Balearic PAU Technology & Engineering II common mistakes
Balearic PAU Technology & Engineering II · Understand the traps and how to avoid them.
When calculating the equivalent resistance of a mixed series-parallel network, the candidate collapses the whole circuit into a single parallel block (or applies the product-over-sum rule to branches that are not actually in parallel), so the total resistance and every downstream current come out wrong.
Candidates memorize R_eq formulas as recipes and never redraw the network node by node. Because the schematic is drawn compactly, they mistake two resistors that merely touch at a junction for a parallel pair, when the junction is actually a shared node feeding a third branch. The habit is reinforced by textbook exercises where the drawing is always already simplified.
What to do instead
Adopt a fixed procedure: label every node with a letter, redraw the circuit with nodes as dots and resistors between them, then reduce strictly from the node pair with no intermediate components. Verify by checking that the equivalent resistance of a parallel group is always smaller than the smallest branch, and that series groups always sum larger than any single element. Practise five mixed networks per session with the redraw step written out explicitly, not done mentally.
The candidate writes the static equilibrium equations but omits one reaction or applies a moment equation about a point that does not eliminate the unknown they intend to find, producing a system that is either indeterminate or solved with a sign error on a couple.
Free-body diagrams are drawn hastily and incompletely: the weight of a bar is placed at its end rather than its centre of gravity, a roller support is given two reaction components, or a cable is treated as a two-force member with a bending moment. Sign conventions for clockwise versus anticlockwise moments are set inconsistently between the diagram and the algebra.
What to do instead
Standardise a drawing protocol: isolate the body, mark every external force with magnitude, direction and point of application, replace supports by exactly the reactions their symbol permits (pin = two components, roller = one normal, fixed = two plus a moment), and draw the weight at the centroid. Write the sign convention in the margin before writing equations, and take moments about the point where the most unknowns intersect. Check the result by taking moments about a second point.
On a mechanism problem the candidate computes the transmission ratio but then applies it in the wrong direction, or treats a belt-and-pulley train and a gear train identically, so the output speed or torque is inverted or scaled by the wrong factor.
The ratio is memorised as 'driven over driver' without attaching it to a physical meaning, and candidates do not track which shaft is input. Direction reversal in external gear meshes and the absence of reversal in belt drives are forgotten because they are learned as trivia rather than as consequences of the geometry.
What to do instead
Always write the ratio as a labelled fraction with the input on the bottom and the output on top, and state in words 'the output turns slower by this factor'. For each mesh, note whether it is external gearing (direction reverses) or a belt or internal gear (direction preserved). Sanity-check torque and speed together: if speed drops, torque must rise by the same factor in an ideal train, so a result where both drop signals an error.
The candidate selects a material by quoting a single property value (for example the highest tensile strength or the lowest density) and declares it optimal, without weighing cost, machinability, corrosion behaviour, recyclability, or the actual stress state of the part.
Selection exercises are treated as lookup tasks and the candidate confuses 'strongest' with 'most suitable'. Ashby-style thinking, where the merit index depends on the loading geometry (tension, bending, buckling), is unfamiliar, and the environmental column of the problem is ignored because it appears at the end of the statement.
What to do instead
Structure every selection answer as a table: candidate materials down the side, the properties that the stated function actually demands across the top (including density-normalised strength if weight matters, and cost per unit of delivered performance), then a justified rejection line for each discarded option. Explicitly name the failure mode the part must resist and tie the chosen property to it. Finish with one sentence on end-of-life and recyclability so the sustainability criterion is visibly addressed.
In pneumatic and hydraulic circuit questions the candidate reads or draws the directional valve symbol incorrectly, confusing the number of positions with the number of ways, or places the actuator and valve so that the circuit cannot extend and retract as described.
The symbolic representation is learned by copying shapes rather than by the rule that each square is a valve position and each connection point on a square is a port. Candidates also forget that the boxes are drawn in the un-actuated rest position, so the spring-side box must show the initial flow path.
What to do instead
Apply the counting rule explicitly: count squares for positions, count distinct port connections on one square for ways, and annotate '5/2' beside the symbol before using it. Trace the flow path through the rest-position square first, then through the actuated square, drawing arrows on the diagram for extend and retract. Confirm that the exhaust ports are shown venting and that the cylinder has both ports connected in each position.
The candidate describes a closed-loop control system but cannot distinguish the sensor, the controller and the actuator in a given block diagram, or claims a system is closed-loop when it merely has a sensor feeding a display with no feedback into the actuation.
The vocabulary of control is absorbed passively from diagrams, and the candidate equates 'has a sensor' with 'has feedback'. The role of the comparator and the error signal is glossed over because it is the least visually prominent part of the block diagram.
What to do instead
For any system, write the loop in words as a chain: setpoint, comparator, controller, actuator, process, sensor, back to comparator. Identify each physical component against that chain before answering. Test openness by asking whether the actuation changes when the sensor reading changes without human intervention; if it does not, the loop is open. Practise on everyday systems such as a thermostat, a servo and a washing machine level sensor.
The candidate confuses machining, forming and joining operations when classifying a manufacturing process, or describes a production sequence that cannot physically produce the stated geometry (for example planning a turned internal thread on a part that is already hardened, or drilling after a finish grinding operation).
Processes are memorised as a list of names without their kinematic capability or their position in a process plan. The candidate does not connect the operation to the machine tool's relative motions, so chip-removal, deformation and material-addition processes blur together.
What to do instead
Classify every process by two questions: does it remove, deform or add material, and what relative motion between tool and workpiece does it require? Build a one-page map of the main families with one worked example each. When sequencing, respect the rule that finishing operations come last and that heat treatment precedes final grinding, and state the reason for each ordering decision in the answer.
On technical drawing and standardisation items the candidate mixes first-angle and third-angle projection, omits hidden lines or centre lines, or applies dimensioning rules incorrectly by over-dimensioning a part or placing dimensions on the drawing rather than on the views that show the feature true size.
Projection conventions are learned symbolically and never practised by drawing the same object both ways. Dimensioning is treated as labelling rather than as a complete and non-redundant specification, so the candidate adds every dimension that can be measured instead of the minimum set needed to manufacture the part.
What to do instead
Redraw the same simple bracket in both projection systems side by side and mark the position of the projection symbol. For dimensioning, follow the rule that each feature is dimensioned once, in the view where it appears true size, and that no dimension is repeated or derivable from others; check by asking whether a machinist could make the part without calculating anything. Include centre lines and note surface finish and tolerance callouts where the statement implies them.
The candidate identifies the correct formula for an AC circuit but ignores phase relationships, treating impedance as a simple arithmetic sum of resistance and reactance, or reporting power without distinguishing active, reactive and apparent power.
Direct-current habits carry over: the candidate adds magnitudes as if R and X were in series on a number line, and the power factor is treated as a correction factor applied at the end rather than as the cosine of the phase angle inherent in the impedance. Complex-number or phasor representation is avoided as unfamiliar.
What to do instead
Force every AC calculation through a phasor or complex-impedance step: write Z = R + jX with the sign of X depending on whether it is inductive or capacitive, then take modulus and argument. Report P, Q and S separately with their units and verify that S squared equals P squared plus Q squared. Check that the power factor lies between zero and one and that the phase angle sign matches whether the circuit is net inductive or net capacitive.
In the engineering project and sustainability questions the candidate answers with generic statements about teamwork, brainstorming or 'being green', without producing a budget breakdown, a Gantt-style schedule, a life-cycle inventory stage list, or a quantified comparison of alternatives.
These domains feel like soft content, so candidates do not prepare structured frameworks and fall back on adjectives. They also run out of time and write the first thing that comes to mind rather than applying a method such as life-cycle analysis stages or a project phase model.
What to do instead
Prepare a small set of reusable skeletons: project phases (need, specification, conceptual design, detailed design, construction, testing, documentation), a budget table with materials, labour and contingency lines, and a life-cycle list of extraction, manufacture, distribution, use and end-of-life. In the exam, instantiate the skeleton with the numbers and materials from the statement and add one explicit comparison, such as energy or cost per unit between two options, so the answer is quantified rather than descriptive.