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It would be a great idea to have some samples of RG-58/U (or other coaxial cable type) available in your laboratory for students to measure themselves. There is nothing like direct, hands-on experimentation to make a point!

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Students should realize by the wording of the question that the voltage signal probably does not arrive at the far end of the cable instantaneously after the switch is closed. Although the speed of light is very, very fast, it is not instant: there will be a measurable time delay.

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Notes:
Students should realize by the wording of the question that the voltage signal probably does not arrive at the far end of the cable instantaneously after the switch is closed. Although 69% of the speed of light is still very, very fast, it is not instant: there will be a measurable time delay.

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This concept will seem very strange to students who are only familiar with resistance in the context of resistors and other simple electrical components, where resistance does not change appreciably over time. In this example, though, the "resistance" of the cable is extremely time-dependent, and the time spans involved are typically very short - so short that measurements made with ohmmeters will not reveal it at all!

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Answering this question requires several steps, and the combining of multiple concepts. It should be apparent from the answer that Ohm's Law (I = ^E/_R) is sufficient for calculating pulse current, but the time delay figure given in the answer may confuse some students. For those students who calculate a time figure that is half as much as the one given in the answer, encourage them to think of why their (incorrect) answer might have been off by 50%. The existence of a 2:1 ratio such as this implies a simple conceptual misunderstanding.
For the RG-58/U cable velocity factor, I obtained two different figures: 0.63 and 0.66, which accounts for the two time delay answers given.

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Challenge your students to think of another electrical component (besides an RG-58/U cable of infinite length) that would behave like this, drawing 480 mA of current from a 24 volt source any time the switch is closed. Hint: you don't have to think very hard!

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Ask your students to compare the behavior of this circuit with that of an unterminated RG-58/U cable. How does this circuit's behavior differ? Why is that?
To phrase the question in a different way, what does the inclusion of a terminating resistor do to the apparent length of the cable? In other words, what length of RG-58/U cable would behave exactly the same as this circuit?

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To help answer this question, it is helpful to ask students how voltage and current relate to each other in an open-circuit condition (maximum voltage, zero current).

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To help answer this question, it is helpful to ask students how voltage and current relate to each other in a short-circuit condition (minimum voltage, maximum current).

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Answering this question is an exercise in qualitative thinking: compare the results of termination with the proper amount of resistance, versus termination with infinite or zero resistance. A terminating resistor of improper value will produce an effect somewhere between these extreme cases.
For instance, compare the cable impedance (as ßeen" by the voltage source after a substantial amount of time) for a properly terminated cable, versus one that is either open-ended or shorted. What would a cable terminated by an improper-value resistor "look" like to the source after the propagation delay time has passed?

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Be sure to ask your students to explain why the characteristic impedance will change in the direction it does, based on the known changes to both capacitance and inductance throughout the cable. It should fairly simple for students to explain why capacitance will increase as the two conductors are brought closer together, but it may not be as apparent why the inductance will decrease. A good "Socratic" question to ask is about magnetic field strength, assuming one end of the cable were shorted, and a DC current source connected to the other end. Be sure to remind them to discuss the right-hand corkscrew rule for current and magnetic fields in their answer to this follow-up question!

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Notes:
Be sure to ask your students to explain why the characteristic impedance will change in the direction it does, based on the known changes to both capacitance and inductance throughout the cable. It should fairly simple for students to explain why capacitance will increase as the two conductors are brought closer together, but it may not be as apparent why the inductance will decrease. A good "Socratic" question to ask is about magnetic field strength, assuming one end of the cable were shorted, and a DC current source connected to the other end. Be sure to remind them to discuss the right-hand corkscrew rule for current and magnetic fields in their answer to this follow-up question!

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This is sort of a "trick" question, designed to make students think about characteristic impedance, and to test their real comprehension of it. If a student properly understands the physics resulting in characteristic impedance, they will realize length has nothing whatsoever to do with it. Although the cable's total capacitance will change as a result of shortening the cable's length, and the cable's total inductance will likewise decrease for the same reason, these electrical changes should not present a conceptual difficulty to students unless they are modeling the cable in terms of one lumped capacitance and one (or two) lumped inductance(s). If they are thinking in these terms, they have not yet fully grasped the reason why characteristic impedance exists at all.

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This question is really a review of Thévenin's theorem as it applies to common-emitter, divider-biased BJT amplifier circuits.
In case anyone asks, the "zig-zags" in the four lines for the cable represent an unspecified distance between those points. In other words, the cable is longer than what may be proportionately represented on the schematic diagram.

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Notes:
This question is really a review of Thévenin's theorem as it applies to common-emitter, divider-biased BJT amplifier circuits, and also impedance transformation as it applies to step-up and step-down transformers.
In case anyone asks, the "zig-zags" in the four lines for the cable represent an unspecified distance between those points. In other words, the cable is longer than what may be proportionately represented on the schematic diagram.

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Understanding this answer requires that students recall the filtering behavior of a series capacitor in an AC circuit.

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The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a ßhow and tell" exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they're learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting component markings, use a multimeter, access datasheets, etc.

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