Aptitude skill lesson
Electrical reasoning: skill lesson
Electrical reasoning is the ability to move confidently between volts, amps, ohms and watts on a circuit you can only see as a diagram, and to say what a fault would look like on a meter before you pick one up. It is the core construct in electrician and powerline apprenticeship entry tests, electronics and telecommunications technician screening, and military electrical batteries. On the job it is the difference between localising a break in three measurements and replacing three good parts. Everything you practise here stays on this device; there is no account and no upload.
Published · last reviewed
Applies to Novus Learn 0.1.0
What changed, and when
- , Replaced the body of all 27 non-judgement lessons with construct-specific material: six worked examples each, carrying the actual arithmetic, the actual inference or the actual procedure, plus expanded objectives, practice tips and glossary. Every 'Related Learn topics' link now points at a real page on this site rather than a generic search. The eight workplace-judgement lessons are unchanged.
- , Rebuilt the lesson page around a sticky contents rail, per-section links, previous/next lesson navigation, and two graded checkpoints drawn from the open practice bank.
- , Repaired the aptitude integrations behind the lessons so each one links to skill-specific practice instead of the unscoped fixture engine.
- , Published one lesson for each of the 35 aptitude skill constructs: objectives, worked examples, practice tips, glossary, Learn topic links, and sources.
These 35 skill lessons are authored and revised as one set, so they share one revision history rather than 35 identical dates.
Objectives
Copy link- Rearrange Ohm's law and the three power forms (P = VI, P = I x I x R, P = V x V / R) to solve for any unknown in a single-source circuit.
- Compute total resistance for series and parallel combinations and predict what happens to supply current when a branch is added or removed.
- Predict the meter readings a specific fault produces. Full supply across an open circuit, zero volts across a good component in a dead series string.
- Identify the standard schematic symbols for cell, battery, fixed and variable resistor, lamp, fuse, switch, ammeter and voltmeter, and place each meter correctly.
- Explain from the I x I x R term why doubling current quadruples conductor heating, and use that to reason about cable sizing and voltage drop.
- Apply a transformer turns ratio in both directions (voltage down, current up) and say why a transformer does nothing on steady DC.
Checkpoint: does the idea land?
Copy linkBefore the worked examples, check that the objectives above actually landed.
Two questions from the open practice bank, answered here and scored on this device. Untimed, ungraded, and not added to your practice history, the full bank is where attempts are recorded.
Examples
Copy linkOhm's law on real components
A 12 V battery feeds a 4 ohm heater element. Current is 12/4 = 3 A, and power is 12 x 3 = 36 W. Put the same element on 24 V and current becomes 6 A while power becomes 24 x 6 = 144 W: doubling the voltage quadruples the power, because both factors in P = VI doubled. Candidates who answer 72 W have doubled one factor and forgotten the other. The nastier version of this item uses lamps. On the same 230 V supply, which has the higher resistance, a 60 W lamp or a 100 W one? Since P = V x V / R, resistance is V x V / P: the 60 W lamp is 52,900/60 = 882 ohms and the 100 W lamp is 52,900/100 = 529 ohms. The more powerful lamp has the LOWER resistance and draws more current, which feels backwards to anyone who reads 'more powerful' as 'more of everything'. Fix the direction of the relationship once: at a fixed voltage, more power means less resistance.
Series and parallel: two rules, two consequences
Three 6 ohm resistors across an 18 V supply. In series the resistances add to 18 ohms, the current is 18/18 = 1 A and it is the same at every point in the loop, and each resistor drops 1 A x 6 ohms = 6 V. The three drops sum to the supply, 6 + 6 + 6 = 18 V, which is the check you should run every time. In parallel each resistor sees the full 18 V, so each branch carries 18/6 = 3 A, the supply delivers 9 A, and the equivalent resistance is 18/9 = 2 ohms. The same 6/3 the reciprocal rule gives. The consequences matter more than the arithmetic. Adding a parallel branch always lowers total resistance and raises total current, which is exactly why plugging a fourth appliance into one socket circuit trips the breaker. Two checks catch nearly every slip: parallel resistance is always less than the smallest branch, and the branch with the smallest resistance always carries the biggest current.
Fault-finding by what stays lit
Six identical lamps in series on 12 V drop 2 V each. Break one filament and the current has nowhere to go: all six go out, which is the old fairy-light string that fails completely for one dead bulb. Finding the break is the counter-intuitive part. Across every good lamp your meter now reads 0 V, because no current is flowing to develop a drop, while across the broken one it reads the full 12 V. Candidates who expect the faulty component to read zero walk straight past it. Wire the same six in parallel instead and each lamp sees the full 12 V on its own branch, so a broken filament kills that lamp alone and the supply current merely drops by one sixth. That contrast is what makes a half-dead string diagnostic. If lamps 1 to 3 are lit and 4 to 6 are dead, they cannot be in series at all, because one break in a series loop kills every lamp; they are parallel branches hanging off a shared feed, and the fault is a break in that feed between position 3 and position 4. Probe the feed there rather than testing lamps: the last lit branch and the first dead one bracket the break.
Voltage dividers, and where each meter belongs
A 2 kilohm and a 3 kilohm resistor in series across 10 V. Total 5 kilohms, current 10/5,000 = 2 mA, which is the same in both. The 3 kilohm resistor drops 0.002 x 3,000 = 6 V and the 2 kilohm drops 4 V, splitting the supply in the ratio of the resistances. The bigger resistor always takes the bigger share. Now the meters. An ammeter goes IN the current path, in series with the component, and is designed with a near-zero resistance so it barely disturbs the circuit. A voltmeter goes ACROSS the component, in parallel, and has a very high resistance for the same reason. Swap them and the consequences are asymmetric: a voltmeter placed in series reads almost the whole supply and the circuit stops working, while an ammeter placed straight across a battery is a near-zero resistance across a source: a deliberate short circuit, a blown fuse at best. That asymmetry is the point of the item.
Why cable size is an I x I x R question
A 2.5 kW heater on 230 V draws 2,500/230 = 10.9 A. Suppose the supply cable has 0.5 ohms of total loop resistance. The voltage lost in the cable is 10.9 x 0.5 = 5.4 V, and the heat dissipated in the cable itself is I x I x R = 10.9 x 10.9 x 0.5 = about 59 W: quietly warming a coiled extension lead. Fit a thicker conductor so the loop resistance halves to 0.25 ohms and both the drop and the heat halve. Double the current instead, to 21.7 A, and the drop doubles but the heat goes up FOURFOLD, to about 236 W, because the current appears twice. Candidates who treat cable loss as proportional to current under-estimate it badly. This squared term is the reason a partly uncoiled drum overheats, the reason long runs need a larger conductor for the same load, and the reason transmission networks push power at high voltage and low current.
Transformers: turns ratio, and why they ignore DC
An ideal transformer has 400 primary turns and 40 secondary turns, a 10:1 ratio, and sits on 230 V AC. The secondary voltage is 230 x 40/400 = 23 V. Power in equals power out in the ideal case, so if the secondary supplies 5 A into a load, the secondary is delivering 23 x 5 = 115 W and the primary must draw 115/230 = 0.5 A. Voltage steps down by the turns ratio, current steps up by the same ratio, and no power is created. Two traps. The first is multiplying rather than dividing, giving 2,300 V from a step-down winding. Always ask which winding has more turns before you touch the arithmetic. The second is the DC version of the question: connect that primary to a 24 V battery and the secondary produces nothing once the initial switch-on transient passes, because a steady current produces a steady flux and only a CHANGING flux induces a secondary voltage. Meanwhile the primary, with only its winding resistance to limit it, draws enough current to burn out. That is also why the mains figure quoted as 230 V is an RMS value, and why the waveform actually peaks near 230 x 1.414 = 325 V.
Checkpoint: can you apply it?
Copy linkNow apply it. These come from a later section of the bank, so they are not more of the same.
Two questions from the open practice bank, answered here and scored on this device. Untimed, ungraded, and not added to your practice history, the full bank is where attempts are recorded.
Practice tips
Copy link- Write the three Ohm's law rearrangements and the three power forms across the top of your rough paper before question one. Most electrical items are a single substitution away once the right form is already in front of you.
- Normalise units first: put everything into volts, amps and ohms, or knowingly pair milliamps with kilohms (which conveniently gives volts). Mixing milliamps with plain ohms produces an answer that is wrong by a factor of a thousand and looks fine.
- Test every parallel result against the smallest branch. If your combined resistance is bigger than the smallest resistor in the group, you have added instead of combined.
- Practise fault items by predicting the meter reading rather than naming the culprit: 'zero volts across each good lamp, full supply across the break' is what you will actually see, and it is what the answer options are built from.
- Drill the schematic symbols in short bursts until they are instant: cell versus battery, fixed versus variable resistor, fuse, normally-open versus normally-closed contacts, lamp, ammeter, voltmeter. These items carry full marks and cost seconds.
- Keep safe-working questions in a separate mental bucket from circuit questions. Isolate, lock off, prove dead, then re-prove the tester is the expected answer even when a cleverer electrical shortcut exists, and attempts you log stay on this device either way.
Glossary
Copy link- Ohm's law
- V = I x R. Voltage across a resistive component equals the current through it multiplied by its resistance; rearranged as I = V/R and R = V/I.
- Potential difference
- The energy transferred per unit charge between two points, measured in volts and always measured ACROSS a component rather than through it.
- Current
- The rate of charge flow, in amperes. It is identical at every point of a series path and divides between parallel branches in inverse proportion to their resistances.
- Resistance
- Opposition to current, in ohms. Series resistances add directly; a parallel combination is always smaller than its smallest member.
- Power
- The rate of energy conversion in watts, obtainable three ways: P = V x I, P = I x I x R when you know the current, and P = V x V / R when you know the voltage.
- Short circuit
- An unintended low-resistance path around a load. Current rises until something limits it, which is precisely the job a fuse or circuit breaker exists to do.
- Open circuit
- A break in the conducting path. Current is zero and, in a live circuit, the full supply voltage appears across the break, the reading that locates it.
- Turns ratio
- Primary turns divided by secondary turns in a transformer. Voltage divides by this ratio while current multiplies by it, so the power either side is nominally equal.
- RMS value
- The steady DC value that would deliver the same heating as a given AC waveform. For a sine wave the peak is about 1.414 times the RMS value.
Sources
Copy link- Circuit examples computed for Novus Learn from first principles using Ohm's law, the series and parallel resistance rules, and the ideal transformer relation.
- Terminology checked against the public Wikipedia articles 'Ohm's law', 'Series and parallel circuits', 'Electric power' and 'Transformer'. Definitions only; every value above is original.
- Novus Learn aptitude construct registry (catalog seed) for the construct scope and related suite mapping.
- Public educational framing only: not affiliated with any official exam board, publisher or employer. Safe-working guidance here is illustrative and never replaces a competent person or your local wiring rules.
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