In this project, students will use special dough to construct electrical circuits that light up. We re going to use two different types of dough, conductive dough and insulating dough. Let s take a minute to talk about what that means. Electricity is the flow of electrons from one place to another. Electrons won t just flow through any material though they only flow through conductive materials. Conductive materials include metals (which is why wires are made of metal) and salty liquids (which is why you shouldn t go swimming in a thunderstorm). Materials that are not conductive are called insulating. Insulating materials stop or slow the flow of electricity. Common insulating materials are plastics and wood. Really pure water (like distilled water) is also an insulator, but tap water has salts in it that make it conductive. Electricity flows in a closed loop, called a circuit. For instance, a circuit for a light bulb that runs from a battery starts from one side of the battery, and flows through a metal wire to a light bulb, then back through a wire to the other end of the battery. In this lesson, we ll make circuits using conducive and insulating dough. The instructions for the dough is below (taken fromhttp://courseweb.stthomas.edu/apthomas/squishycircuits/index.htm). For the conductive dough, you will need: 1 cup Water 1 1/2 cups Flour 1/4 cup Salt Page 1 of 8 WCCUSD STEM (06/15/15)
3 Tbsp. Cream of Tartar 1 Tbsp. Vegetable Oil Food Coloring To start, mix all the ingredients together in a pot over medium heat, stirring constantly. The mixture will start to boil and get chunky. It will also start sticking together. When it forms a ball in the center of the pot, take it out of the pot and put it on a floured surface. Wait for the ball to cool for a few minutes, then kneed in flour until the ball has reached the right consistency. You can start working with it immediately, or stick it in a plastic bag for later. It will keep for a few weeks. For the insulating dough, you will need: 1 1/2 cup Flour (plus a little more for later) 1/2 cup Sugar 3 Tbsp. Vegetable Oil 1/2 cup Deionized (or Distilled) Water Wash your pot out really well, drying off any water with a paper towel. Mix all the ingredients together and stir. You may add heat if you like, but it s not really necessary. When everything is mixed, remove the dough from the pot. Kneed it together. If the dough is too runny, kneed in more flour until it has the correct consistency. It will be stickier than the insulating dough. The dough is ready to use, but you can store it in a plastic bag for a few weeks. Now it s time to make some circuits. For this part, we re going to use a 4- AA battery pack for power (you can also use a 9V battery if you want), and light emitting diodes (LEDs for short). You can complete a circuit by using conductive dough. Have the students start by getting a single LED to light up. They could use the dough to connect the wires to the LED. The correct layout is below. Page 2 of 8 WCCUSD STEM (06/15/15)
You can see that the wires from the battery both lead to conductive dough, and that the LED is inserted into the dough. LEDs only work when the electricity flows through them in one direction. If your LED isn t lighting up, try switching the wires around. The longer of the two wires from the LED should go to the red (positive) wire coming off the battery. In practice, you ll probably find that trial and error is easier than picking the long wire on the LED, especially if you end up cutting or folding the wires. It s important to make sure that the two pieces of conductive dough are kept separate. If they connect, electricity will flow through the dough rather than through the LED. You can make sure that the two pieces are separated by adding a bit of insulating dough in the middle, like so: Page 3 of 8 WCCUSD STEM (06/15/15)
You can also light up more than one LED at once. There are two ways to do this: series circuits and parallel circuits. You can have the students construct both types to get a feel for how the work. In a parallel circuit, you can bridge the gap between two pieces of conductive dough with more than one LED. You can have the students try this out after they have gotten a single LED to light up. Page 4 of 8 WCCUSD STEM (06/15/15)
In a parallel arrangement, you can use about as many LEDs as you want. This is because the voltage (the force that pushes the electrons through the conductive dough and the LEDs) is the same for every LED in a parallel circuit. Once the students have gotten their parallel circuit to work, you can have them arrange their LEDs in a series circuit, as shown below. Page 5 of 8 WCCUSD STEM (06/15/15)
In a series circuit, the LEDs are connected end- to- end. The number of LEDs that you can put together in a series circuit will depend on the size of your battery and the type of LED you are using. With a four battery pack, you can probably get two LEDs to work. With 9V battery, you can probably get three LEDs to work. The reason for this is that in a series circuit, the voltage is split across LEDs. Most LEDs need about 2V to work, so if you have two LEDs, you need 4V. A normal AA battery has 1.5V, so you need two AA batteries to power a single LED in series. Because of this, you may want to limit the students to trying out 2 LEDs. They will notice that the first LED is brighter than the second. That is because some of the voltage is used up by the first LED before it reaches the second. Now, you can have the students use the basic ideas that they have learned to make more creative dough designs! Just make sure that the wires from the LED each touch only one piece of conductive dough. It is okay if the LED wires touch the insulating dough, so you can embed the LED in insulating dough if you are careful about it. Here is a flower And a fish: Page 6 of 8 WCCUSD STEM (06/15/15)
Notice that in both cases, the two pieces of conductive dough are not allowed to touch each other. That is critical to get the whole thing to work properly. It might be a good idea to have the students draw out their designs first, and have them trace the flow of electricity with a pencil. They should make sure in their drawing that the only closed look that the electricity creates is through the conductive dough. An example is shown below: Of course, you can use more than one LED for making creative shapes as well. This works especially well for making eyes for animals. You can connect two eyes in series quite easily. A couple of troubleshooting points: The dough will stay conductive for a day or two after it is left out. Once it dries out, it will stop being conductive. Keeping it in a bag will help it stay conductive for longer. The more dough the electricity has to travel through, the worse the circuit will work. Conductive dough is not as good at conducting electricity as wires are. It s best to limit the size of the students creations in order to make sure that they work properly. The insulating dough in particular will be much stickier than play- dough. You may want to use parchment paper to keep the dough from sticking to desks. If you want to reuse the dough after the students have used it, make sure that the conductive and insulting doughs don t mix. Throw out any mixed parts. Make sure that you do not directly connect the wire to the LED. Doing so will burn out the LED. You need some distance of travel through the dough in order to make sure that the LED doesn t burn out. You might want to clean the LEDs off between uses with rubbing alcohol or water. The salt in the conductive dough will case the LED wire to rust. Page 7 of 8 WCCUSD STEM (06/15/15)
It s easy to lose the electrical connection between a wire and the dough. You might want to consider using a long stretch of wire, or folding the wire over a few times in order to ensure a good connection. Burned out LEDs look just like working LEDs. It is probably best to check the LEDs after each use to see which ones are working and throw out the rest. LEDs are relatively inexpensive when you buy them in bulk. Make sure that the students do not connect the red and black wire directly to each other. Doing so can destroy the battery, or the wire, and can cause either to get extremely hot. It s not quite as bad to connect both wires to the same piece of conductive dough, but it s definitely not as bad. Page 8 of 8 WCCUSD STEM (06/15/15)