Comprehensive Guide to AP Physics 2 Unit 3: Electric Circuits

Electric Current

The rate at which electric charge flows through a cross-section of a conductor.

Drift Velocity

The net motion of charge carriers, primarily due to an electric field.

Conventional Current

The flow of positive charge from high potential to low potential.

Electron Flow

The actual movement of electrons from low potential to high potential.

Ampere (A)

The unit of electric current, equivalent to one coulomb per second.

Resistance (R)

The opposition to the flow of electric charge.

Resistivity (ρ)

An intrinsic material property that quantifies how much a material resists electron flow.

Geometric Formula for Resistance

R = ρL/A; describes how resistance depends on material and geometry of the conductor.

Temperature and Resistivity

As temperature increases, resistivity generally increases due to increased atomic vibrations.

Ohm's Law

The relationship between voltage, current, and resistance; expressed as I = ΔV/R.

Ohmic Devices

Materials that follow Ohm's Law, with constant resistance regardless of voltage.

Non-Ohmic Devices

Materials whose resistance changes with voltage or current, like filament bulbs.

Electric Power (P)

The rate at which electrical energy is converted into other forms; calculated as P = IV.

Terminal Voltage (VT)

The actual voltage across the terminals of a battery when in operation.

Internal Resistance (r)

The resistance within a battery that causes a drop in terminal voltage.

Kirchhoff's Junction Rule

The sum of currents entering a junction equals the sum of currents leaving it.

Kirchhoff's Loop Rule

The sum of potential changes around a closed loop must be zero.

Series Circuits

Components arranged in a single path, where current is the same through all components.

Parallel Circuits

Components arranged in separate branches, sharing the same voltage across nodes.

Equivalent Resistance for Series

Req = R1 + R_2 + …; total resistance in a series circuit.

Equivalent Capacitance for Parallel

Ceq = C1 + C_2; total capacitance in a parallel circuit.

Time Constant (τ)

The time it takes for a capacitor to charge to about 63% of the maximum voltage.

Voltage across Capacitor during Charging

V_c(t) = ε(1 - e^{-t/RC}); describes the voltage behavior during the charging process.

Current in RC Circuit during Discharging

I(t) = I_0 e^{-t/RC}; describes how current decreases over time when discharging.

Voltage Gain in Loop

Crossing a battery from - to + results in a gain of voltage.

Voltage Drop across Resistor

Crossing a resistor in the direction of current results in a drop of voltage.

Physical Meaning of EMF (ε)

The maximum voltage a battery can provide when no current flows.

Potential Difference (ΔV)

The voltage difference across two points in a circuit.

Power Equation via Ohm's Law

Power can also be expressed as P = I^2 R or P = V^2 / R.

Current Splitting in Parallel Circuits

In parallel circuits, total current splits based on the resistances of each branch.

Brightness and Power Relationship

The brightness of a light bulb in a circuit is determined by the power it dissipates.

Capacitors in Series

The total capacitance is less than any individual capacitor's capacitance.

Charging Behavior of Capacitors

At t=0, an uncharged capacitor acts like a wire; at t=infinity, it acts like an open circuit.

Confusion between Series and Parallel Rules

Students often mistakenly apply resistance rules for capacitors instead of resistors and vice versa.

Using V=IR Correctly

Ohm's law must be applied to the specific component, not to the whole circuit.

Understanding Junction Rule

Current at a junction can split; it does not necessarily divide evenly among branches.

Steady State in RC Circuits

In DC steady state, the current through a fully charged capacitor is zero.

Voltage Drop across Battery

As current increases, the battery's terminal voltage decreases due to internal resistance.

Measurement Tools: Voltmeter

Device to measure potential difference, connected in parallel with infinite resistance.

Measurement Tools: Ammeter

Device used to measure current, connected in series with zero resistance.