AP Physics C: Electricity and Magnetism Flashcards: Circuits with Resistors and Inductors (LR Circuits)
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
In a DC circuit that has reached a steady state, what is the current through the inductor if the battery has emf E and the total resistance is R?
In a steady state, the inductor acts as a wire with zero resistance, so the total resistance is just R and the current is I = E/R.
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In a DC circuit that has reached a steady state, what is the current through the inductor if the battery has emf E and the total resistance is R?
In a steady state, the inductor acts as a wire with zero resistance, so the total resistance is just R and the current is I = E/R.
What is the time constant (τ) of an LR circuit?
The time constant (τ) is a measure of how quickly an LR circuit will reach a steady state and is described by the equation τ = L/R.
What two electrical properties cause voltage drops in a non-steady-state series LR circuit with a battery?
The voltage drops are caused by the current through the resistor (IR) and the changing current through the inductor (L dI/dt).
What fundamental physics rule is applied to derive the governing equation for a series LR circuit?
Kirchhoff’s loop rule is applied to a series LR circuit to derive the differential equation that describes the current in the loop.
What differential equation describes the current in a series LR circuit with a battery, according to Kirchhoff's loop rule?
The differential equation that describes the current (I) in the loop is E = IR + L(dI/dt), where E is the emf of the battery.
What is the formula for the time constant (τ) of an LR circuit?
The time constant is described with the equation τ = L/R.
What components are found in an LR circuit?
An LR circuit contains a combination of resistors and a single inductor.
How does an inductor behave in a DC circuit a long time after the circuit is closed?
After a time much greater than the time constant of the circuit, an inductor behaves as a conducting wire with zero resistance.
If the inductance (L) in an LR circuit is increased while the resistance (R) is kept constant, how is the time to reach a steady state affected?
Since τ = L/R, increasing the inductance (L) will increase the time constant, meaning the circuit will take longer to reach a steady state.
What does a smaller time constant (τ) imply about an LR circuit?
A smaller time constant implies that the LR circuit will reach its steady state more quickly.