DC circuits

Meaning: A DC circuit has a constant emf source, such as a battery, driving charge around a closed loop in one direction. The junction rule (current in equals current out) reflects conservation of charge; the loop rule (potential changes around a loop sum to zero) reflects conservation of energy. Series resistors carry the same current and add: Req = R₁ + R₂. Parallel resistors share the same voltage, and 1/Req = 1/R₁ + 1/R₂, so adding a branch lowers total resistance. A fully charged capacitor carries no current in its branch.

Equation: V = IR, where V is potential difference (volts), I is current (amperes), and R is resistance (ohms, Ω).

On the MCAT: A passage gives a circuit diagram. Reduce it to one equivalent resistance, find the battery current from V = IR, then work back to the current, voltage, or power (P = I²R) in one resistor. A typical prediction: if one bulb is removed, bulbs in series go out, while bulbs in parallel are unaffected.

Common confusion: Students think current is used up as it passes through a resistor. Current is the same entering and leaving; what drops is electric potential (energy per charge). Also, the equivalent resistance of parallel resistors is smaller than the smallest branch, not larger.