Electric Circuits practice quiz

Key ideaI = dQ/dt, and R = ρL/A. Ohm’s law, V = IR, applies to ohmic materials.

Common mistakeThinking a longer wire has less resistance, or that current is used up as it flows around a circuit.

Key ideaIn series the current is the same and the voltages add; in parallel the voltage is the same and the currents add. Kirchhoff’s loop and junction rules handle everything else.

Common mistakeCombining resistors that are neither in series nor in parallel, or getting signs wrong going around a loop.

Key ideaAn uncharged capacitor acts like a wire at first. A fully charged capacitor carries no current, like a break in the circuit.

Common mistakeLetting steady current flow through a charged capacitor branch.

Key ideaThe time constant is $\tau = RC$. A charging capacitor follows $Q = Q_{\max}(1 - e^{-t/\tau})$, and the current decays as $e^{-t/\tau}$.

Common mistakeUsing the wrong R in the time constant. It is the equivalent resistance seen from the capacitor’s terminals (with the battery replaced by a wire).