Study guides › C/P › Physics
Circuits & Capacitors
Circuits move charge and energy through resistors and capacitors. You will use Ohm's law, combine resistors and capacitors in series and parallel, apply Kirchhoff's rules, calculate power, and read RC charging curves.
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What this chapter covers
- Current, Resistance & Ohm's Law. Every circuit problem starts with V = IR, and the MCAT loves to ask how resistance changes when a wire is stretched or thickened.
- Series, Parallel & Kirchhoff's Rules. Reducing a network to one equivalent resistance, then working back out, solves most MCAT circuit problems.
- Power, Energy & Meters. Power questions hinge on picking the right form of P, and meter questions hinge on how ideal meters are wired.
- Capacitors & Dielectrics. Capacitors store energy in an electric field, and the MCAT asks what changes when a dielectric is inserted with the battery on or off.
- RC Circuits & Time Constants. RC questions come as graphs. Know what the capacitor does at t = 0 and at long times, and what 63% and 37% mean.
Key points to know
- I = ΔQ/Δt; conventional current runs + to −
- V = IR for ohmic conductors; 1 Ω = 1 V/A
- Series: same current, voltages add, R_eq = ΣR
- Parallel: same voltage, currents add, 1/R_eq = Σ1/R
- P = IV = I²R = V²/R; the watt is J/s
- Series: bigger R gets more power; parallel: smaller R gets more power
- C = Q/V; parallel plate C = κε₀A/d
- U = ½CV² = ½QV = Q²/2C
Try a practice question
In a copper wire connected to a battery, which statement is correct?
- Electrons drift toward the + terminal, opposite to conventional current
- Electrons drift in the same direction as conventional current
- Protons move through the wire toward the − terminal
- Charge is used up as it passes through the wire
Show the answer
A. Electrons are negative, so they drift toward the positive terminal, opposite to the defined direction of current. Charge is conserved and the same current flows out as flows in.
Common questions
What does this chapter cover?
Circuits & Capacitors covers Current, Resistance & Ohm's Law, Series, Parallel & Kirchhoff's Rules, Power, Energy & Meters, Capacitors & Dielectrics, RC Circuits & Time Constants.
What should I know first?
I = ΔQ/Δt; conventional current runs + to − V = IR for ohmic conductors; 1 Ω = 1 V/A Series: same current, voltages add, R_eq = ΣR
25 original practice questions accompany this chapter in the app. Spaced review brings missed ideas back at the right time; it helps memory but cannot promise you will not miss an idea again.