Education:Project 2 - Parallel and Series

From Stuy Techtonics Wiki

Materials

• (2x) LED (Visible)
• (2x) 100-150 ohm resistor
• Solid core wire
• Wire strippers
• Breadboard (optional)

Overview

In this project: You will play around with different types of circuits, learn the math behind each kind, and be introduced to the Breadboard.

Lesson and resources

There are two basic types of circuits, called series circuits and parallel circuits.

Take a look at the image on the left. This is an example of a series circuit. Electricity flows along a single path, going through both of the lightbulbs. Try building this circuit with your materials (don't forget to include the resistors, or you'll burn out the LEDs!). What happens when you disconnect one of the LEDs? The other one will go out as well, because you've broken (or "opened") the only path that the electricity could travel along.

Next, look at the image on the right - this is a parallel circuit. Now the electricity has two different paths it can take, one going to each bulb. Trying building this circuit as well. Notice that when you disconnect one of the LEDs, the other one stays lit up! Even though you've broken one part of the circuit, the other LED still forms a complete circuit with the battery.

You may have noticed that both LEDs appear slightly brighter when they're connected in parallel than when they're connected in series. To explain why this is, we have to introduce a very important equation called Ohm's law:

<math>V=IR</math>

This law explains the relationship between voltage (V), current (I), and resistance(R). Note that voltage is measured in volts (V), current is measured in amperes (A), and resistance is measured in ohms (Ω). Say you have a 6 battery connected to a 2Ω resistor. How much current is there flowing through the circuit? I=V/R, so we divide 6 volts by 2 ohms, and we get 3 amps.

<math>6V/2Ω=3A</math>

Easy, right? Now let's try it with two resistors. When you have more than one resistor in a circuit, you need to calculate the "equivalent resistance", which is the combined resistance of all the resistors in the circuit. If you're connecting several resistors together in series, you find their equivalent resistance by simply adding their resistances together. The equation for this is:

<math>R_eq = R_1 + R_2 + R_3...</math>

Imagine you connect a 2Ω resistor and a 1Ω resistor to each other in series, and hook them up to a 6V battery. Based on this equation, what will the current in the circuit be? First you find that the equivalent resistance is 3Ω, by adding together 2Ω and 1Ω. Remember that <math>I=V/R_eq</math>, so we divide 6V by 3Ω and the result is 2A.

When we connect multiple resistors in parallel, however, we have to use a slightly different formula to find the equivalent resistance:

<math>1/R_eq = 1/R_1 + 1/R_2 + 1/R_3...</math>

Now imagine we're taking our 2Ω and 1Ω resistors and connecting them in parallel, instead of in a line. First we add the reciprocals of the resistances: 1/2 + 1/1, which equals 3/2. 3/2 isn't the equivalent resistance - it's the reciprocal of the equivalent resistance. So we divide 1 by 3/2, and get 2/3Ω. THIS is the equivalent resistance. Now when we want to calculate the current, we divide 6V by 2/3Ω and get 4A.

The current in the circuit is a lot greater when we connect the resistors in parallel than when we connected them in series. This why the LEDs looked brighter when they were connected in parallel - there was more current flowing through them!

Exercise:

Use a multimeter to measure the voltage of your battery and the resistances of all the components in your circuit. Calculate the current when the components are connected in a series or a parallel circuit. Use the multimeter to check your answers.