Chapter 19, Problem 84PP

Spiderweb conductivity. 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 mm length of thread between two electrical contacts. The researchers stretched the thread in 1 mm increments to more than twice its original length, and then allowed it to return to its original length, again in 1 mm increments. Some of the resistance measurements are given in the table:

Resistance of thread (109 Ω)	9	19	41	63	102	76	50	24
Length of thread (mm)	5	7	9	11	13	9	7	5

84. What is the best explanation for the behavior exhibited in the data?

1. A.    Longer threads can carry more current than shorter threads and so make better electrical conductors.
2. B.     The thread stops being a conductor when it is stretched to 13 mm due to breaks that occur in the thin coating.
3. 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.
4. D.    The resistance of the thread increases with distance from the end of the thread.