This is one of the required topics in the Electrical Repairs Course. We will cover only material that is needed to repair truck electrical problems. We will not go into scientific detail beyond what you need for understanding of truck electrical problems.

This topic will cover direct current (DC) circuits only, because that is what your truck battery provides to all your truck electrical devices. Alternating current (AC) is used in your home, is much more complicated, and will not be covered in these topics. Good luck with your new knowledge; the rewards are well worth the effort.

Electric Charge & Atoms

Electricity is a physical phenomenon involving positive and negative charge. When these charges are in motion they may produce heat, light and magnetism. When charges are not in motion, static electricity can manifest itself as a force such as clothes clinging to each other when they are removed from a dryer. Static electricity in clouds results in lightening strikes from cloud to cloud or from cloud to earth.

All substances are composed of atoms. Each atom has negative charges called electrons, which orbit the atom nucleus, which contains positive charges called protons. In most atoms, the positive and negative charges are equal and balanced. If an atom has 10 electrons, it also typically has 10 protons.

The electrons orbit the atom nucleus, therefore, they can be easily torn away by external energy. A charge potential is created when electrons from one atom are moved to another atom. Some energy is required to make that electron change atoms, and once the change takes place, that charge is the stored energy which was required to make the change and move that electron.

Friction (rubbing) and chemical reactions (batteries) can create electrical charge differences between two materials. In clouds on hot summer afternoons, the friction of the air masses moving against each other, rubs electrons from one air mass and deposits them onto another air mass. Now the two air masses have an electrical potential difference (charge difference). Dust or moisture in the air masses increases the friction and increases the potential difference build up. Lightening strikes are the result of a tremendous potential difference. Batteries create a much smaller potential difference between the two battery plates due to the chemical reaction of the plates with the battery electrolyte (acid).


Conductors are used to move electrical current to another location. Most metals have an abundance of free electrons, which allows electrons to flow easily from one atom to the next atom. Some of these metals are Gold, Silver, Copper, and Iron. Gold is the best conductor and iron is the least conductor of these listed metals.

Conductors need very little potential difference to make electrons move (current flow (amps)) within them. Most truck wiring is made of copper, which is cheaper than silver or gold. Copper is the best choice for conductors on a truck because it is cheaper than gold and silver, and has less resistance that iron wire.

The bigger the diameter of the copper wire, the more atoms there are per unit of distance to provide electron transfers. Therefore, the larger the wire, the less resistance the wire has (smaller ohms). Your truck's starter requires massive current to crank the engine (about 500 amps), therefore the starter cables are very large in diameter so that they can pass the massive quantity of electrons with very little resistance. On the other hand, the dome light requires very little current (about 1/2 amp), so it has a wire which is very small in diameter.


Insulators are used to block electrical current. Most conductors are sheathed in an insulation (insulator) material to prevent the electrical current from taking an alternate path while traveling to its destination.

Insulator substances have very few free electrons. This prevents easy electron flow from one atom to the next atom. Glass, plastic, and rubber are examples of good insulators. Other good insulators are air and water! Yes, pure water is an insulator. Only the minerals in the water make it conduct electricity. Think about it for a moment, don't you put water in your battery? If it was a conductor, wouldn't it short out the battery plates? This is why you are supposed to use distilled water in batteries. Distilled water has no minerals to short out the battery plates.

By the way, that water in your bath tub has many minerals in it, so you don't want to place any electrical appliances near by!

Stored Energy

Batteries provide a way to store electrical energy in chemical form. When electrical energy is required, the battery converts chemical energy into electrical energy as needed. The lead acid battery, is composed of lead plates and acid electrolyte, and is used for all automotive and trucks applications.

Each truck battery cell produces 2 volts; therefore 12-volt batteries have 6 cells. The two plates of the battery cell are labeled plus (+) and minus (-). As the lead acid battery is charged, the charging energy is stored within the electrolyte (acid) of the battery, and a 2-volt potential difference exists between the two battery plates. When the battery is in use (or discharged), the electrolyte and the battery plates work together to provide free electrons to do the battery's work. In other words, the energy stored within the battery is converted chemically into electron potential difference between the two plates, which performs work outside of the battery.

Conventional Current & Electron Current

Back in the early days of electrical study, scientists decided that electrical current flowed from the positive battery plate to the negative battery plate. This concept is still accepted today, and is referred to as conventional current theory.

When scientists finally learned the atom's construction, it became quite obvious that the electron (negative) charge is the part to break away from the atom, since the proton (positive) charge is contained within the nucleus of the atom. Electron flow is the true electrical current. So this current concept was aptly named electron current theory. Both theories abound today, so check the book you are reading to see if they are using conventional theory or electron theory.

Like electrical charges repel and unlike electrical charges attract. So the excess electrons at the negative plate of the battery cell would just love to get over to the positive plate where there is a shortage of electrons. All they need is some path to make the jump between the two plates. Once these electrons have a path, electrical work is done, and eventually the battery will become discharged and neutralized of its stored energy (same charge on both plates).

The following illustration shows how electrons pass from one atom to another and the resulting electron comes out the other end of the wire. The electron passes through the wire from atom to atom, much like the energy of the pool stick passes from one ball to the next, with the 8 ball flying off the end. When the first ball is struck by the pool stick, another ball pops out the other end of the string of balls.

It is the same with electron flow in a conductor (wire). As each electron is stuffed into an atom at one end of the wire, another electron is forced out of the wire from another atom at the other end. Note the positive charge on the left side of the wire and the negative charge on the right side of the wire. This charge difference, or potential difference, or voltage difference, (they all mean the same thing) is what forces the electrons to pass through the wire. Electrons have a negative charge, so they rush away from the like negative charge and travel through the wire to get to the unlike positive charge. When the electrons are forced through the wire, work is done. It took energy to force the electrons through all those atoms. Therefore the wire has some Resistance (opposition to current flow).


The above illustration shows the difference between conventional current flow and electron current flow. All references to electrical current at this website will refer to electron current flow, because that is how it really works. In either case, the light bulb still lights!

Electrical Work (Power)

Work is done by electricity when electrons are forced to move through some resistance to discharge a potential difference. In the above battery and light bulb example, electrons were forced to move through the wire and through the light bulb. This flow of electrons is called current, and is measured in amperes. The wire's opposition to the electron flow and the bulb's opposition to electron flow is called resistance, and is measured in ohms.

When current is forced through a resistance, work is said to have been done. Electrical work is measured in Watts and the power is represented by "P" for power. Approximately 760 Watts equals one horsepower.

Any device, which uses electrical power, is called an electrical load, so a headlamp, which is illuminated, is an electrical load. Electrical power is electrical work being done. Just like it takes power to move your truck, it also takes power to light that light bulb. Current must be pushed from the negative battery terminal, through the bulb, and back to the positive terminal of the battery, and through the electrolyte, to make the bulb light.

Electrical Symbols & Ohm's Law

The electrical symbol for power is "P". The electrical symbol for voltage is "E" (E for voltage??). The electrical symbol for resistance is "R" (they got one right). The electrical symbol for current is "I" (go figure!).

A man named Charlie Ohm spent most of his weekends studying electricity and came up with a set of rules which are called Ohm's Law. This rules are used to evaluate any electrical circuit. His rules define the relationship between voltage, resistance, current, and power. His rules are really quite simple. They are:

I = E / R (current = voltage divided by resistance)
E = I * R ( voltage = current multiplied by resistance)
R = E / I ( resistance = voltage divided by current)
P = E * I ( power = voltage multiplied by current)
P = (E * E) / R (power = voltage squared divided by resistance)
P = I * I * R (power = current multiplied by current multiplied by resistance)

Simple Series Circuit

All electrical circuits have three elements. A source of voltage (potential difference), an electrical load (something which takes current (amps)), and a path for the current to flow through (conductors). The following electrical circuit consists of a 12-volt battery, a headlamp bulb, and the wires (conductors) which connect the battery terminals to the bulb.

Two things determine the power consumed by an electrical load, the potential difference (voltage) of the source, and the current (amps) delivered into the load resistance.

The voltage of the circuit shown below is a 12-volt truck battery. This circuit is called a series circuit because the same current passes through all components of the circuit. The electrical resistance of this load is the filament within the head lamp light bulb.

In the circuit above, notice the Voltmeter and the Ammeter. The voltmeter measures the potential difference across two points. In this example, the voltmeter is measuring the voltage across the light bulb. The ammeter measures the current passing through the ammeter. Detailed theory of meters can be found in our meter theory topic, and proper use of meters can be found in our topic called using multimeters .

The filament within the light bulb is made of a special metal, which passes current easier than an insulator, but harder than a conductor. This special metal is called tungsten. The 12-volt potential difference of the battery, forces electrons through the tungsten, but the tungsten resists this current flow, resulting in work being done by the battery to overcome the tungsten resistance. This work is converted into heat which heats the tungsten until it gets white hot, and gives off light. This heating of the tungsten requires power from the battery, and the unit of electrical power is watts. Notice that the light bulb rating is 36 Watts.

The resistance of this light bulb is specified as 4-ohms. Let us use Ohm's Law and determine what current the amp meter will indicate.

I = E / R (therefore, current "I" = 12-Volts divided by 4-Ohms = 3-Amps)

There, that wasn't so hard was it. We know that the bulb is rated at 36 Watts, so lets see how much power the bulb is using in this circuit.

P = I * I * R (therefore, power "P" = 3-Amps multiplied by 3-Amps multiplied by 4-Ohms = 36-Watts)

The bulb is operating at its designed power rating in this circuit. What is the voltage reading of the voltmeter?

E = I * R (therefore, voltage "E" = 3-Amps multiplied by 4-Ohms = 12-Volts.

You have just seen how Ohm's Law is used in real life. Any time you know two circuit parameters, you can calculate the third circuit parameter, using Ohm's Law.

This concludes the basic electricity topic. We have covered a lot of material which will be used in the next topic. We avoided much scientific detail because you don't need that detail to fix truck electrical problems. You do need an understanding of each area that we presented here. If you are unsure about anything in this topic, reread that area until it makes sense. Everything else electrical is based upon the material presented in this topic.

We suggest that your next topic be Basic Electricity-2, where you will learn the other important concepts of electricity. If you have any problems or comments, feel free to contact webRider.

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