Inside This Unit: The Full Breakdown
This unit analyzes electric circuits: current and resistance, Ohm’s law, series and parallel resistors, Kirchhoff’s rules, capacitors, and power.
Why it matters
Circuit analysis is a core, heavily tested skill. Knowing how current and voltage behave in series vs. parallel is essential for both MCQ and FRQ.
Key concepts
- Ohm’s law V = IR relates voltage, current, and resistance for ohmic devices.
- Series: resistances add and current is shared; parallel: voltage is shared and total resistance drops.
- Kirchhoff’s rules express charge (junction) and energy (loop) conservation.
- Power dissipated is P = IV = I²R = V²/R.
Resistor Networks
In series, the same current flows through each resistor and resistances add; in parallel, each branch shares the same voltage and total resistance is less than the smallest branch. Reducing a network to an equivalent resistance lets you find total current from the source.
Kirchhoff’s Rules, Power, and Capacitors
The junction rule (charge conservation) and loop rule (energy conservation) solve multi-loop circuits. Power dissipated follows P = I²R or V²/R. Capacitors store charge and energy (C = Q/V), and real batteries have internal resistance that lowers terminal voltage under load.
AP exam tip
Before computing, label whether each element is in series or parallel — getting the topology right is more important than memorizing formulas.
Connections to other units
- Unit 3: Potential difference from electrostatics drives circuit current.
- Unit 5: Changing currents and fields link circuits to induction.
