ConceptElectromagnetism
← All conceptsDC circuits
current I=q/t, resistance R=ρ·L/A, Ohm's law U=IR, series (equal I, voltage divides; R=sum) and parallel (equal U, current divides; 1/R=sum, two-resistor shortcut), power P=UI=I2R=U2/R, Joule heating Q=I2Rt, EMF & internal resistance (terminal voltage, max power at R=r), non-ohmic devices (motor Pmech=UI-I2R)
21 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Adding parallel R raises totalThinks adding a resistor in parallel increases the overall resistance instead of decreasing it.→
- Current equal in parallel branchesAssumes equal current through unequal parallel branches instead of the current dividing in inverse proportion to resistance.→
- Current used up in circuitBelieves current is consumed by components so less returns to the battery than left it.→
- EMF equals terminal voltage alwaysTreats EMF and terminal voltage as identical even when current is flowing.→
- Internal resistance ignoredSets terminal voltage equal to EMF, ignoring the Ir drop across the source's internal resistance.→
- Metal resistance drops with heatBelieves a metallic conductor's resistance falls as its temperature rises.→
- Output power assumed monotonic in RBelieves the power delivered to the external resistor keeps rising as R increases, missing that it peaks when R equals the internal resistance r.→
- Parallel R exceeds smallest branchReports an equivalent parallel resistance larger than the smallest branch resistance.→
- Parallel resistance added directlyAdds parallel resistances directly instead of combining reciprocals (or misuses the two-resistor product-over-sum shortcut).→
- Power formula variant mismatchedPicks the wrong P formula among UI, I2R, U2/R for the known quantities, e.g. uses I2R when only U and R are given.→
- Power scaling taken as linearAssumes power changes in proportion to voltage (or current) rather than to V2 (or I2) when R is fixed, e.g. doubling V doubles instead of quadruples P.→
- R independent of lengthIgnores the length dependence in R=ρ·L/A when comparing wires.→
- R proportional to areaTreats resistance as increasing with cross-sectional area instead of R proportional to L/A.→
- R read as U/I proportionalityTreats R=U/I as meaning resistance changes when the applied voltage changes, rather than R being a fixed property of the ohmic component.→
- Rated power used at wrong voltageUses a device's rated power directly at a different operating voltage, instead of treating R as fixed and recomputing P=U2/R.→
- Resistivity equals resistanceUses resistivity ρ and resistance R interchangeably, ignoring the L/A geometry factors.→
- Running motor treated as ohmicApplies U=IR or P=U2/R to a running motor, ignoring that it does mechanical work so Pmech=UI-I2R and it is non-ohmic.→
- Stretched-wire area change ignoredWhen a wire is stretched to double its length, doubles R but ignores that the area halves, missing the net factor of 4.→
- U=IR rearranged wrongRearranges Ohm's law incorrectly, e.g. writes I=UR or R=UI.→
- Voltage divider ratio invertedInverts the series voltage-divider ratio, assigning the larger share of voltage to the smaller resistor.→
- Voltage equal across series resistorsAssumes the same voltage across each series resistor regardless of their resistances.→
