Castilla-La Mancha PAU Technology and Engineering II (Tecnología e Ingeniería II - UCLM 2026) common mistakes
Castilla-La Mancha PAU Technology and Engineering II (Tecnología e Ingeniería II - UCLM 2026) · Understand the traps and how to avoid them.
Using the yield strength (límite elástico) as the allowable stress when the problem asks for a safety coefficient, or dividing by n instead of multiplying, so the calculated section comes out too small and the design is unsafe.
Candidates memorise σ = F/S and then apply σ_adm = σ_e without checking whether the statement gives n, or they confuse the definition σ_adm = σ_e/n with the reversed expression. In UCLM problems the safety coefficient is often buried in the wording ('con un coeficiente de seguridad de 2,5') rather than in a formula, so it is easily skipped.
What to do instead
Before any calculation, write a one-line data block: F, material, σ_e, n, and the target (dimension or verification). Then state explicitly σ_adm = σ_e/n, substitute numbers with units, and only afterwards solve for S. Finish with a verification sentence: 'σ_trabajo = ... ≤ σ_adm = ..., cumple'. If n is not given, say so and solve with σ_adm = σ_e.
Confusing the three simple machines/mechanisms formulas: applying the pulley-block relation to a gear train, or using the wheel-radius ratio for a differential or a screw jack, and mixing velocity ratio with force ratio.
The transmission formulas (i = Z2/Z1 = n1/n2 = M2/M1, pulley blocks, screw: F·p = 2π·R·Q, differential) look similar and are taught close together, so under time pressure candidates grab the most recent one. They also forget that the velocity ratio and the force ratio are reciprocals unless efficiency is 100 %.
What to do instead
Build a one-page formula card at revision time with a sketch next to each mechanism and the physical meaning of each ratio. In the exam, first draw the mechanism, label the driver and driven elements, then choose the formula by asking 'what is connected to what?'. Always state the efficiency η and use M2 = M1·i·η for force/torque, n2 = n1/i for speed.
Solving beam bending problems without a correct free-body diagram: forgetting the reaction at one support, omitting the weight of the beam, or placing the point load at the wrong distance, so the bending moment diagram is wrong from the start.
Candidates jump straight to M = F·d formulas memorised for standard cases (simply supported with central load) and force the data into them even when the load is not central or the beam is a cantilever. Sign conventions and the difference between 'moment at a section' and 'maximum moment' are also weak.
What to do instead
Always draw the beam, the supports with their unknown reactions, and every load with its distance. Apply ΣF = 0 and ΣM = 0 about one support to obtain reactions before computing any internal moment. Then compute M(x) at the critical sections (supports, under point loads, at zero shear) and sketch the diagram. Check the result with a limiting case (load at mid-span vs. at the support).
Treating a three-phase load as if it were single-phase: using P = V·I·cos φ instead of P = √3·V_L·I_L·cos φ, or mixing phase and line voltages in star and delta connections.
Single-phase formulas are learned first and are simpler, and the √3 factor is easy to drop. In star/delta problems candidates forget that in star V_L = √3·V_f and I_L = I_f, while in delta V_L = V_f and I_L = √3·I_f, so they substitute the wrong voltage into the power or impedance formula.
What to do instead
Write the connection type at the top of the solution ('estrella' or 'triángulo') and list the two relations V_L/V_f and I_L/I_f for that connection before calculating. Use P = √3·V_L·I_L·cos φ as the default and only convert to phase values when the load impedance per phase is given. Verify with the per-phase power: P_total = 3·V_f·I_f·cos φ must give the same number.
Drawing or interpreting a circuit diagram with the wrong meter connection or polarity: ammeter in parallel, voltmeter in series, or ignoring the sign of the induced e.m.f. in a coil when applying Lenz's law.
Candidates reproduce symbols from memory without recalling the ideal-meter condition (ammeter: zero resistance, in series; voltmeter: infinite resistance, in parallel), and in electromagnetic induction problems they compute the magnitude of ε = −N·dΦ/dt but then assign the current direction arbitrarily.
What to do instead
State the ideal-meter rule explicitly in the answer and, when asked to place a meter, justify it in one sentence. For induction, apply the three-step routine: (1) determine the direction of the change of flux, (2) the induced current opposes that change (Lenz), (3) use the right-hand rule to give the current sense. Report the sign of ε from Faraday's law and interpret it in words.
Choosing a material for a given application by quoting a single property ('aluminium because it is light') without comparing the relevant index, cost, and the competing candidates, and without mentioning recyclability or availability.
Selection questions look like general-knowledge questions, so candidates answer with a slogan rather than an argument. They also ignore that the exam expects the Ashby-type logic: property index, constraints, and objective (mass, cost, stiffness).
What to do instead
Answer with a structured three-line argument: (1) constraints of the application (strength, density, corrosion, temperature, cost), (2) property or index that must be maximised or minimised (e.g. σ/ρ for a light stiff tie, E/ρ for a light stiff beam), (3) the chosen material compared with at least one alternative and a sentence on recycling/availability. Never give a material without a comparative justification.
Describing a manufacturing process by name only ('se hace por extrusión') without stating the material state, the tooling, the temperature regime, and the resulting geometry or defect risk that the question is testing.
Candidates learn process names as vocabulary and cannot connect them to the physical principle (plastic deformation, solidification, material removal, joining). Consequently they cannot answer the 'why this process and not another' part, which carries most of the marks.
What to do instead
For every process, prepare a five-point card: principle, material/state (hot or cold, thermoplastic or thermoset), tooling, typical part geometry, and characteristic defect (e.g. extrusion: die wear, surface tearing; deep drawing: earing, wrinkling; welding: porosity, HAZ). In the exam, answer with those five points and finish by naming one alternative process and why it is worse here.
Presenting the automation/control answer as a block diagram with no signal definition: drawing a generic feedback loop without identifying the sensor, the actuator, the set point, the controlled variable, and the type of control (open/closed loop, on-off, PID).
Block diagrams are memorised as a picture, not as a causal model, so candidates cannot label the elements or explain what happens when a disturbance appears. The exam typically asks for the loop description of a concrete process (level, temperature, position), which exposes this gap.
What to do instead
Practise describing five real loops (tank level, oven temperature, conveyor position, motor speed, greenhouse humidity) with the same template: variable controlada, consigna, sensor/transductor, comparador, regulador, actuador, perturbaciones. Draw the loop with arrows and label every block. Then state whether the action is todo-nada or proporcional and what would happen without feedback.
In technical drawing, dimensioning a part with redundant or contradictory dimensions, omitting the scale or the units, or failing to use the correct views and section representation according to the standards.
Candidates focus on making the drawing look right and ignore that dimensioning is a normative language: each dimension must appear once, must not close a chain redundantly, and must follow UNE/ISO criteria for arrows, symmetry and section hatching. Time pressure leads to omitting scale and units.
What to do instead
Apply a checklist to every drawing: (1) all views aligned and consistent (no hidden line contradictions), (2) each dimension once only, no closed chains, (3) functional dimensions from the datum, (4) units and scale in the title block, (5) sections hatched at 45° and labelled. Before handing in, re-read the drawing as if you had to manufacture the part and ask whether any dimension is missing or duplicated.
Answering the energy/project question with a list of generalities ('use renewable energies, they are better for the environment') without quantifying efficiency, energy payback, installed power, or the project phases and documents required.
These are the discursive questions at the end of the paper, often attempted with little time left, and candidates fall back on environmental slogans. They also confuse the documents of a project (memoria, planos, pliego de condiciones, presupuesto) or omit the viabilidad and normativa stages.
What to do instead
Prepare two compact frameworks. Energy: source → conversion chain → efficiency of each stage → installed power and annual energy → comparison with the fossil alternative including CO2 avoided and payback. Project: necesidad → estudio de viabilidad → anteproyecto → proyecto (memoria, planos, pliego, presupuesto, seguridad y salud) → ejecución → explotación. In the exam, answer with the framework and put at least one number or standard reference in each paragraph.