Basic Electrical Theory

 Basic Electrical Theory (Voltage, Current, Resistance)

Understanding basic electrical theory is essential for working with any electrical system, especially in vehicles. The three core concepts are Voltage, Current, and Resistance, which are related by Ohm’s Law.

Voltage (V) – "Electrical Pressure"

 Definition:

Voltage is the electrical potential difference between two points. It pushes electrons through a circuit—just like water pressure pushes water through a pipe.

 Unit: Volts (V)

1 volt = 1 joule of energy per coulomb of charge.

 In Automotive Systems:

Most car batteries supply 12 volts (or 24V in heavy-duty vehicles).

Sensors and modules often use 5V reference signals.

➤ Example:

If a battery has 12V, it means there is a 12-volt potential difference between the positive and negative terminals, ready to push current through a circuit.

Current (I) – "Flow of Electricity"

 Definition:

Current is the flow of electric charge (electrons) through a conductor.

 Unit: Amperes (A)

1 ampere = 1 coulomb of charge per second.

 Types of Current:

DC (Direct Current): Used in automotive systems. Current flows in one direction.

AC (Alternating Current): Used in household electricity. Not common in vehicles except in alternators.

 In Automotive Systems:

A headlight may draw around 5 amps.

Small sensors might draw only milliamps (1/1000 of an amp).

➤ Example:

A fuel pump may draw 4A when operating. That’s the amount of electrical current flowing through the circuit to make it work.

 Resistance (R) – "Opposition to Flow"

➤ Definition:

Resistance is the opposition to current flow in a circuit. All conductors have some resistance.

 Unit: Ohms (Ω)

Higher resistance = lower current for the same voltage.

 Factors That Affect Resistance:

Material (e.g., copper has low resistance)

Length of wire (longer = more resistance)

Thickness (thicker = less resistance)

Temperature (heat usually increases resistance)

 In Automotive Systems:

Good ground connections = low resistance

A faulty wire or corroded connector = high resistance → can cause voltage drops and malfunction

➤ Example:

A wire with 6Ω resistance connected to a 12V battery allows 2A of current to flow (Ohm’s Law: I = V / R → I = 12V / 6Ω).

 The Relationship: Ohm’s Law

V=I×R\boxed{V = I \times R}

This law describes how voltage, current, and resistance relate. You can calculate any one if you know the other two.

 Real-Life Automotive Example

Let’s say you have a fog light:

Voltage: 12V (car battery)

Resistance of the light: 6 ohms

Calculate the Current:

I=VR=12V6Ω=2AI = \frac{V}{R} = \frac{12V}{6Ω} = 2A

So the light draws 2 amps of current.

 Quick Tips for Technicians:

Always measure voltage with a multimeter to confirm power is present.

Check current draw to detect short circuits or high resistance.

Use resistance checks to test fuses, wires, sensors, or switches.