Electrical quantities form the foundation of understanding and working with electric circuits in any professional field. Mastery of these basics enables technicians and professionals in healthcare facilities, banks, farms, and other sectors to safely and effectively operate, troubleshoot, and maintain electrical equipment. This chapter covers the essential units, components, symbols, quantities, specifications, and tools needed to apply electrical principles across diverse Kenyan workplaces.

1.1 Basic SI Units Electrical Quantities

Electrical quantities are measured using standard units defined by the International System of Units (SI). Accurate measurement and understanding of these units ensure consistency and safety in electrical work across all industries, from hospital equipment servicing to agricultural irrigation systems.

1.1.1 Overview of SI Units

The SI system standardizes measurements globally, including electrical quantities. The three fundamental electrical quantities are voltage, current, and resistance, each with a specific SI unit.

• The volt (V) measures electrical potential difference or electromotive force.
• The ampere (A) measures electric current, the flow of electric charge.
• The ohm (Ω) measures electrical resistance, opposition to current flow.

Other derived units include the watt (W) for power and the coulomb (C) for electric charge.

Worked Examples

Example 1: A hospital uses a device with a potential difference of 12 volts. What is the unit measuring this quantity?

Given: Voltage = 12 volts

The SI unit for voltage is the volt (V).

Answer: 12 V

Example 2: A water pump at a county farm draws a current of 5 amperes. Identify the SI unit for current.

Given: Current = 5 amperes

The SI unit for current is the ampere (A).

Answer: 5 A

Example 3: A resistance coil in a hotel’s water heater has a resistance of 20 ohms. What is the SI unit for resistance?

Given: Resistance = 20 ohms

The SI unit for resistance is the ohm (Ω).

Answer: 20 Ω

1.1.2 Classification of Components: Passive, Active, and Electromechanical

Electrical components are classified based on their function and behavior in circuits. This classification guides selection and application in various sectors such as banks’ ATM machines or university laboratory equipment.

• Passive components do not supply energy but consume or store it. Examples include resistors, capacitors, and inductors.
• Active components can supply energy and control current flow, such as transistors and integrated circuits.
• Electromechanical components convert electrical energy to mechanical motion or vice versa. Examples include relays, switches, and motors.

Worked Examples

Example 1: Identify the classification of a resistor used in a county government office’s lighting circuit.

Given: Component = Resistor

Resistors are passive components because they consume energy by resisting current flow.

Answer: Passive component

Example 2: A transistor in a SACCO’s security alarm system amplifies signals. What type of component is it?

Given: Component = Transistor

Transistors supply energy to control current and are therefore active components.

Answer: Active component

Example 3: A relay used in a retail shop’s electrical control panel operates mechanically with electrical input. Classify it.

Given: Component = Relay

Relays convert electrical signals to mechanical movement, thus electromechanical components.

Answer: Electromechanical component

1.1.3 Introduction to Circuit Symbols and Schematic Diagrams

Circuit symbols provide a universal language to represent electrical components, while schematic diagrams map out the connections in a circuit. Understanding these is essential for troubleshooting and installation in any sector, such as hospitals or educational institutions.

• Each component has a unique symbol (e.g., resistor, capacitor, battery).
• Schematic diagrams show how components connect electrically.
• Diagrams use lines for conductors and symbols for components.
• Proper interpretation avoids errors during assembly or maintenance.
• Software tools and printed diagrams assist in complex circuit understanding.

Worked Examples

Example 1: Identify the symbol for a resistor in a schematic used in a university physics laboratory.

Given: Component = Resistor

The resistor symbol is a zigzag line.

Answer: Zigzag line symbol

Example 2: What symbol represents a battery in a hospital’s emergency power system schematic?

Given: Component = Battery

The battery symbol consists of alternating long and short parallel lines.

Answer: Alternating long and short line pair

Example 3: In a retail store’s circuit diagram, which symbol indicates a switch?

Given: Component = Switch

A break in a line with a pivot point represents a switch.

Answer: Break in line with pivot symbol

1.1.4 Basic Electrical Quantities and Units (Voltage, Current, Resistance)

Voltage, current, and resistance are the core quantities in any electrical circuit. Calculations involving these quantities help professionals in sectors like county referral hospitals or cooperative societies ensure devices operate correctly.

• Voltage (V): Potential difference driving current.
• Current (I): Rate of charge flow.
• Resistance (R): Opposition to current.
• Ohm’s Law relates these: V = I × R
• Accurate measurement informs safe and efficient design.

Worked Examples

Example 1: A hospital device has a current of 2 A and resistance of 10 Ω. Find the voltage.

Given: \(I = 2\,A\), \(R = 10\,\Omega\)

V = I \times R

V = 2 \times 10

V = 20\,V

Answer: 20 V

Example 2: A SACCO’s ATM machine operates at 240 V with a resistance of 120 Ω. Find the current.

Given: \(V = 240\,V\), \(R = 120\,\Omega\)

I = \frac{V}{R}

I = \frac{240}{120}

I = 2\,A

Answer: 2 A

Example 3: A cooperative’s irrigation motor draws 15 A at 230 V. Calculate the resistance.

Given: \(I = 15\,A\), \(V = 230\,V\)

R = \frac{V}{I}

R = \frac{230}{15}

R = 15.33\,\Omega

Answer: 15.33 Ω

1.1.5 Understanding Component Specifications

Component specifications define ratings such as voltage, current, power, and tolerance, ensuring correct application in systems like bank security or county government offices.

• Voltage rating: Maximum safe operating voltage.
• Current rating: Maximum continuous current.
• Power rating: Maximum power dissipation (watts).
• Tolerance: Acceptable variation range.
• Temperature rating: Operating temperature limits.

Worked Examples

Example 1: A component rated at 250 V and 0.5 A is used in a retail store. Can it safely operate at 230 V and 0.3 A?

Given: Rated voltage = 250 V, operating voltage = 230 V, rated current = 0.5 A, operating current = 0.3 A

Since operating voltage and current are below ratings, the component is safe.

Answer: Safe to operate

Example 2: A resistor has a power rating of 0.25 W and resistance of 100 Ω. What is the maximum current it can safely carry?

Given: \(P = 0.25\,W\), \(R = 100\,\Omega\)

P = I^{2} \times R

I = \sqrt{\frac{P}{R}}

I = \sqrt{\frac{0.25}{100}}

I = \sqrt{0.0025}

I = 0.05\,A

Answer: 0.05 A

Example 3: A capacitor with a voltage rating of 50 V is used in a hospital circuit operating at 60 V. Is this acceptable?

Given: Rated voltage = 50 V, operating voltage = 60 V

Operating voltage exceeds rating; use is unsafe.

Answer: Not safe to use

1.1.6 Overview of Testing and Measurement Tools (Multimeters, Oscilloscopes)

Testing tools are essential for diagnosing and maintaining electrical systems in places like universities or farms. Correct tool choice and use ensure accuracy and safety.

• Multimeters can be analog or digital.
• Oscilloscopes allow visualization of AC signals and transients.
• Proper calibration and handling prevent damage.
• Safety precautions include using correct settings and protective gear.
• Regular maintenance extends tool lifespan.

Worked Examples

Example 1: Measuring voltage across a bank’s UPS battery using a multimeter set to 0-300 V DC range.

Given: Multimeter set to 300 V DC

Procedure: Connect probes across battery terminals, read value.

If display shows 12.5 V, that is battery voltage.

Answer: Battery voltage = 12.5 V

Example 2: Using an oscilloscope to view a 50 Hz AC waveform in a county hospital’s power supply.

Given: AC frequency = 50 Hz

Set oscilloscope time base to 20 ms/div (1/50 Hz = 20 ms period), observe sine wave.

Answer: Sine waveform displayed with 20 ms period

Example 3: Measuring current in a SACCO’s lighting circuit using a multimeter in series with the load.

Given: Expected current ~5 A

Set multimeter to 10 A range, break circuit, connect meter in series, read current.

If meter shows 4.8 A, that is the circuit current.

Answer: Circuit current = 4.8 A

Chapter Summary

This chapter introduced the fundamental electrical quantities and their basic SI units, including power measured in watts, current in amperes, resistance in ohms, and voltage in volts. It explained the differences between conductors and insulators by examining the properties of metals and non-metals and their specific applications in electrical circuits. Key electrical quantities such as charge, force, work, and power were defined along with their units, and calculations involving these quantities were demonstrated. Ohm’s Law was explored to establish the relationship between voltage, current, and resistance, with practical applications and problem-solving techniques. The chapter also covered basic electrical and electronic measurements, focusing on the use of multimeters and ammeters, as well as the importance of proper measurement calibration. Furthermore, it classified electrical components into passive, active, and electromechanical types and introduced common circuit symbols and schematic diagrams. The properties and functions of capacitors and inductors were described, including their types, values, behavior in DC and AC circuits, and their roles in filtering and energy storage. Finally, the chapter touched on basic transistor circuits used as switches and amplifiers, with an emphasis on hands-on activities involving simple diode and transistor testing.