Electrical quantities underpin all electronics engineering work in Kenya, from designing circuits to powering communication devices. Mastering the correct application of these quantities ensures precision, safety, and efficiency in electronic system development and maintenance. This chapter begins by establishing a clear understanding of the International System of Units (SI units), which form the universal language for measuring electrical quantities. Kenyan electronics engineers rely on SI units to communicate specifications, perform calculations, and comply with international standards.

1.1 The meaning of SI unit

The International System of Units (SI) is the globally accepted metric system for measuring physical quantities, including electrical parameters fundamental to electronics engineering. In Kenya, adherence to SI units ensures interoperability of electronic components and systems, whether in telecommunications, industrial automation, or consumer electronics. SI units provide standardization that eliminates ambiguity in measurements such as voltage, current, resistance, and power, which are critical for design accuracy and regulatory compliance.

1.1.1 Introduction to SI Units and Their Role in Electronics Engineering

SI units represent a standardized set of units defined internationally to measure physical quantities consistently. The system is based on seven base units from which all other units derive, allowing electronics engineers to quantify electrical parameters precisely. Understanding SI units enables engineers to interpret datasheets, perform accurate calculations, and communicate technical information effectively. The seven base SI units include the meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Among these, the ampere is particularly important in electronics as it quantifies electric current flow.

Worked Examples

Example 1: Calculate the current flowing through a circuit if a charge of 12 coulombs passes a point in 4 seconds.
Given: \( Q = 12\, C \), \( t = 4\, s \)$$ I = \frac{Q}{t} $$$$ I = \frac{12}{4} $$$$ I = 3\, A $$ Answer: 3 amperes

Example 2: A resistor has a resistance of 10 ohms and a current of 2 amperes flows through it. Calculate the voltage across the resistor.
Given: \( R = 10\, \Omega \), \( I = 2\, A \)$$ V = I \times R $$$$ V = 2 \times 10 $$$$ V = 20\, V $$ Answer: 20 volts

Example 3: A capacitor stores a charge of \( 5 \times 10^{-6} \) coulombs when connected to a 10-volt source. Calculate the capacitance.
Given: \( Q = 5 \times 10^{-6} \, C \), \( V = 10\, V \)$$ C = \frac{Q}{V} $$$$ C = \frac{5 \times 10^{-6}}{10} $$$$ C = 5 \times 10^{-7}\, F $$ Answer: 0.5 microfarads

1.1.2 The SI Base Unit for Electric Current: The Ampere

The ampere (A) is the SI base unit for electric current and is defined by the flow of electric charge per unit time. It quantifies the amount of charge passing a point in an electric circuit every second. The ampere is essential for specifying current ratings of components such as transistors, resistors, and integrated circuits in electronics engineering. Precisely measuring current in amperes enables engineers to design circuits that operate safely within component limits, preventing damage and ensuring performance.

Worked Examples

Example 1: A circuit carries a current of 0.5 amperes. How much charge passes through the circuit in 10 seconds?
Given: \( I = 0.5\, A \), \( t = 10\, s \)$$ Q = I \times t $$$$ Q = 0.5 \times 10 $$$$ Q = 5\, C $$ Answer: 5 coulombs

Example 2: If a current of 3 amperes flows through a wire for 15 seconds, calculate the total charge transferred.
Given: \( I = 3\, A \), \( t = 15\, s \)$$ Q = I \times t $$$$ Q = 3 \times 15 $$$$ Q = 45\, C $$ Answer: 45 coulombs

Example 3: A device draws 0.75 amperes for 20 seconds. Calculate the total charge transferred.
Given: \( I = 0.75\, A \), \( t = 20\, s \)$$ Q = I \times t $$$$ Q = 0.75 \times 20 $$$$ Q = 15\, C $$ Answer: 15 coulombs

1.1.3 Derived SI Units for Electrical Quantities: Voltage, Resistance, and Power

Derived SI units combine base units to measure electrical quantities critical to electronics engineering. Voltage (V) is the electric potential difference measured in volts, defined as one joule per coulomb. Resistance (Ω) measures opposition to current flow, defined in ohms, which is volts per ampere. Power (W) quantifies the rate of energy transfer, measured in watts, defined as one joule per second or volts times amperes. Understanding these derived units allows engineers to analyze circuit behavior, design efficient components, and troubleshoot effectively.

Worked Examples

Example 1: Calculate the power dissipated in a resistor when a current of 2 amperes flows through a resistor of 5 ohms.
Given: \( I = 2\, A \), \( R = 5\, \Omega \)$$ P = I^2 \times R $$$$ P = 2^2 \times 5 $$$$ P = 4 \times 5 $$$$ P = 20\, W $$ Answer: 20 watts

Example 2: Find the voltage across a resistor of 8 ohms when a current of 1.5 amperes flows through it.
Given: \( R = 8\, \Omega \), \( I = 1.5\, A \)$$ V = I \times R $$$$ V = 1.5 \times 8 $$$$ V = 12\, V $$ Answer: 12 volts

Example 3: A resistor dissipates 45 watts of power when a voltage of 15 volts is applied. Calculate the current flowing through the resistor.
Given: \( P = 45\, W \), \( V = 15\, V \)$$ I = \frac{P}{V} $$$$ I = \frac{45}{15} $$$$ I = 3\, A $$ Answer: 3 amperes

1.1.4 Multiples and Submultiples of SI Units in Electronics Engineering

In electronics engineering, SI units are commonly expressed in multiples and submultiples to handle the very large or very small quantities encountered. Prefixes such as kilo (k, \(10^3\)), milli (m, \(10^{-3}\)), micro (μ, \(10^{-6}\)), nano (n, \(10^{-9}\)), and mega (M, \(10^6\)) are used to scale units conveniently. This practice simplifies calculations and specifications, making it easier to communicate values such as resistance in kilo-ohms or capacitance in microfarads. Kenyan electronics engineers must be proficient in converting between these prefixes to interpret data sheets and perform accurate measurements.

Worked Examples

Example 1: Convert 4.7 kilo-ohms to ohms.
Given: \( 1\, k\Omega = 10^3\, \Omega \)$$ 4.7\, k\Omega = 4.7 \times 10^3\, \Omega $$$$ = 4700\, \Omega $$ Answer: 4700 ohms

Example 2: Express 0.0022 farads in microfarads.
Given: \( 1\, \mu F = 10^{-6}\, F \)$$ 0.0022\, F = 0.0022 \times \frac{1}{10^{-6}}\, \mu F $$$$ = 2200\, \mu F $$ Answer: 2200 microfarads

Example 3: A voltage of 5 millivolts is applied. Convert this to volts.
Given: \( 1\, mV = 10^{-3}\, V \)$$ 5\, mV = 5 \times 10^{-3}\, V $$$$ = 0.005\, V $$ Answer: 0.005 volts

Chapter Summary

This chapter introduced the concept of the International System of Units (SI units) and their significance in standardizing electrical measurements. It covered the SI units for various electrical parameters such as current, voltage, resistance, and power, establishing a clear foundation for accurate calculations. The chapter then explained Ohm's law, which relates voltage, current, and resistance in an electrical circuit. Following this, it demonstrated how to perform calculations involving these electrical quantities, emphasizing practical problem-solving skills. Finally, the chapter described the instruments commonly used to measure electrical parameters, highlighting their functions and applications in real-world electrical work. Together, these topics provide a comprehensive understanding of fundamental electrical principles and their practical application.