Valencian Community PAU Technology & Engineering II common mistakes
Valencian Community PAU Technology & Engineering II · Understand the traps and how to avoid them.
In stress–strain problems the candidate reads the elastic limit off the curve at the point where the line stops being perfectly straight, instead of identifying the proportional limit and then the yield point (0.2% offset when there is no clear yield), and reports the wrong allowable stress.
Textbook diagrams are drawn schematically with an exaggerated curved transition, so candidates equate 'end of straight line' with 'yield' and never apply the offset construction; they also confuse the ultimate strength with the working stress.
What to do instead
Before calculating, always annotate the graph: mark proportional limit, elastic limit, yield (with 0.2% offset construction drawn on the strain axis), ultimate and breaking point. Compute the safety factor as yield stress divided by allowable stress and state which stress you used. Practise on at least three curves, one with a pronounced yield plateau and one without.
When choosing a material, the candidate lists generic properties ('strong', 'light', 'cheap') without linking them to the specific service requirement, and does not compare candidates on a quantitative criterion such as specific strength (σ/ρ) or stiffness-to-weight ratio.
Selection questions are answered from memory of material names rather than from a requirement-driven method; candidates have not internalised that the property that matters is the one dictated by the dominant failure mode of the part.
What to do instead
Use a fixed three-step template in every answer: (1) identify the dominant requirement (strength, stiffness, wear, corrosion, conductivity, cost), (2) compute the relevant index with given data, e.g. specific strength in kN·m/kg, (3) justify the choice against at least one rejected alternative. Write the rejected alternative and the reason explicitly, since that is where marks are awarded.
In tensile test interpretation the candidate mixes units and magnitudes: reports stress in kg instead of N/mm² (MPa), forgets to divide force by the original cross-section, or uses the instantaneous area when the question asks for engineering stress.
Routine use of 'kilos' in everyday language and of diameter instead of area in quick calculations; the distinction between engineering and true stress is often memorised as a definition but not applied numerically.
What to do instead
Force yourself to write the formula with units before substituting: σ = F/A₀ with F in N and A₀ in mm², giving MPa directly. Convert diameter to area with A₀ = πd²/4 in a separate line, and keep three significant figures. Do two drill problems converting between d, A, F and σ in both directions.
In structural analysis the candidate omits or misdirects reaction components: forgets the horizontal reaction at a pinned support, treats a roller as fixed, or draws the bending moment diagram on the wrong side of the beam without sign convention.
Support symbols are memorised visually but their kinematic meaning (restrained degrees of freedom) is not, and sign conventions are applied inconsistently between the free-body diagram and the moment diagram.
What to do instead
State the convention at the top of the answer ('sagging positive, moments drawn on the tension side') and keep it throughout. For each support write explicitly which translations and rotations are blocked before writing equilibrium. Check the result with a quick order-of-magnitude estimate, e.g. wL²/8 for a simply supported beam under uniform load.
In mechanism problems the candidate computes velocity ratios or torques without accounting for efficiency, friction or the direction of power flow, and applies the ideal gear ratio to a worm or screw drive where the efficiency is far from unity.
Classroom exercises usually assume ideal machines, so the candidate transfers the ideal formula to real devices; the meaning of efficiency as output/input power is known verbally but not inserted into the chain of calculation.
What to do instead
Always write the power chain: P_in × η_total = P_out, with η_total as the product of stage efficiencies. For screw and worm drives quote the typical low efficiency explicitly rather than assuming 1. Check that the computed output torque is lower than the ideal value; if it is higher, an efficiency has been inverted.
In DC circuit problems the candidate applies Ohm's law to a whole branch containing several resistors without reducing it, or misapplies Kirchhoff's voltage law with inconsistent sign conventions around the loop.
The candidate pattern-matches 'V = I·R' to any two numbers in the diagram; loop sign conventions are treated as arbitrary rather than fixed by the chosen traversal direction.
What to do instead
Adopt a written procedure: label all currents with assumed directions, reduce series and parallel groups, then apply the node or mesh method with a declared traversal direction. After solving, verify with a power balance (power delivered by sources equals power dissipated in resistors) - this catches almost every sign error.
When reading or drawing electrical schematics the candidate confuses contact types (normally open vs normally closed), does not identify the coil designation (KM1, KA1) as the link between power and control circuits, and cannot trace a self-holding (latching) circuit.
Symbols are learned as isolated pictures without the functional rule that a contact drawn in the rest position is the de-energised state; the auxiliary contact in parallel with the start button is memorised as a shape, not as a logic function.
What to do instead
For every control circuit, redraw it as a truth table: list each pushbutton and each contact state, then evaluate the coil. Practise the start/stop with self-holding circuit until you can explain why the auxiliary contact must be in parallel with the start button and in series with the stop button. Label every device with its standard designation.
In combinational logic the candidate writes the truth table correctly but derives the Boolean expression from the wrong rows (takes the 0-rows for a sum-of-products without negating), or simplifies with Karnaugh maps by grouping cells that are not adjacent or not in powers of two.
The rule 'group ones for SOP' is remembered, but the mapping between minterms and cell positions in the map is not secure, especially with four variables and Gray-code ordering of rows and columns.
What to do instead
Always label the Karnaugh map axes with the Gray sequence (00, 01, 11, 10) and write the minterm numbers inside the cells before grouping. Restrict groups to 1, 2, 4, 8 cells and make them as large as possible; allow wrap-around. Verify the simplified expression by substituting two or three input combinations back into both the original and simplified forms.
In energy and power questions the candidate calculates efficiency or energy consumption without converting units consistently (kWh vs J, W vs kW, hours vs seconds), and mixes calorific value per kg with per m³ of fuel gas.
Everyday units (kWh on electricity bills) coexist with SI units in formulas, and the candidate substitutes numbers into a remembered formula without tracking the unit of each quantity.
What to do instead
Write the unit next to every substituted number and cancel them explicitly, e.g. 1 kWh = 3.6 MJ. Convert all quantities to SI before computing, then convert the result back to the unit the question asks for. For fuels, state whether the calorific value is on a mass or volume basis and confirm the fuel is in the matching phase (gas in m³ at normal conditions).
In technical drawing the candidate produces views with correct outlines but omits hidden lines, centrelines, symmetry axes and the correct line hierarchy (thick continuous for visible edges, thin continuous for dimension and projection lines), and places dimensions that duplicate or contradict each other.
The drawing is treated as a picture to be imitated rather than a standardised language; line thickness and type are seen as cosmetic, and dimensioning rules (no redundant dimensions, dimensions outside the view, no dimensioning to hidden lines) are not practised.
What to do instead
Keep a one-page line-type and dimensioning checklist and apply it to every drawing before submission: visible edges thick, hidden edges dashed, axes dash-dot, dimensions thin with arrowheads touching the extension lines. Check that each dimension appears exactly once and that no dimension is closed into a chain that could contradict the overall dimension. Redraw one standard part (a bracket with holes and a slot) from scratch weekly.