Three resistances of 5 ohms, 10 ohms, and 25 ohms are connected in series across a 120-volt source. What is the voltage measured across the 25-ohm resistance?
- 75 volts
- B120 volts
Putting the whole supply across one element is what happens in a parallel arrangement. Series elements share the source voltage between them, so no single element can ever see the entire supply.
- C40 volts
This splits the source into three equal parts. Voltage divides in proportion to resistance, not by a head count, so equal shares occur only when all three resistances happen to be equal.
- D30 volts
This is the drop across the 10-ohm element rather than the 25-ohm one. Reading the proportion off the wrong resistance is an easy slip when three values are in play and the clock is running.
Why A is correct
Series means a single path, so every element carries the identical 3 amperes and each takes voltage in proportion to its own resistance. The 25-ohm element holds 25 of the 40 total ohms, so it takes 25 fortieths of 120 volts, which is 75 volts. The largest resistance takes the largest share of voltage and therefore produces the most heat.
What this question is testing
Two habits are being checked at once: totaling series resistances before doing anything else, and remembering that current is the quantity that stays constant. The distractors reward candidates who split the voltage evenly among the elements or who quietly assume a parallel arrangement, both common errors under time pressure.
On the job
Series wiring turns up in heating elements, control circuits, and, unintentionally, in every loose connection you will ever chase. A loose splice puts resistance in series with the load, claims its proportional share of the voltage, and turns that share into heat right at the joint. That is exactly why a bad termination glows while the equipment downstream runs low, and why voltage readings taken across a suspect connection find the fault so quickly.
Memory technique
Series shares current, parallel shares voltage.
Exam tip
Work series problems in order: total resistance first, then circuit current, then back to any individual drop. Jumping straight to the drop invites an even-split error.
Where to look it up
Closed-book theory with no article involved.