# ⚡ Cheatsheet: Electrical Engineering Basics: Solve DC Circuits for Exams

> This cheatsheet is part of the paid section. Sign in and purchase the course to view the full content.

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**