Ohm's law links V, I and R
Ohm's law links the three basic quantities: V = I x R (voltage = current x resistance), V in volts, I in amps, R in ohms. Any two give the third: I = V / R, R = V / I, V = I x R. The Ohm's law triangle (V on top, I and R below) shows whether to multiply or divide -- cover the quantity you want. Example: 6 V across 300 ohm gives I = 6 / 300 = 0.02 A (20 mA). Its main design use is sizing a resistor to limit current for a component such as an LED.
Electrical power: P = V x I
Power is the electrical energy converted each second, in watts (W) (1 W = 1 J/s). Basic form: P = V x I. Example: a lamp at 12 V drawing 0.5 A gives P = 12 x 0.5 = 6 W. With Ohm's law it is also P = I squared x R and P = V squared / R; e.g. 0.5 A through 12 ohm gives 0.5 x 0.5 x 12 = 3 W. Power sets the power rating a resistor needs -- it must shed at least this heat or it overheats.
Ammeter in series, voltmeter in parallel
Three instruments test a circuit. An ammeter measures current and goes in series; its near-zero resistance barely disturbs the circuit. A voltmeter measures voltage and goes in parallel, across the component; its high resistance diverts almost no current. A multimeter is one switchable instrument doing any of these -- the everyday fault-finding tool. Wire a meter wrongly and the reading is wrong or the meter is damaged.
Drawn from real examiner reports.
LED resistor: subtract the LED voltage first
The LED series resistor comes from Ohm's law on the resistor alone: R = (Vsupply - Vled) / I. Only the voltage LEFT after the LED's drop (about 2 V) sits across the resistor. On 6 V, LED dropping 2 V, at 20 mA (0.02 A): 6 - 2 = 4 V, then R = 4 / 0.02 = 200 ohm. Skipping the subtraction gives 6 / 0.02 = 300 ohm. Subtract first, then divide, and give the unit.
Flagged s23 P42 Q12(a)(vi): some candidates lost a mark on the current-limiting resistor by not subtracting the given LED voltage drop from the supply before dividing by the current.
Prefix slips: mega, milli and micro
Every answer must carry its unit or sub-unit (ohm, kilohm, volt, milliamp, watt), and prefix slips wreck a calculation. Mega is x1000000, milli is /1000, micro is /1000000, and the micro symbol is often unrecognised. For example 20 mA is 0.02 A, not 20 A. Convert every value to its base unit before substituting.
Flagged w23 P42 Q7(b): candidates muddled mega with milli and did not recognise the micro symbol.
Meter connected the wrong way round
An ammeter goes in series -- it measures the current through, its low resistance barely disturbing the circuit. A voltmeter goes in parallel -- it measures the voltage across a component, its high resistance diverting almost no current. Swapping them is a fault: an ammeter in parallel nearly short-circuits it; a voltmeter in series nearly stops the current.
Voltmeter wired in series like an ammeter
A testable wrong belief: that both meters connect the same way, in series. Only the ammeter goes in series -- it measures the current THROUGH the path, and its near-zero resistance leaves the circuit undisturbed. A voltmeter measures the voltage ACROSS a component, so it goes in parallel; its high resistance barely diverts current.
Finding current by multiplying V by R
A testable error: rearranging Ohm's law backwards and multiplying voltage by resistance to get current. Correct is V = I x R, so to find current you divide: I = V / R. Bigger resistance means LESS current for the same voltage, not more. A 6 V supply across 300 ohm gives 6 / 300 = 0.02 A (20 mA). Only voltage is found by multiplying.
Dropping the square in P = I squared R
The derived power forms contain a square that is easy to drop. In P = I squared x R square the current first: 0.5 A through 12 ohm gives 0.5 x 0.5 x 12 = 3 W, not 6 W. In P = V squared / R square the voltage: 9 V across 45 ohm gives 81 / 45 = 1.8 W, not 0.2 W. Square first, then multiply or divide, and check the unit is watts.
"Wear PPE" is too vague for soldering
Health-and-safety marks for circuit building need process-specific precautions, not a blanket "wear PPE". Credited points: ventilation or fume extraction (fumes are harmful); return the hot iron (tip ~350 C) to its stand, not the bench; clamp the board; handle corrosive etchant with care; let joints cool. Name the hazard and the exact control.
Breadboard vs PCB for the final product
A breadboard is only for temporary prototyping: its push-in connections work loose, so it is unreliable. A soldered PCB has permanent joints and etched tracks that make every board identical -- compact, robust and repeatable, right for a finished or batch product. Do not choose a breadboard for the sold product, or treat a PCB as only a prototype.
Lay the calculation out in stages
Calculations are marked for METHOD, not just the number. Write the formula, substitute the numbers (convert prefixes first: 20 mA -> 0.02 A), then the answer with its unit. Staged working banks method marks even if a later slip spoils the answer.
Name process-specific soldering safety
For a safety question barring PPE, name precautions tied to the soldering process, each with its hazard: ventilation or fume extraction (fumes harmful); hot iron in its stand (tip ~350 C); board clamped; etchant handled with care. Two such points beat a vague "be careful".
Match the power formula to the data
Three power forms avoid finding a missing quantity first. Voltage and current? P = V x I. Current and resistance? P = I squared x R. Voltage and resistance? P = V squared / R. Pick the form fitting the data; all come from P = V x I plus Ohm's law.
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