Bac F3 Bénin common mistakes
Bac F3 Bénin · Understand the traps and how to avoid them.
When computing the equivalent resistance of a mixed series-parallel network, the candidate collapses the whole circuit into a single sum R1+R2+R3 instead of first identifying which branches are truly in parallel and which are in series.
The candidate memorised the two formulas separately but never learned to redraw the network node by node, so visual proximity of components is mistaken for electrical connection.
What to do instead
Adopt a fixed procedure: label every node with a letter, redraw the circuit with nodes as dots and branches between them, then reduce from the innermost parallel group outward, writing the intermediate value at each step before continuing.
The candidate writes the transformer secondary voltage as U2 = U1 x N2/N1 without checking whether the given turns are primary or secondary, and forgets that the relation applies to induced emf, not to terminal voltage under load.
The formula is recalled as a single symbol pattern with no attached meaning, and the candidate does not distinguish no-load emf from on-load terminal voltage affected by winding impedance.
What to do instead
Before substituting, write the known quantities with their names and units in a small table, state explicitly which winding is primary, and add a sentence noting whether the machine is assumed ideal; if not, apply the voltage-drop correction.
Faced with a logic circuit or a truth table, the candidate fills the output column by intuition from the gate symbols rather than by evaluating intermediate nodes, and produces a column that does not match any consistent Boolean expression.
The candidate reads the schematic left to right as a picture instead of as a sequence of operations, and skips writing the intermediate variable at each gate output.
What to do instead
Annotate the diagram: name each gate output (S1, S2, ...), write its Boolean expression beside it, then substitute rows of the truth table into those expressions one column at a time before stating the final output.
In automation questions the candidate describes the operating cycle in vague prose ('the motor starts, then the door opens') without producing the grafcet or the ladder rung that implements it, so no points are earned for the sequential part.
The candidate treats the question as a reading-comprehension exercise and has not practised translating a described cycle into formal steps with transition conditions.
What to do instead
Train a standard conversion: list each action as a step, write the condition that ends it as a transition (sensor, timer, pushbutton), then draw the grafcet with initial step, transitions and divergence/convergence, and finally transcribe each step into a self-holding rung.
When selecting a protective device, the candidate picks a circuit breaker rating equal to the calculated load current, ignoring the need for a margin above the load and coordination with the cable's current-carrying capacity.
The candidate reduces sizing to a single equality and has not internalised the double inequality that a protection device must satisfy with respect to both the load and the conductor.
What to do instead
Always write the two conditions explicitly (load current below device rating, device rating below cable ampacity, adjusted for installation method and ambient temperature) and justify the chosen standard rating from the manufacturer's series.
In earthing and protection questions the candidate confuses the role of the neutral conductor with that of the protective conductor, and describes a TN or TT arrangement using the wrong conductor for the fault current return path.
The candidate learned the regime names as labels without a mental picture of where the fault loop closes, so the conductors become interchangeable in the answer.
What to do instead
Draw the fault loop for each regime on a small sketch, marking the fault point, the return path and the device that must trip; state in one sentence which conductor carries the fault current and why the exposed conductive part is at a safe potential.
The candidate computes a numerical result and stops, giving an answer with no unit, an implausible order of magnitude, or a sign that contradicts the physical situation described.
The candidate treats mathematics as a separate subject from the technical problem and does not habitually re-read the result against the physical context.
What to do instead
End every calculation with three checks written out: unit consistency of each term, order-of-magnitude plausibility against the given data, and sign or direction meaning; if a check fails, return to the equation rather than adjusting the number.
In applied physics problems involving energy, power or efficiency, the candidate mixes watts, kilowatts and hours, and reports an energy in kilowatts where kilowatt-hours were required.
Unit conversions are done by habit at the end rather than carried through the working, so the dimension of the intermediate quantities is never verified.
What to do instead
Convert all data to coherent SI units in a first line before any calculation, keep units attached to every intermediate value, and convert to the requested practical unit only in the final line, showing the conversion factor used.
On a schematic or drawing question the candidate produces a diagram with unlabelled terminals, missing reference designators, and crossing lines that cannot be distinguished from junctions, so the drawing cannot be read by a third party.
The candidate draws to satisfy themselves rather than to a convention, and has not been required to read back their own drawing as if it belonged to someone else.
What to do instead
Apply a checklist before handing in: every component carries its designator and value, every terminal is numbered, junctions are marked with a dot, crossings without connection are avoided or bridged, and the drawing is accompanied by a short legend.
When reporting a measurement, the candidate writes the raw display reading with no indication of range, instrument, uncertainty, or the conditions of the measurement, and draws a conclusion from a single reading.
The candidate sees the meter reading as the answer itself rather than as data that must be qualified, and has not been trained to record a measurement protocol.
What to do instead
Use a fixed reporting format: quantity measured, instrument and range, reading, estimated uncertainty from the instrument class and scale, conditions, then the result written as value plus or minus uncertainty with unit; repeat the reading at least twice and comment on the spread.
In the written technical expression section the candidate answers with a single unpunctuated block of sentences, mixes the informal register of spoken French with technical vocabulary, and never states a conclusion.
The candidate prepares the technical content only and treats the writing as a formality, so no time is allocated to planning the structure before writing.
What to do instead
Spend two minutes on a plan: one sentence stating the object of the note, a short development with one idea per paragraph and a connector between them, and an explicit concluding sentence; then reread once for punctuation and once for register.