Basic Electricity

Electricity
Electricity is defined by three basic elements: Voltage, Current and Resistance.

Voltage
Voltage is a measure of electromotive force, sometimes referred to as electrical "pressure". It can be described as the difference in potential (potential for the flow of electricity) between any two points in a circuit.

A typical automobile battery, for example, has a difference in potential of about 12 Volts between the positive (+) terminal and the negative (-) terminal.

The basic units of electrical potential are Volts (V). Very low Voltages are expressed as millivolts (mV).
1 V = 1000 mV; 1 mV = 0.001 V

Current
Current is the term describing the flow of electricity through a conductor. In a complete circuit, potential (Voltage) will cause current to flow from positive (+) to negative (-).

The basic units of current flow are amperes or amps (A). Small amounts of current flow are often measured in milliamps (mA).
1 A = 1000 mA; 1 mA = 0.001 A

Resistance
Resistance resists or opposes the flow of electricity. Conductors are made from materials of low resistance that allow electricity to flow easily. Insulators are materials of very high resistance that inhibit the low of electricity.

The basic unit of resistance is the Ohm. High resistance values are often expressed as Kilo ohms (K ohm).
1K ohm = 1000 ohm

Resistance vs. Current Flow
The basic rule of electricity (Ohm's Law) states that one unit of force (1 Volt) is required for one unit of current (1 am) to flow against one unit of resistance (1 Ohm). From Ohm's Law, we also know that:
Voltage = Current x Resistance

When Voltage is approximately constant, as in an automobile electrical system, current and resistance affect each other. As resistance increases, there will be less current flow. And lower resistance will permit higher current flow.

Higher resistance = lower current flow
Example: Corrosion on a headlight connector (higher resistance) causes the light to be dim (lower current flow).

Lower resistance = higher current flow
Example: A damaged wire shorted to ground (GND) (lower resistance) overloads circuit capacity (higher current flow) and blows a fuse.

Definition of a Circuit

A Complete Circuit:

Four things are required for current to flow in any electrical circuit, and for that circuit, and for that circuit to function as intended:
- Power Source (Voltage)
- Conductors (wires, printed circuits, etc.)
- Load or Consumer (a user of electrical power)
- Complete Circuit (a connection to ground)

Open Circuits
An open circuit is an incomplete circuit. An open circuit occurs when some kind of malfunction interrupts the circuit path and prevents current flow. Some common causes of open circuits are:
- Broken wire
- Loose or disconnected connector
- Loose or damaged connector terminal
- Corrosion
- Malfunctioning fuse or component

Test for an open circuit by checking continuity using an Ohmmeter (multimeter), or by checking for Voltage at various points of the circuit using a test light or Voltmeter (multimeter). See Checking Wiring and Components.

Short Circuits
A short circuit is an unintended complete circuit. A short circuit occurs when some kind of malfunction causes current flow to follow the wrong path.

A short circuit to ground (GND) (grounded circuit) may prevent Voltage from reaching a component. If Voltage is shorted directly to ground (GND), bypassing any load, the unrestricted current flow will damage fuses wires or components. Some common causes of short circuits are:
- Damaged wire or wiring harness
- Malfunctioning insulation
- Internally damaged component
- Incorrect connection

Test for a short circuit to ground (GND) using a multimeter or a test light to indicate circuit malfunctions and abnormal current flow paths. See Checking for Short Circuit to Ground (GND).