Catalonia PAU Technology and Engineering II common mistakes
Catalonia PAU Technology and Engineering II · Understand the traps and how to avoid them.
In circuit analysis problems, the candidate applies Ohm's law directly to the total voltage and total resistance without first deciding whether the network is series, parallel or mixed, and without reducing it step by step, so a single wrong reduction propagates through every later value.
The exam habit of 'find the formula and plug in' works for single-component drills, and candidates transfer it to multi-branch networks where the equivalent resistance must be built in stages. Time pressure in PAU-style papers rewards the first number that appears, and students rarely re-check whether the same current really flows through every element they treated as series.
What to do instead
Adopt a fixed written protocol before any arithmetic: mark nodes, label each branch current with an assumed direction, then reduce from the innermost parallel group outward, writing the equivalent resistance at each stage. Verify with a power balance (sum of I²R equals source power) and with Kirchhoff's voltage law around one loop. Practise three mixed networks per session until the reduction is automatic.
When drawing orthographic views, the candidate places hidden features as solid lines, omits hidden lines entirely, or misaligns the front, plan and side views so that the projection correspondence between them is broken.
Candidates learn the three-view layout as a picture to copy rather than as a projection rule, so they do not check that each point in the plan sits directly below the corresponding point in the front view. Hidden edges are mentally 'invisible' and get dropped, and the standard line-type hierarchy (visible over hidden over centre) is not applied.
What to do instead
Before drawing, construct a 45° mitre line and project every feature through it so plan and side view heights and depths are forced to agree. Then apply the line hierarchy deliberately: draw all visible outlines, then add dashed lines for every edge not seen from that direction, then centre lines. Self-check by counting edges on the isometric sketch and confirming each appears in at least two views.
In gear, pulley and belt transmission problems, the candidate uses the ratio of diameters or teeth but forgets that torque and speed vary inversely, and mixes up which wheel is driver and which is driven, producing a torque or output speed in the wrong direction of change.
The numerical ratio is easy to compute and feels like the answer, so the physical meaning of the ratio is skipped. Students also memorise 'big wheel turns slowly' without linking it to the conservation of power, so when a gear train has more than two stages they multiply ratios in the wrong order or invert one stage.
What to do instead
Write the relation explicitly as n1·z1 = n2·z2 and P = T·ω before substituting, and state in words 'the driven wheel is larger, so it turns slower and with higher torque'. For multi-stage trains, draw the shaft line and label each stage driver/driven, then multiply stage ratios in sequence. Sanity-check the final speed against the input: a reduction train must always slow down.
When asked to design or interpret a pneumatic or hydraulic circuit, the candidate draws the actuator and directional valve but omits the pressure-relief valve, the filter-regulator-lubricator unit, or the return line to tank, producing a circuit that cannot work or cannot be read.
Circuit diagrams are treated as a sketch of the main components rather than a complete functional system. Candidates focus on the actuator motion requested by the question and forget that every real circuit needs pressure limitation and, in hydraulics, a path back to the reservoir. Symbol conventions for the valve centre positions are also shaky.
What to do instead
Memorise a checklist for every circuit: energy source, pressure relief, conditioning (filter/regulator), directional control valve with correct number of positions and ways, actuator, and return path. Draw the valve in its rest position and verify the centre configuration blocks or bypasses as intended. Practise reading a supplied circuit symbol by symbol and naming the function of each element before answering.
In materials selection questions, the candidate answers with a single property ('steel because it is strong') instead of matching a set of required properties to the service conditions, and ignores the environmental or recyclability criterion when the question asks for it.
Property tables are memorised as isolated facts and the selection method (identify function, constraints, objective) is not internalised. Candidates also default to the most familiar material rather than the one that satisfies the stated constraints, and they treat environmental impact as a separate topic rather than a selection criterion.
What to do instead
Answer in three explicit steps: list the constraints from the problem statement (load type, temperature, weight, corrosion, cost, end-of-life), eliminate materials that fail any constraint, then justify the survivor against the objective. When sustainability is mentioned, add recyclability, embodied energy or toxicity as a named criterion. Write the justification in full sentences so the reasoning, not just the material name, earns marks.
In control-system questions, the candidate labels a system open or closed loop by whether it 'looks automatic' rather than by whether feedback is compared with a reference, and cannot identify the sensor, controller and actuator in a block diagram.
Everyday examples (a washing machine, a thermostat) are memorised with their classification but the defining criterion, comparison of output with setpoint, is not applied to new systems. Block diagrams are read left to right as a flow rather than as a signal path with a feedback branch, so the subtraction junction is missed.
What to do instead
Apply a single test: is the measured output fed back and subtracted from the reference to generate the control signal? If yes, closed loop. Then decompose any diagram into reference, comparator, controller, actuator, process and sensor, and trace the feedback arrow back to the comparator. Practise with unfamiliar examples (a drone holding altitude, a tank filling to a mark) so classification comes from structure, not memory.
In energy and efficiency problems, the candidate computes efficiency as output over input using mismatched units (kWh in the numerator, J in the denominator) or forgets that a chain of conversions multiplies efficiencies rather than adding them.
Unit conversion is treated as a formality and done at the end rather than at the start, and percentages are handled loosely. For multi-stage systems, students average the efficiencies because that feels like a sensible 'overall' figure, missing that each stage loses a fraction of what reaches it.
What to do instead
Convert every quantity to SI at the start and carry units through the calculation. For chains, write η_total = η1·η2·η3 and compute the energy remaining after each stage in sequence, which makes the multiplication self-evident. Check plausibility: overall efficiency must be lower than the worst single stage, and always below 100 %.
In manufacturing questions, the candidate names a process without matching it to the material and batch size, for example recommending sand casting for a short run of thin steel sheet parts or forging for a complex hollow shape, and does not distinguish forming from machining from joining.
Process names are learned as vocabulary with one example each, and the selection criteria (material, geometry, tolerance, quantity, cost) are not used. Candidates also conflate categories, describing welding as a forming process or turning as a joining operation, which reveals a missing map of the field.
What to do instead
Build a decision table with rows for material class, batch size, shape complexity and tolerance, and columns for the main processes, then practise placing a new part in it. Classify every process explicitly as forming, machining, joining or surface treatment before discussing it. In answers, state the process, then justify it against at least two of the criteria from the question.
In product design and project questions, the candidate jumps straight to a single solution and writes a description of it, skipping the specification of requirements, the generation of alternatives and the evaluation criteria that the question is actually assessing.
Design is perceived as a creative act whose output is the product, so the method (brief, specifications, alternatives, weighted evaluation, documentation) is seen as padding. Under exam time pressure, describing one solution feels efficient, and candidates do not realise that marks are allocated to the process stages.
What to do instead
Structure the answer with visible headings: problem statement, list of requirements with measurable targets, at least two alternative concepts, a comparison table against weighted criteria, chosen solution with justification, and a note on documentation or testing. Even in a short answer, include one alternative and one explicit criterion, because that is where the method marks sit.
In AC circuit questions, the candidate treats impedance as a plain resistance, adds reactances arithmetically without regard to phase, and reports a current magnitude without the phase angle or power factor the question asked for.
DC habits carry over: resistance adds directly and current equals voltage over resistance. Complex impedance is learned as a formula but not as a geometric sum, so series R and X are added as R + X instead of as a vector sum. Power factor is often forgotten entirely because it has no counterpart in DC.
What to do instead
Always draw the impedance triangle or phasor diagram first, then compute Z as the modulus of the vector sum and the angle from the arctangent. Report magnitude and phase together, and derive power factor as cos φ, stating whether it is lagging or leading. Check with a limiting case: a purely resistive circuit must give φ = 0 and power factor 1.