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Some types of spiders build webs that consist of threads made of dry silk coated with a solution of a variety of compounds. This coating leaves the threads, which are used to capture prey, hygroscopic-that is, they attract water from the atmosphere. It has been hypothesized that this aqueous coating makes the threads good electrical conductors. To test the electrical properties of coated thread, researchers placed a \(5 \mathrm{~mm}\) length of thread between two electrical contacts. The researchers stretched the thread in \(1 \mathrm{~mm}\) increments to more than twice its original length, and then allowed it to return to its original length, again in \(1 \mathrm{~mm}\) increments. Some of the resistance measurements are given in the table: $$\begin{array}{l|cccccccc}\begin{array}{l}\text { Resistance of } \\\\\text { thread }\left(10^{9} \Omega\right)\end{array} & 9 & 19 & 41 & 63 & 102 & 76 & 50 & 24 \\ \hline \begin{array}{l}\text { Length of } \\\\\text { thread }(\mathrm{mm})\end{array} & 5 & 7 & 9 & 11 & 13 & 9 & 7 & 5\end{array}$$ What is the best explanation for the behavior exhibited in the data? A. Longer threads can carry more current than shorter threads and so make better electrical conductors. B. The thread stops being a conductor when it is stretched to 13 \(\mathrm{mm}\) due to breaks that occur in the thin coating. C. As the thread is stretched, the coating thins and its resistance increases; as the thread is relaxed, the coating returns nearly to its original state. D. The resistance of the thread increases with distance from the end of the thread.

Step by step solution

01

Understand the Data

Observe that the table shows the resistance and corresponding lengths of the stretched spider silk. We see that resistance values increase as the thread is stretched from 5 mm to 13 mm and decrease as it returns back to its original length of 5 mm. This is a crucial observation in determining the behavior of the thread.

02

Analyze the Hypotheses

Consider each explanation option: Option A suggests that longer threads carry more current and make better conductors, but data shows increased resistance with length, which is a characteristic of poorer conductors. Option B suggests a permanent loss of conductivity at 13 mm, but the decrease in resistance as the thread contracts back to 5 mm contradicts this. Option C suggests that the thinning coating causes resistance to increase with stretching, aligning closely with the observed data. Option D suggests increased resistance by distance from thread end, but data reflect changes with length adjustments.

03

Match Observation with Hypotheses

The observation is that resistance increases when length increases and decreases when the length is reduced. This behavior does not directly relate to the length of the wire (Option D) or permanent coating breaks (Option B). It aligns well with option C, as the increased resistance upon stretching the thread followed by a decrease upon contraction suggests a reversible change, likely due to the thinning and re-congesting of a conductive coating.

04

Select the Best Explanation

The hypothesis that fits the observed data is C: "As the thread is stretched, the coating thins and its resistance increases; as the thread is relaxed, the coating returns nearly to its original state." This accounts for reversible increases and decreases in resistance corresponding to the lengthened and shortened states of the thread.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Hygroscopic Properties

Some materials have the unique ability to attract and hold water molecules from the surrounding environment. This property is known as "hygroscopicity." Spider silk often exhibits this characteristic when coated with certain solutions. This hygroscopic nature makes the silk threads ideal for capturing prey because the moisture can add weight or tackiness, enhancing the effectiveness of the web. Additionally, from an electrical perspective, the presence of water increases conductivity. Water is an excellent medium for ions, which are essential carriers of electric charge. Therefore, when the spider silk threads attract water, they can potentially become better conductors of electricity.

Resistance Measurement

Resistance measurement is a fundamental concept in evaluating electrical properties. It involves determining how much a material opposes the flow of electric current. In the context of spider silk, measuring resistance helps researchers understand how the silk's properties change under varying conditions. In the given exercise, resistance was measured for different lengths of spider silk. As the silk is stretched, its resistance tends to increase, suggesting that the structural integrity and coating of the silk play a critical role in its electrical properties. Accurate resistance measurement can offer insights into the material composition, thickness, and even the behavior of the coating when subjected to physical changes.

Conductive Coatings

Conductive coatings are thin layers of material applied to surfaces to enhance electrical conductivity. In biological materials like spider silk, these coatings can drastically alter electrical performance. The silk threads are coated with a solution that forms a conductive layer when it dries. This layer acts as a medium for electrical charge to flow through the silk. When the silk thread is stretched, this coating may become thinner, increasing the resistance. Upon releasing the thread, the coating can return closer to its initial thickness and conductivity level. Understanding the properties of such conductive coatings is essential in optimizing their application in various fields, such as flexible electronics and sensors.

Spider Silk Electrical Testing

Spider silk drawn from a web, particularly when coated, holds potential technological applications due to its unique properties. Electrical testing involves placing the silk between two contacts and measuring its resistance as it is stretched and relaxed. This testing methodology helps in assessing changes in its conductive properties under mechanical stress. In the described experiment, researchers measured the resistance of a 5 mm silk thread, stretched it incrementally, and observed changes in resistance. Such tests are pivotal in understanding how natural materials can be engineered for use in bioelectronic devices, providing potential breakthroughs in creating flexible, biodegradable conductors. These insights pave the way for innovative applications in sustainable electronics.