplifier) but the concept is the same.
Click HERE to make this applet fill this window.
Georg Simon Ohm:
James Watt:
James Prescott Joule:
The 'Joule' as a unit of measure: 
You should remember:
1.If you have any 2 of the 4 electrical properties, you can find the other properties through Ohm's law. 
