Predict circuit readings from competing mechanisms

Start with explanations that predict different readings

A device stops behaving as expected. Replacing parts at random might eventually change it, but it would not explain the failure. In this investigation, first draw the stated circuit and write competing explanations. Then choose a measurement whose possible outcomes would help distinguish them.

The nominal circuit has an ideal 6 V source, a 100 Ω resistor R1 followed by a 200 Ω resistor R2, and a return wire. Define return as 0 V. Positive conventional current travels from the positive source terminal through R1, the junction and R2 to return. Terminal voltage means source-positive potential minus return potential. Junction voltage means the intermediate junction’s potential minus return, so it is the voltage across R2 with the stated polarity.

Calculate the healthy reference

For the healthy series path, the resistance is 100 + 200 = 300 Ω. Current is I = V/R = 6/300 = 0.020 A = 20 mA. R1’s drop is 0.020 × 100 = 2 V, leaving the junction at 6 − 2 = 4 V. R2’s drop is 0.020 × 200 = 4 V, bringing the potential back to return. The two drops sum to the source voltage.

A resistor does not use up the current as it transfers energy. The same steady loop current passes both healthy resistors. OpenStax’s series-circuit explanation supports these circuit relationships. The following cases and numerical diagnostic task are original synthetic constructions, not recorded instrument observations.

Give each candidate a mechanism

The supplied hypothesis set has five possibilities: healthy wiring; an open R1; an open R2; a bypass around R2; or a weak ideal source fixed at 3 V instead of 6 V. Treat the last as a changed source value, not a model of battery aging or internal resistance.

An open R1 interrupts the path upstream of the junction. Loop current is zero. The junction still connects to return through intact R2, whose zero-current drop is zero; therefore the junction is at 0 V. The source still has 6 V across its terminals.

An open R2 also makes loop current zero. Now the junction remains connected to source positive through intact R1. R1’s drop is zero, so the junction is at 6 V. The selected open cases do not leave the measured junction floating; its connection matters.

A bypass around R2 connects the junction directly to return. Its voltage is 0 V, but current is not zero: the 6 V source drives R1 alone, giving 6/100 = 60 mA. This is an ideal simulated topology, not a physical wiring instruction. A 3 V ideal source with both nominal resistors gives 3/300 = 10 mA and junction voltage 2 V.

Put prediction before the reading

Create a hypothesis table with terminal voltage, junction voltage and current columns. Before a probe is taken, write the possible values and why they differ. Select an anonymous case A–E and a next probe in the dedicated diagnostic notebook. Enter your prediction, then request the reading. Do not use the case letter as a reason for a diagnosis: the labels are identifiers, and their public mapping is not a secret assessment.

Each case allows two distinct readings. Selecting another case or reloading keeps the prior case’s history. Reopening a recorded probe does not create another independent observation or spend another slot. Generic lesson captures do not stand in for the dedicated prediction-and-reading record. If you have already seen a case’s answer, retain and disclose that exposure rather than pretending to start fresh.

Transfer: preserve ambiguity

A new paper record states only “terminal voltage 6 V; current 0 mA.” List every compatible supplied mechanism. Predict the junction reading for each and explain what the available evidence cannot yet decide.

Check the open-path transfer

Both open R1 and open R2 remain compatible. Open R1 predicts junction voltage 0 V; open R2 predicts 6 V. Zero loop current identifies neither location alone. If both readings have already consumed the notebook budget, retain the unresolved pair rather than manufacture a third reading. This public answer demonstrates the reasoning pattern, not independent mastery.

Keep your original table even after eliminating candidates. Return to series circuits for the physical relationships, or continue to the measurement budget to plan a useful probe sequence.