Basic Electrical Concepts

Introduction

Before diving into analog electronics, it is essential to master the fundamental electrical quantities and laws that govern all circuits. These concepts — voltage, current, resistance, power, and energy — form the building blocks of every analog system, from simple resistor networks to complex integrated circuits.

Fundamental Electrical Quantities

Charge (Q)

Electric charge is the fundamental property of matter that causes electromagnetic interaction. The unit is the Coulomb (C).

Current (I)

Current is the rate of flow of electric charge through a conductor:

Voltage (V)

Voltage is the electrical potential difference between two points — the work done per unit charge:

Resistance (R)

Resistance opposes the flow of current:

Ohm's Law

The most fundamental relationship in electronics:

Ohm's Law Triangle

Power and Energy

Electrical Power

Electrical Energy

Kirchhoff's Laws

Kirchhoff's Current Law (KCL)

The algebraic sum of all currents entering a node equals zero:

Kirchhoff's Voltage Law (KVL)

The algebraic sum of all voltages around any closed loop equals zero:

Series and Parallel Circuits

Series Connection

Parallel Connection

Voltage and Current Dividers

Voltage Divider

Current Divider

For two parallel resistors:

Numerical Examples

Example 1: Ohm's Law Application

Problem: A 12V battery is connected to a 4.7 kΩ resistor. Find the current and power dissipated.

Solution:

Example 2: Voltage Divider

Problem: Design a voltage divider to produce 3.3V from a 5V supply using standard resistor values. Maximum current draw should be under 1 mA.

Solution:

Step 1: Choose minimum total resistance

Step 2: Use the voltage divider formula

Step 3: Choose R2 = 6.8 kΩ (standard value)

Choose R1 = 3.3 kΩ (nearest standard value)

Step 4: Verify

Example 3: KVL and KCL

Problem: In the circuit below, find currents I1, I2, I3.

Solution:

Applying KCL at the center node: I1 = I2 + I3

Loop 1 (left): 10 - 2×I1 - 6×I2 = 0 Loop 2 (right): 5 - 4×I3 - 6×I2 = 0

Substituting I1 = I2 + I3:

From (1): I3 = (10 - 8I2)/2 = 5 - 4I2 Substituting in (2): 5 - 6I2 - 4(5 - 4I2) = 0

Therefore: I3 = 5 - 4(1.5) = -1 A (flows in opposite direction) I1 = 1.5 + (-1) = 0.5 A

AC Circuit Fundamentals

Reactance

Impedance

Power in AC Circuits

Interview Questions

1. What is the difference between EMF and potential difference?

EMF is the energy supplied by a source per unit charge (open-circuit voltage), while potential difference is the voltage across a component when current flows. EMF includes internal resistance effects.

1. Why does current flow from higher to lower potential?

Conventional current flows from higher to lower potential because positive charges (by convention) move toward lower energy states. Electrons actually move in the opposite direction.

1. Explain why resistance increases with temperature in conductors but decreases in semiconductors.

In conductors, higher temperature increases lattice vibrations, impeding electron flow. In semiconductors, higher temperature generates more electron-hole pairs, increasing available charge carriers and reducing resistance.

1. What happens if you short-circuit an ideal voltage source?

An ideal voltage source with zero internal resistance would supply infinite current during a short circuit (I = V/0 = ∞). In practice, real sources have internal resistance that limits current, and fuses/circuit breakers provide protection.

1. Derive the formula for two resistors in parallel.

For two resistors R1 and R2 in parallel, the same voltage V appears across both. Total current I = V/R1 + V/R2 = V(1/R1 + 1/R2). Since R_eq = V/I, we get 1/R_eq = 1/R1 + 1/R2, giving R_eq = (R1×R2)/(R1+R2).

Summary

These fundamental concepts — Ohm's law, Kirchhoff's laws, and series/parallel analysis — form the essential toolkit for analyzing any analog circuit. Mastering these basics enables you to tackle increasingly complex circuits involving active devices like transistors and op-amps.