PAU Technology & Engineering II (Cantabria) common mistakes
PAU Technology & Engineering II (Cantabria) · Understand the traps and how to avoid them.
When asked to select a material, the candidate picks the one with the highest value of a single property (for example, the strongest steel or the lightest aluminium) and ignores the remaining requirements, such as density, cost, machinability, corrosion resistance, or electrical conductivity.
The candidate reduces material selection to a single 'best' property and does not build a list of requirements first. Textbooks often present property tables one column at a time, so the candidate never practises weighing several criteria against one another, and the exam phrasing 'which material is most suitable' invites a single-answer reflex.
What to do instead
Before choosing, write the functional requirements as a short list: what must the part do, what loads act on it, what environment does it face, and what are the cost and manufacturing constraints. Then score each candidate material against every requirement and justify the trade-off explicitly. Practise with Ashby-style charts and with short exercises in which two properties conflict (for example, specific stiffness against cost).
When solving a statics problem, the candidate writes the equilibrium equations but mixes sign conventions, forgets a reaction, or applies a moment equation about a point that does not eliminate the unknown the candidate is trying to find, so the numeric result is wrong even though the method is correct.
The candidate works directly on the calculator without drawing a complete free-body diagram, so distributed loads, weights of members, and reactions at supports such as a roller are omitted or given the wrong direction. Sign errors accumulate silently because the candidate never checks the result.
What to do instead
Always draw the free-body diagram first: isolate the body, replace every support with its reaction, and replace distributed loads with their resultant at the centroid. State the sign convention at the top of the page, write sum of forces and sum of moments about a point that removes the most unknowns, and finish with a check using a second moment point. Train until the check catches errors routinely.
When calculating stresses and deformations, the candidate substitutes the wrong geometric property (using the diameter instead of the radius, or the section modulus where the second moment of area is required) or forgets to convert millimetres to metres, producing answers off by factors of 2, 4, or 1000.
Formulas are memorised as symbols without a mental picture of what each quantity means. Unit conversion is treated as an afterthought, and the candidate does not estimate the order of magnitude of the result, so a stress of 2000 MPa in ordinary steel passes unnoticed.
What to do instead
Before substituting, write the formula, name each symbol and its unit, and sketch the cross-section with the relevant dimension marked. Convert every quantity to SI base units at the start. Estimate the answer mentally (for example, a 10 mm bar carrying 10 kN gives roughly 130 MPa) and compare it with the calculated value; investigate any discrepancy before moving on.
When analysing mechanisms, the candidate confuses velocity ratio with mechanical advantage, applies the pulley or gear ratio in the wrong direction, or treats a worm gear or a planetary train as if it were a simple two-gear train.
The candidate memorises ratio formulas without tracking which shaft is the input and which is the output. Compound trains, where the ratio is the product of the stage ratios, and reversibility (a worm drive does not back-drive) are poorly practised, and the candidate does not check whether the answer makes physical sense.
What to do instead
For every mechanism, draw a block diagram of the shafts and label input, output, and each stage ratio with its direction of rotation. Compute the overall ratio as the product of the stage ratios, then state whether speed increases and torque decreases or the reverse. Verify with a limiting case, such as a ratio of 1 giving equal input and output speeds, and remember that a self-locking worm gear cannot be driven from the output.
When solving DC or AC circuits, the candidate applies Ohm's law to a whole network as if it were a single resistor, ignores the internal resistance of a source, or in AC problems adds resistances and reactances arithmetically instead of as phasors.
Series and parallel rules are learned as formulas rather than as a procedure. The candidate does not simplify the circuit step by step or redraw it, and in AC the candidate treats impedance as a real number, missing that the magnitude is the square root of the sum of squares and that the phase angle matters for power factor.
What to do instead
Redraw the circuit at each simplification step and mark the equivalent resistance or impedance. Apply Kirchhoff's laws to check the result, and compute the power balance (power delivered by sources equals power dissipated). For AC, write impedances as complex numbers or draw a phasor diagram, then calculate the magnitude and phase separately and interpret the power factor.
When asked to design or interpret a logic circuit, the candidate builds the truth table from the written description incorrectly, or implements a function with the wrong gates because the candidate did not simplify it or confused NAND with NOR behaviour.
The candidate jumps to drawing gates before writing the truth table, and negations in the statement ('the alarm sounds unless the door is closed') are misread. De Morgan's laws and Karnaugh maps are known in theory but not applied under time pressure.
What to do instead
Always write the truth table first, one row per input combination, with the output column filled from the sentence. Simplify with a Karnaugh map or Boolean algebra, then draw the circuit. Verify by evaluating two or three rows directly through the drawn circuit and comparing with the table. Practise translating 'unless', 'only if', and 'at least two of' into Boolean expressions.
When describing a control system, the candidate labels an example as open loop when it is closed loop (or the reverse), or cannot identify the sensor, controller, actuator, and controlled variable in a given process, and confuses feedforward with feedback.
The definitions are memorised as slogans without a physical example. The candidate does not check whether the system measures the output and acts on the error, which is the decisive test, and the vocabulary of setpoint, error, and disturbance is not firmly attached to a block diagram.
What to do instead
For each example, draw the block diagram and ask one question: is the controlled variable measured and compared with a reference? If yes, it is closed loop. Label every block with its physical device (thermostat, relay, motor, valve) and identify the disturbance. Practise with domestic and industrial examples until the classification is automatic.
When drawing or interpreting pneumatic and hydraulic circuits, the candidate uses the wrong symbol for a valve or actuator, misreads the number of positions and ways in a directional valve designation, or ignores the difference in behaviour caused by the compressibility of air versus the incompressibility of oil.
Symbols are learned from a chart without practising reading complete circuits. The candidate does not decode designations such as 5/2 or 3/2 systematically, and the candidate treats pneumatics and hydraulics as interchangeable, missing differences in force, precision, and speed control.
What to do instead
Learn to read valve designations as positions over ways and to trace the flow path through each position. Redraw given circuits symbol by symbol and label the actuation method (manual, solenoid, pilot). Compare a pneumatic and a hydraulic version of the same circuit and state why hydraulic systems handle higher forces and pneumatic systems are faster and cleaner. Simulate the circuit to confirm the sequence.
When answering manufacturing questions, the candidate recommends a process that cannot produce the required geometry or tolerance (for example, casting a sharp internal undercut or expecting a lathe to cut a square pocket), or confuses the purpose of metrology instruments and quality tools.
Processes are memorised as definitions rather than as capabilities with limits on shape, surface finish, tolerance, and batch size. The candidate does not connect the drawing's features to the process that can create them, and quality concepts such as tolerance, calibration, and control charts are recalled loosely.
What to do instead
Build a capability table for each process: achievable tolerance, surface finish, typical batch size, and shapes it cannot make. For each practice part, list its features and match each feature to a process, justifying the choice. Separately, match each measuring instrument to its resolution and range, and practise distinguishing tolerance from deviation and calibration from verification.
In the technical project section, the candidate lists documents or budget lines without linking them to the project phases, omits the bill of materials or the contingency, or describes ICT tools generically instead of naming what each tool produces in the project.
The project methodology is studied as a list of headings to memorise rather than as a sequence of deliverables. The candidate has not practised producing a small project file, so the relationship between specification, drawings, budget, and planning is not internalised.
What to do instead
Work through a complete miniature project: specification, alternatives with selection criteria, drawings, bill of materials, budget with labour and contingency, and schedule. For each document, state which phase produces it and who uses it. For ICT tools, name the output each one generates (CAD drawing, spreadsheet budget, simulation result) rather than the tool alone.