Circuits: every key term you need (+ practice quiz)
26 flashcard terms for AP Physics 2 Unit 4, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 25-question quiz — free, no account needed.
V = IR where V = voltage (volts), I = current (amperes), R = resistance (ohms). Describes electrical relationship.
Resistance
R = ρL/A where ρ = resistivity, L = length, A = cross-section. Longer wire = higher R. Thicker wire = lower R.
Power
P = IV = I²R = V²/R. Power dissipated as heat. Measured in watts (W).
Series Circuits
Components in single loop. I same everywhere. V_total = V₁ + V₂ + ... R_total = R₁ + R₂ + ...
Parallel Circuits
Components in branches. V same everywhere. I_total = I₁ + I₂ + ... 1/R_total = 1/R₁ + 1/R₂ + ...
Kirchhoff's Laws
Junction rule: current in = current out. Loop rule: sum of voltages around loop = 0. Essential for circuit analysis.
EMF vs Terminal Voltage
EMF (ε): theoretical voltage source. Terminal V: reduced by internal resistance. V_terminal = ε - Ir.
Capacitance
C = Q/V where Q = charge, V = voltage. Capacitor stores charge. Unit: farad (F).
Capacitor Energy
U = ½QV = ½CV² = ½Q²/C. Energy stored in electric field.
Dielectric
Material between capacitor plates; increases capacitance. C = κC₀ where κ = dielectric constant.
Unit 4 Summary
Circuits: Ohm's law (V=IR). Series: R adds, I same. Parallel: V same, 1/R adds. Capacitors store charge and energy.
Resistivity Geometry
R = ρL/A. Stretching a wire to twice its length halves its area (volume conserved), so R rises by a factor of 4. Doubling the diameter quarters R.
Internal Resistance and Load
Terminal voltage V = ε − Ir drops as current rises. Maximum power to a load occurs when R_load = r; short-circuit current is ε/r.
Power in Series vs Parallel
Series: same I, so P = I²R — larger R glows brighter. Parallel: same V, so P = V²/R — smaller R glows brighter. Always ask which quantity is shared.
Ammeter and Voltmeter Placement
Ideal ammeter (zero resistance) goes in series; ideal voltmeter (infinite resistance) goes in parallel. A voltmeter placed in series stops the current; an ammeter in parallel shorts the element.
Loop Rule as Energy Conservation
Sum of ΔV around any closed loop is zero because potential is single-valued. Crossing a resistor in the direction of current is −IR; crossing a battery from − to + is +ε.
Junction Rule as Charge Conservation
Current into a node equals current out; charge does not accumulate in steady state. Assign directions arbitrarily; a negative solution means the guess was backward.
Adding a Parallel Branch
Adding a resistor in parallel lowers the total resistance, so total current from the battery rises and (with internal resistance) terminal voltage drops slightly, dimming bulbs elsewhere in series.
At t = 0 an uncharged capacitor acts as a wire (V_C = 0, max current); as t → ∞ it acts as an open switch (I = 0, V_C = its branch voltage). Time constant τ = RC sets the scale.
Capacitor Discharge
Q(t) = Q₀e^(−t/RC); after one τ about 37% remains, after 5τ less than 1%. Current follows the same decay; energy ½Q²/C is dissipated as heat in R.
Capacitors in Series and Parallel
Series: same Q on each, 1/C_eq = Σ1/C_i, voltages divide inversely with C. Parallel: same V, C_eq = ΣC_i, charge divides in proportion to C. Opposite to resistors.
Dielectric Inserted: Battery Connected vs Disconnected
Battery attached (V fixed): C rises κ-fold, Q rises, U = ½CV² rises. Battery removed (Q fixed): C rises, V drops by κ, U = Q²/2C drops — the slab is pulled in and energy goes into that work.
Energy Density in a Capacitor
Stored energy ½CV² can be written as ½ε₀E²(volume): energy resides in the field between the plates. Doubling plate separation at fixed Q doubles the stored energy (you did work pulling plates apart).
Wheatstone / Balanced Bridge
If R₁/R₂ = R₃/R₄, the middle branch carries no current because both ends sit at the same potential; that resistor can be removed without changing anything.
Non-Ohmic Elements
A filament bulb's I–V curve bends over because resistance rises with temperature; diodes conduct one way. Ohm's law is a property of materials, not a universal law.
Ground and Reference Potential
Choosing a node as 0 V changes no currents; only differences matter. Two grounded points in a circuit are at the same potential and may be treated as connected.