Circuit analysis and magnetism concepts and equations relevant to the MCAT physical sciences section. Front: the term or equation. Back: definition or equation with a brief usage note.
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- Ohm's Law (equation)
- V = IR; voltage across a resistor equals current times resistance.
- Resistance of a wire (resistivity equation)
- R = (rho x L) / A; resistance increases with length and resistivity, decreases with cross-sectional area.
- Electric power in a circuit (basic formula)
- P = IV; power equals current times voltage.
- Electric power in terms of current and resistance
- P = I^2 R; useful when voltage is unknown but current and resistance are known.
- Electric power in terms of voltage and resistance
- P = V^2 / R; useful when current is unknown but voltage and resistance are known.
- Total resistance of resistors in series
- R_total = R1 + R2 + R3 + ...; series resistances simply add.
- Total resistance of resistors in parallel
- 1/R_total = 1/R1 + 1/R2 + 1/R3 + ...; parallel total resistance is always less than the smallest individual resistor.
- Capacitance (definition)
- C = Q/V; capacitance is the ratio of stored charge to voltage across a capacitor.
- Total capacitance of capacitors in parallel
- C_total = C1 + C2 + C3 + ...; parallel capacitances simply add.
- Total capacitance of capacitors in series
- 1/C_total = 1/C1 + 1/C2 + 1/C3 + ...; series total capacitance is always less than the smallest individual capacitor.
- Energy stored in a charged capacitor
- U = 1/2 C V^2; energy stored depends on capacitance and the square of the voltage.
- Kirchhoff's Current Law (junction rule)
- The sum of currents entering a junction equals the sum of currents leaving it; charge is conserved at every node.
- Kirchhoff's Voltage Law (loop rule)
- The sum of voltage changes around any closed loop in a circuit equals zero; energy is conserved around a loop.
- RC circuit time constant
- tau = RC; the time for a charging or discharging capacitor's voltage to change by about 63 percent toward its final value.
- Terminal voltage of a battery with internal resistance
- V_terminal = EMF - I r; terminal voltage is less than EMF because of the voltage drop across internal resistance r.