Circuits, current, voltage, and resistance, plus how moving charge generates a magnetic field — Ohm's law and the difference between series and parallel circuits carry most of the weight here. Every question comes with a written walkthrough of the circuit or field relationship involved.
Parallel resistances don't simply add — the reciprocal of the total equals the sum of the reciprocals, which means total resistance in a parallel circuit is always less than the smallest individual resistor.
Current splits between parallel branches based on each branch's resistance — it's voltage, not current, that stays the same across parallel branches.
In series, current stays the same and voltage divides; in parallel, voltage stays the same and current divides — the two circuits swap which quantity is shared.
Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.
A circuit has a resistance of 5 ohms and a current of 3 amps flowing through it. What is the voltage across the circuit?
A — 15 V. Ohm's Law: V = IR = (3)(5) = 15 V. 1.67 V (B) comes from dividing resistance by current instead of multiplying — computing 5/3 ≈ 1.67 instead of 3×5.
Two resistors, 4 ohms and 6 ohms, are connected in series to a 20V battery. What is the total current flowing through the circuit?
A — 2 A. In series, resistances add directly: 4+6=10 ohms total. Applying Ohm's Law: I = V/R = 20/10 = 2 A. 5 A (B) comes from dividing the voltage by only one resistor (20/4=5) instead of the total series resistance.
Two resistors, 6 ohms and 3 ohms, are connected in parallel across a 12V battery. What is the total current supplied by the battery?
A — 6 A. Combine the parallel resistances first: 1/R_total = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so R_total = 2 ohms. Applying Ohm's Law to the whole circuit: I = V/R = 12/2 = 6 A. 1.33 A (C) comes from adding the resistances as if they were in series (6+3=9 ohms) and computing 12/9 ≈ 1.33 — the wrong combination rule for a parallel circuit.
Circuit problems reward correctly identifying series versus parallel before doing any arithmetic — the combination rule you use depends entirely on that one classification.
Before calculating anything, decide whether resistors are in series or parallel, since that determines which combination rule applies. Untimed practice is where that classification habit sticks.
Move to timed sessions once series and parallel combination rules are automatic.
Pair electricity and magnetism with work, energy, and power in a mock — electric power calculations use the same energy-rate concept.
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