Cheatsheet: Electrical Engineering Basics: Solve DC Circuits for Exams

Scope: Modules 1 to 3 (Circuit Theory, Electronics Core, Power Systems Essentials). Use this as a lookup card during problem sets and exam review.

Module 1: Circuit Theory Essentials

1.1 DC Circuit Variables and Ohm's Law

Quantity	Symbol	Unit	Note
Voltage	$V$	volt (V)	Energy per unit charge
Current	$I$	ampere (A)	Charge flow rate
Resistance	$R$	ohm ($\Omega$)	Opposition to current
Power	$P$	watt (W)	Energy per second

Core equations

$$ V = IR \qquad P = VI = I^2 R = \frac{V^2}{R} $$

Prefix conversions (watch these on exams)

Prefix	Multiplier	Example
k	$10^3$	4.7 k$\Omega$ = 4700 $\Omega$
m	$10^{-3}$	20 mA = 0.020 A
µ	$10^{-6}$	100 µA = $1\times10^{-4}$ A
n	$10^{-9}$	10 nF = $1\times10^{-8}$ F

[!TIP] Convert every quantity to base SI units (V, A, $\Omega$, W) before plugging into a formula. Unit slips are the most common lost mark.

Power sign convention: passive sign convention means current enters the positive terminal, so $P = VI$ is power absorbed. A negative result means the element is delivering power.

1.2 Kirchhoff's Laws and Series/Parallel Networks

Law	Statement	Form
KCL	Sum of currents into a node = sum out	$\sum I_{in} = \sum I_{out}$
KVL	Sum of voltages around a closed loop = 0	$\sum V_{loop} = 0$

Series vs parallel rules

Configuration	Resistance	Current	Voltage
Series	$R_{eq} = R_1 + R_2 + \dots$	Same through all	Divides
Parallel	$\dfrac{1}{R_{eq}} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dots$	Divides	Same across all

Two resistors in parallel shortcut: $R_{eq} = \dfrac{R_1 R_2}{R_1 + R_2}$.

Voltage divider (series): $V_x = V_{total}\dfrac{R_x}{R_{total}}$

Current divider (parallel): $I_x = I_{total}\dfrac{R_{other}}{R_x + R_{other}}$

Problem routine

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