Electric Paint: Light Up Your Painting

Introduction

Electric paint is a fun replacement for traditional copper wires when making an "artistic" circuit that includes batteries and lights, like the one in Figure 1. Typically, "artistic" circuits include a battery, which stores electrical energy, and light-emitting diodes, or LEDs. An LED requires a certain amount of electrical currentflowing through it to light up. The current flowing through the LED is determined by the voltage supplied by the battery and the resistance of the circuit (to learn more about these terms, see the references in the Bibliography section). If there is not enough current, the LED will be very dim or not light up at all. If there is too much current, the LED can actually burn out! So, normally, when building a circuit with LEDs, it is very important to select properly sized resistors to match the LEDs and battery voltage. This process is not so simple with electric paint, however. How do you know the resistance of a strip of electric paint? What happens if you make the strip of paint longer or wider?

Figure 1. An example of a circuit using electric paint. Here, the lines of paint take a long, curvy path to the light from the battery to the LED, instead of going in a short, straight line. Do you think this affects their resistance?

You can predict how the geometry of a strip of paint will affect its resistance using a property called resistivity. Resistivity is a material property that defines how much a certain material resists the flow of electricity, independent of the size of the piece of material (this makes it an intrinsic property, meaning it only depends on the type of material, but not the amount of material). This is different from resistance, which does depend on the amount of material (which makes it anextrinsic property). For example, imagine two copper wires—one very long and thin, and one very short and wide, as shown in Figure 2. The resistance of the long, thin wire will be higher than the short, wide wire (it is harder for current to flow through the long, thin wire); but they are both made out of the same material (copper), so their resistivity is the same.

Resistance and resistivity are related by Equation 1:

Equation 1:

R=ρLA

where:

• R is resistance in ohms (Ω).
• ρ is resistivity in ohm·meters (Ω·m).
• L is the length of the material sample in meters (m).
• A is the cross-sectional area of the material sample in square meters (m²).

Figure 2. An example of how material geometry affects resistance. The resistivity (ρ) of both copper wires is the same, since they are made from the same material. However, according to Equation 1, the resistance of the long, thin wire (R₁) is higher since it has a longer length (L₁) and smaller cross-sectional area (A₁).

Note that Equation 1 applies regardless of the shape of the cross-section. For example, it does not matter if the cross section is circular, triangular, or a square. All that matters is the total cross-sectional area, A. Given this information, what do you think will happen to the resistance if you change the length or width of a thin coat of electric paint on a piece of paper, and how could that be important for your "artistic" circuit? Read the following Terms and Concepts, Questions, and Bibliography sections for more background information to help you do your research and come up with a hypothesis.

Terms and Concepts

• Light-emitting diode (LED)
• Current
• Voltage
• Resistance
• Resistor
• Resistivity
• Intrinsic property
• Extrinsic property
• Trace
• Polarity

Questions

• What is the difference between resistance and resistivity?
• What is the difference between intrinsic and extrinsic properties?
• Why does the resistance of electric paint matter when making a paper circuit?
• What will happen to the resistance if you keep the width and thickness of a strip of electric paint constant, but increase its length?
• What will happen to the resistance if you keep the length and thickness of a strip of conductive paint constant, but increase its width?

Materials and Equipment

• Bare Conductive electric paint (50 mL), available from SparkFun Electronics
• Multimeter, available from SparkFun Electronics
• Metric ruler
• Pencil or pen
• Thin paintbrush
• Newspaper to protect your work surface
• Several pieces of printer or construction paper
• Lab notebook

Optional: If you want to build your own paper circuit with LEDs and a battery, you will also need:

• 9 V battery, available from SparkFun Electronics
• Alligator clip leads, available from SparkFun Electronics
• Assorted LEDs (size and color of your choice), available from SparkFun Electronics
• Needle-nose pliers

Experimental Procedure

This experiment will consist of two parts:

• Keeping their width constant at 5 mm, measure the resistance of strips of conductive paint varying from 50 mm to 250 mm in length.
• Keeping their length constant at 100 mm, measure the resistance of strips of conductive paint varying from 5 mm to 25 mm in width.

1. Create two data tables, like Tables 1 and 2, in your lab notebook. There are two different sources for error in this experiment (explained later in the procedure), which is why you will be doing multiple measurements for multiple samples at each length (or width).

		Resistance (Ω) for 5 mm wide paint strips
Strip length (mm)	Sample Number	Trial 1	Trial 2	Trial 3	Average (for sample)	Average (for all 3 samples)
50	1
	2
	3
100	1
	2
	3
150	1
	2
	3
200	1
	2
	3
250	1
	2
	3

Table 1. Data table for recording resistance values for constant-width paint samples.

		Resistance (Ω) for 100 mm long paint strips
Strip width (mm)	Sample Number	Trial 1	Trial 2	Trial 3	Average (for sample)	Average (for all 3 samples)
5	1
	2
	3
10	1
	2
	3
15	1
	2
	3
20	1
	2
	3
25	1
	2
	3

Table 2. Data table for recording resistance values for constant-length paint samples.

2. Lay down newspaper to protect your work surface and prepare your paint samples.
   1. Use a pencil and ruler to trace outlines for three each of the following size samples. If you run out of room on a piece of paper, use additional pieces as necessary.
      1. Constant width: 5×50 mm, 5×100 mm, 5×150 mm, 5×200 mm, 5×250 mm.
      2. Constant length: 5×100 mm, 10×100 mm, 15×100 mm, 20×100 mm, 25×100 mm.
      3. Figure 3 shows an example layout of constant-width paint samples.
   2. Use your paintbrush to fill in each area with a thin, even layer of conductive paint.
      1. It is very important to use a consistent thickness for your coating of paint for each sample. Remember that resistance depends on cross-sectional area. So, if you have thick, gloppy sections of paint in one sample, and extremely thin, stretched-out sections of paint in another, this will affect your results. Since it is impossible to get perfectly identical strips each time, this is why you will prepare three samples of each size.
      2. Note: You could use the edge of a scrap piece of paper as a "shield" to avoid painting outside of the lines.
   3. Wait at least 15 minutes for the paint to dry. If the paint is still not dry to the touch after 15 minutes, wait longer. The resistivity of electric paint decreases as it dries, so it is important to wait for the paint to dry fully before taking your measurements.

Figure 3. An example layout for the constant-width paint samples.

3. Use a multimeter to measure the resistance of each sample, as shown in Figure 4.
   1. Place the multimeter's probe tips at opposite ends of each sample, holding them at an angle so the angled surface of the probe tip is fully in contact with the paint (instead of holding them vertically so only the point is in contact), and press down firmly onto the paper. Since there can be some variation in exactly where you place the probe tips, how you angle them, and how hard you press on the paper, this can introduce some error into your experiment, and is why you will conduct three measurements for each strip.
   2. Take three measurements for each paint sample, resulting in nine total trials, and record all of your results in Tables 1 and 2.

Figure 4. Measure the resistance of a paint sample by pressing the multimeter's probe tips into opposite ends of the strip.

4. Analyze your data.
   1. Calculate an average resistance across the three trials for each of your paint samples.
   2. Calculate an overall average resistance for each sample size (three samples with three trials each, nine trials total).
   3. For your constant-width data, create a graph with sample length on the x-axis and resistance on the y-axis.
   4. For your constant-length data, create a graph with sample width on the x-axis and resistance on the y-axis.
   5. How does resistance vary as you change the length or width of a strip of paint? How does this compare to your predictions, and what you would expect from Equation 1 in the Introduction?
   6. If you are designing a circuit and the resistance of a strip of paint is too low, what can you do to increase it?
   7. If you are designing a circuit and the resistance of a strip of paint is too high, what can you do to decrease it?

Optional: Build Your Own Paper Circuit

If you purchased a 9 V battery, LEDs, and alligator clip leads, you can use electric paint to make your own paper circuit by following these steps.

1. Carefully bend the leads (the thin metal legs) of an LED sideways so the LED can sit flat on a piece of paper (needle-nose pliers will make this much easier to do), as shown in Figure 5.

Figure 5. Bending the leads of an LED into either a zig-zag or spiral shape will help it stand up on a piece of paper.

2. Use electric paint to create two circuit traces, or electrically conductive paths (if you have ever looked inside a computer or cell phone, you may have seen the tiny copper traces on the circuit boards inside), leading from the edge of the paper to the leads of the LED, as shown in Figure 6. You can draw any shape, but the lines should not intersect, or this will create a short circuit. Paint over the leads of the LED, which will "glue" it to the paper. Wait at least 15 minutes for the paint to dry.

Figure 6. An example spiral pattern with electric paint. Note how there are two separate traces that each connect to one lead of the LED.

3. Attach alligator clips to the terminals of the 9 V battery and your paint traces, as shown in Figure 7.
   1. Attach the red clip to the positive terminal and the black clip to the negative terminal of the battery.
   2. Attach the other ends of the clips to the paint traces at the edge of your paper.
   3. If your LED does not light up, reverse the alligator clip connections on the paper. LEDs have polarity, meaning they have a positive and negative side. The longer lead of the LED is positive, and the shorter lead is negative. They must be connected to the positive and negative battery terminals, respectively.

  
Figure 7. Connect one end of the alligator clip leads to your 9 V battery terminals and the other ends to traces of electric paint.

4. If your LED still does not light up, or it lights up but is rather dim, it could be because the resistance of your paint traces was too high. If the LED flashed very brightly and then went out, it probably burned out because the resistance was too low (you will need to use a new LED). Based on the results of your experiment, how can you adjust the resistance of your paint traces appropriately?
   1. Note: Do not connect the LED leads directly to the 9 V battery with the alligator clips, as this will cause the LED to burn out.