Circuit basics: Resistance (2)

Last time I blogged about resistance. Today we are going to look at resistance within a circuit. Everything has basically to do with Ohm's Law, which states states:

the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them. (http://en.wikipedia.org/wiki/Ohm%27s_law)

This means that you can calculate the current if you know the voltage and the resistance. Also true is that you can calculate the voltage if you know the current and the resistance, and finally the resistance if you know the current and the voltage.

The formula for Ohm's law is: U = I x R, where U is voltage (in Volts), I is current (in Ampere) and R is resistance (in Ohm). You can also write this as I = U / R or R = U / I.

Given the following circuit:

Here we have the three variables:  U = 9 Volts, R= 450 Ohm and I = 0.020 Ampere (0.020 A = 20 mA). Let's check:

I x R = U
0.020 x 450 = 9 Volts


Ok, here are some circuits, which value is the missing variable? Rhe answers are at the bottom.

1. Given: U = 9, I = 0.0257. What is R?

2. Given: R= 350, I = 0.0143. What is U?

3. Given: U = 9, R=650. What is I?

4. How much resistance does the LED generate?

5. This circuit has a current of 23.6 mA. How much Ohm is the resistance of the resistor?






1. 9 / 0.0257 = 350 Ohm
2. 350 x 0.0143 = 5 Volts
3. 9 / 650 = 13.8  mA = 0.014 A
4. 1.9 / 0.0103 =   184 Ohm
5. As we have learned, the sum of the components in series is the total voltage. So  9 - 1.92 = 7.08 V.  7.08 / 0.0236 = 300 Ohm