When you have a good handle on Ohm's Law, you'll be ready to put it into practice. Ohm's Law is like a master key, unlocking the secrets to electronic circuits.
![image0.jpg](https://www.dummies.com/wp-content/uploads/489386.image0.jpg)
In the simple circuit shown here, a 9-volt battery is applied across a 1
![image1.png](https://www.dummies.com/wp-content/uploads/489387.image1.png)
You calculate the current through the resistor as follows:
![image2.jpg](https://www.dummies.com/wp-content/uploads/489388.image2.jpg)
If you add a 220 ohm resistor in series, as shown here, you're restricting the current even more.
![To calculate the current through this circuit, determine the equivalent resistance and apply Ohm's](https://www.dummies.com/wp-content/uploads/489389.image3.jpg)
To calculate the current flowing through the circuit, you need to determine the total resistance that the 9-volt battery is facing in the circuit. Because the resistors are in series, the resistances add up, for a total equivalent resistance of
![image4.png](https://www.dummies.com/wp-content/uploads/489390.image4.png)
You use this equivalent resistance to calculate the new current, as follows:
![image5.jpg](https://www.dummies.com/wp-content/uploads/489391.image5.jpg)
By adding the extra resistor, you've reduced the current in your circuit from 9 mA to 7.4 mA.
That double squiggle symbol in the equation just given means "is approximately equal to," and it was used because the current was rounded to the nearest tenth of a milliamp. It's usually okay to round off the tinier parts of values in electronics — unless you're working on the electronics that control a subatomic particle smasher or other high-precision industrial device.