Automotive mechatronics professionals routinely handle electrical systems in vehicles, from battery management to sensor circuits and actuators. Mastery of basic electrical quantities is vital for diagnosing faults, designing upgrades, and ensuring safety and efficiency in modern automotive electrical and electronic systems. Understanding SI units for electrical parameters allows precise measurement, troubleshooting, and compliance with global standards in Kenya's automotive sector.

1.1 SI Unit of Various Types of Electrical Parameters

1.1.1 Voltage: SI Unit and Calculations

Voltage is the electrical potential difference between two points and is the driving force behind current flow in automotive circuits. The SI unit for voltage is the volt (V).

The relationship between voltage (\(V\)), current (\(I\)), and resistance (\(R\)) is given by Ohm's Law:$$ V = I \times R $$

Worked Examples

Example 1:
A vehicle headlight circuit draws \(2\,\text{A}\) and has a resistance of \(6\,\Omega\). Find the voltage across the circuit.

Given: \(I = 2\,\text{A}\), \(R = 6\,\Omega\)

$$ V = I \times R $$$$ V = 2 \times 6 $$$$ V = 12\,\text{V} $$ Answer: \(12\,\text{V}\)

Example 2:
An automotive sensor operates at \(9\,\text{V}\) and has an internal resistance of \(3\,\Omega\). What is the current through the sensor?

Given: \(V = 9\,\text{V}\), \(R = 3\,\Omega\)

$$ I = \frac{V}{R} $$$$ I = \frac{9}{3} $$$$ I = 3\,\text{A} $$ Answer: \(3\,\text{A}\)

Example 3:
A relay coil in a vehicle has a resistance of \(50\,\Omega\) and is powered by a \(24\,\text{V}\) supply. Calculate the current.

Given: \(V = 24\,\text{V}\), \(R = 50\,\Omega\)

$$ I = \frac{V}{R} $$$$ I = \frac{24}{50} $$$$ I = 0.48\,\text{A} $$ Answer: \(0.48\,\text{A}\)

Example 4:
A heated rear window circuit requires \(10\,\text{A}\) and the resistance is \(1.2\,\Omega\). Determine the voltage needed.

Given: \(I = 10\,\text{A}\), \(R = 1.2\,\Omega\)

$$ V = I \times R $$$$ V = 10 \times 1.2 $$$$ V = 12\,\text{V} $$ Answer: \(12\,\text{V}\)

Example 5:
A car's ECU (Engine Control Unit) is supplied with \(5\,\text{V}\) and draws \(0.2\,\text{A}\). Find the resistance.

Given: \(V = 5\,\text{V}\), \(I = 0.2\,\text{A}\)

$$ R = \frac{V}{I} $$$$ R = \frac{5}{0.2} $$$$ R = 25\,\Omega $$ Answer: \(25\,\Omega\)

1.1.2 Current: SI Unit and Calculations

Current is the rate of flow of electric charge through a conductor. The SI unit is the ampere (A).

The relationship between charge (\(Q\)) and current (\(I\)) is:$$ I = \frac{Q}{t} $$ where \(t\) is time in seconds.

Worked Examples

Example 1:
A battery supplies \(60\,\text{C}\) of charge in \(10\,\text{s}\). Find the current.

Given: \(Q = 60\,\text{C}\), \(t = 10\,\text{s}\)

$$ I = \frac{Q}{t} $$$$ I = \frac{60}{10} $$$$ I = 6\,\text{A} $$ Answer: \(6\,\text{A}\)

Example 2:
A fuse in a car circuit blows when \(120\,\text{C}\) passes through it in \(30\,\text{s}\). What is the current?

Given: \(Q = 120\,\text{C}\), \(t = 30\,\text{s}\)

$$ I = \frac{120}{30} $$$$ I = 4\,\text{A} $$ Answer: \(4\,\text{A}\)

Example 3:
A vehicle's air conditioning system operates for \(15\,\text{s}\), passing \(225\,\text{C}\) of charge. Calculate the current.

Given: \(Q = 225\,\text{C}\), \(t = 15\,\text{s}\)

$$ I = \frac{225}{15} $$$$ I = 15\,\text{A} $$ Answer: \(15\,\text{A}\)

Example 4:
A car radio draws \(2\,\text{A}\) for \(120\,\text{s}\). Determine the total charge transferred.

Given: \(I = 2\,\text{A}\), \(t = 120\,\text{s}\)

$$ Q = I \times t $$$$ Q = 2 \times 120 $$$$ Q = 240\,\text{C} $$ Answer: \(240\,\text{C}\)

Example 5:
A battery delivers \(3\,\text{A}\) for \(50\,\text{s}\). How much charge is supplied?

Given: \(I = 3\,\text{A}\), \(t = 50\,\text{s}\)

$$ Q = I \times t $$$$ Q = 3 \times 50 $$$$ Q = 150\,\text{C} $$ Answer: \(150\,\text{C}\)

1.1.3 Resistance: SI Unit and Calculations

Resistance is the opposition to current flow in a circuit. The SI unit is the ohm (\Omega).

The relationship between voltage, current, and resistance is:$$ R = \frac{V}{I} $$

Worked Examples

Example 1:
A vehicle indicator circuit operates at \(12\,\text{V}\) and draws \(0.5\,\text{A}\). Find the resistance.

Given: \(V = 12\,\text{V}\), \(I = 0.5\,\text{A}\)

$$ R = \frac{12}{0.5} $$$$ R = 24\,\Omega $$ Answer: \(24\,\Omega\)

Example 2:
A wiper motor draws \(1.5\,\text{A}\) from a \(12\,\text{V}\) battery. Calculate the resistance.

Given: \(V = 12\,\text{V}\), \(I = 1.5\,\text{A}\)

$$ R = \frac{12}{1.5} $$$$ R = 8\,\Omega $$ Answer: \(8\,\Omega\)

Example 3:
A car heater uses \(8\,\text{A}\) and operates at \(24\,\text{V}\). Find the resistance.

Given: \(V = 24\,\text{V}\), \(I = 8\,\text{A}\)

$$ R = \frac{24}{8} $$$$ R = 3\,\Omega $$ Answer: \(3\,\Omega\)

Example 4:
A sensor circuit is powered by \(5\,\text{V}\) and draws \(0.1\,\text{A}\). What is the resistance?

Given: \(V = 5\,\text{V}\), \(I = 0.1\,\text{A}\)

$$ R = \frac{5}{0.1} $$$$ R = 50\,\Omega $$ Answer: \(50\,\Omega\)

Example 5:
A vehicle's brake light circuit operates at \(12\,\text{V}\) and draws \(2\,\text{A}\). Calculate the resistance.

Given: \(V = 12\,\text{V}\), \(I = 2\,\text{A}\)

$$ R = \frac{12}{2} $$$$ R = 6\,\Omega $$ Answer: \(6\,\Omega\)

1.1.4 Power: SI Unit and Calculations

Power is the rate at which electrical energy is converted to another form, such as heat, light, or motion. The SI unit is the watt (W).

The relationship between power (\(P\)), voltage (\(V\)), and current (\(I\)) is:$$ P = V \times I $$

Worked Examples

Example 1:
A car headlight operates at \(12\,\text{V}\) and draws \(4\,\text{A}\). Calculate the power consumed.

Given: \(V = 12\,\text{V}\), \(I = 4\,\text{A}\)

$$ P = V \times I $$$$ P = 12 \times 4 $$$$ P = 48\,\text{W} $$ Answer: \(48\,\text{W}\)

Example 2:
A vehicle audio system draws \(2\,\text{A}\) from a \(12\,\text{V}\) battery. Find the power output.

Given: \(V = 12\,\text{V}\), \(I = 2\,\text{A}\)

$$ P = 12 \times 2 $$$$ P = 24\,\text{W} $$ Answer: \(24\,\text{W}\)

Example 3:
A cooling fan operates at \(24\,\text{V}\) and draws \(3\,\text{A}\). What is the power consumed?

Given: \(V = 24\,\text{V}\), \(I = 3\,\text{A}\)

$$ P = 24 \times 3 $$$$ P = 72\,\text{W} $$ Answer: \(72\,\text{W}\)

Example 4:
A vehicle sensor operates at \(5\,\text{V}\) and draws \(0.2\,\text{A}\). Calculate the power.

Given: \(V = 5\,\text{V}\), \(I = 0.2\,\text{A}\)

$$ P = 5 \times 0.2 $$$$ P = 1\,\text{W} $$ Answer: \(1\,\text{W}\)

Example 5:
A starter motor uses \(12\,\text{V}\) and draws \(100\,\text{A}\). Find the power.

Given: \(V = 12\,\text{V}\), \(I = 100\,\text{A}\)

$$ P = 12 \times 100 $$$$ P = 1200\,\text{W} $$ Answer: \(1200\,\text{W}\)

Chapter Summary

This chapter explored the fundamental concepts of electrical quantities by first reviewing the SI units associated with key electrical parameters such as current, voltage, resistance, and power. Learners practiced using these units accurately in calculations, building a strong foundation for interpreting and solving real-world electrical problems. The chapter then focused on step-by-step calculations involving electrical parameters, including the application of Ohm's Law and the power formula to determine unknown values in various scenarios. Through progressively challenging examples, students learned to confidently solve for current, voltage, resistance, and power in both simple and complex circuits. Additionally, the chapter introduced the main instruments used to measure these electrical quantities, such as ammeters, voltmeters, and wattmeters, highlighting their roles and correct application in the field. By mastering these concepts and calculation techniques, students are well prepared to measure, analyze, and troubleshoot electrical systems with precision. The chapter established a solid basis for further study in electrical and electronics principles, emphasizing both theoretical understanding and practical problem-solving skills.