Chapter 3: Voltage, Current, and Resistance

 

Electric Current: Definition and Units

Electric current is defined as the rate of flow of electric charge past a given point in a circuit. Mathematically: I = Q / t where I is current in Amperes (A), Q is charge in Coulombs (C), and t is time in seconds (s).

One ampere equals one coulomb of charge flowing past a point per second. In practical electronics, we often deal with milliamperes (mA, 10⁻³ A), microamperes (μA, 10⁻⁶ A), and even nanoamperes (nA, 10⁻⁹ A).

There are two conventions for describing current direction. Conventional current is defined as flowing from positive to negative terminal (opposite to actual electron flow). Electron current flows from negative to positive. In circuit analysis, conventional current is universally used.

 

Voltage: The Driving Force

Voltage, also called electromotive force (EMF) or potential difference, is the energy per unit charge required to move a charge between two points. The unit of voltage is the Volt (V), defined as one joule of energy per coulomb of charge: V = W / Q

A battery creates a potential difference by undergoing chemical reactions that separate charges, creating a surplus of electrons at the negative terminal and a deficit at the positive terminal. This potential difference drives current through any connected external circuit.

Voltage is always measured between two points — it is a relative quantity. When we say a node is 'at 5V,' we implicitly mean 5V relative to a reference point, usually called ground (0V).

 

Resistance: Opposition to Current Flow

Resistance is the opposition that a material offers to the flow of electric current. The unit of resistance is the Ohm (Ω). A resistor with a resistance of 1Ω allows 1A of current to flow when 1V is applied across it.

Resistance in a conductor depends on four factors: material (expressed as resistivity ρ in Ω·m), length (L), cross-sectional area (A), and temperature. The formula relating these is: R = ρ × L / A

For most metallic conductors, resistance increases with temperature because thermal vibrations of the lattice impede electron movement. This is called a positive temperature coefficient (PTC). Semiconductors typically show a negative temperature coefficient (NTC) — their resistance decreases as temperature rises.

In practical electronics, resistors are the most commonly used components. They come in various types including carbon composition, metal film, wire-wound, and surface-mount (SMD). Their values are often identified using a color-coding system printed as bands on the resistor body.

 

Color Code for Resistors

The resistor color code is a system of colored bands that indicate the resistance value and tolerance of a resistor. Each color corresponds to a digit: Black = 0, Brown = 1, Red = 2, Orange = 3, Yellow = 4, Green = 5, Blue = 6, Violet = 7, Gray = 8, White = 9.

For a four-band resistor: the first two bands are the first two significant digits, the third band is the multiplier (power of 10), and the fourth band is the tolerance (Gold = ±5%, Silver = ±10%). For example, Red-Violet-Orange-Gold = 27 × 10³ Ω ± 5% = 27 kΩ ± 5%.

Five-band resistors add a third significant digit for greater precision. Memorizing the color code is an essential skill for any electronics practitioner.