Circuits · AP · 5 min
Bulbs and resistors transfer energy; they do not consume charge.
01 The trap
Wrong path
Students think current is smaller after passing through a resistor in a series circuit.
Why it feels right
A bulb gets dimmer when resistance changes, so it feels like current is being spent.
02 Correct model
Current is charge per time. In a single series loop, the same charge flow passes through every element; voltage drops track energy transfer.
03 Mini-example
Prompt
Two bulbs are in series. Is the current after the first bulb smaller?
Common wrong answer
Yes, the bulb used some current.
Correct reasoning
No. Series current is the same everywhere in the loop.
Diagnostic cue
If a circuit question says 'after the resistor,' check whether you are confusing current with energy.
04 Guided practice
Warm-up isolates the principle. Bridge changes the context. Contest-style requires a complete setup on less familiar geometry.
Two resistors are in series with a battery. Is the current after the first resistor smaller than before it?
Current is charge per time.
No. In a single series loop, the same current passes through every element. The voltage drop changes, not the current before and after a resistor.
Trap: Treating current as fuel that gets spent.
Two resistors are connected in parallel across a battery. What is the same across each branch?
Both branches connect to the same two nodes.
The potential difference is the same across each branch. The currents can differ depending on resistance.
Trap: Applying the series-current rule to parallel branches.
A second identical bulb is added in series with the first. What happens to the current through the battery, qualitatively?
Equivalent resistance increases.
The battery current decreases because total resistance increases. The original bulb dims because the series current through it is smaller, not because current is used up before reaching it.
Trap: Explaining dimming as current consumption inside the first bulb.
Two unequal resistors are connected in parallel to an ideal battery. Which quantity is equal for the two resistors: current, voltage, resistance, or power?
Parallel elements touch the same two nodes.
The voltage is equal across the two resistors. The currents and powers generally differ because I = V/R and P = V²/R.
Trap: Transferring the series-current rule into a parallel circuit.
A bulb is connected in parallel with an ideal wire. What happens to the bulb in the ideal circuit model?
Elements in parallel share the same voltage.
The bulb goes out in the ideal model. The wire shorts the bulb, making the potential difference across the bulb zero, so no power is dissipated by the bulb.
Trap: Saying current chooses both paths equally because they are physically parallel.
A resistor is connected to an ideal battery. An identical resistor is then added in parallel. What happens to the current supplied by the battery?
Two equal resistors in parallel have half the resistance of one.
The equivalent resistance halves, so the ideal battery supplies twice the original total current. Each branch carries the original single-resistor current.
Trap: Keeping source current fixed and splitting it between the branches.
05 Mock checks
A cross-unit check for the AP Physics 2 habit that matters most: choosing the right model before calculating.
9 questions · 27 pts
A short conceptual check for current, voltage, and series/parallel reasoning.
4 questions · 10 pts