AC Electricity Basics: Current, Voltage, Resistance, Ohm’s Law
Alternating Current (AC) Fundamentals
Alternating Current (AC) is the electrical current whose value and direction vary cyclically in a circuit. This type of current is generated in power plants and is the one we find in household plug sockets. Most electrical and household appliances are powered by alternating current.
The figure below shows how the direction and value of domestic AC vary:
Key Electrical Variables
Current (I)
Current (I) is the variable that tells us how many electrons flow through the cross-section of a conductor per unit of time. Its unit of measurement in the International System is the ampere (A).
In circuits, current is indicated by arrows around the circuit. To find out which direction the arrows should run in, we need to know that electrons travel from the negative terminal of the power source to its positive terminal (true direction). However, when we are solving problems, the direction of the current is taken as the flow of the load from the positive terminal to the negative terminal (conventional direction).
Even today, the conventional direction of electrical current is defined as the flow from the positive terminal to the negative terminal.
Voltage (V) or Electric Potential Difference
Voltage (V) is the variable that tells us the difference in electrical energy per unit of charge between two points in a circuit. This difference can also be called the electric potential difference, potential drop, or electric tension. Its unit of measurement in the International System is the volt (V).
To indicate the energy delivered by power sources to the circuit per unit of charge, the term electromotive force is also often used; this is also measured in volts.
Resistance (R)
Resistance (R) is the variable that tells us the opposition to the passage of an electric current. Its unit of measurement in the International System is the ohm (Ω).
In real life, all of the elements in a circuit have resistance. However, in this unit we will consider the resistance of conductors and power sources to be negligible.
Ohm’s Law: The Relationship Between Variables
The three main electrical variables are related by Ohm’s Law, which states:
The current in a circuit is directly proportional to the electric potential difference impressed across its ends and inversely proportional to the total resistance offered by the external circuit.
This equation is applied to the complete circuit and to each load device in the circuit, as we will see later. We can calculate the other variables by rearranging the equation:
The table below summarizes the main variables and units, together with their most common multiples and sub-multiples, although some are not used in real circuits because they are too large (such as kA) or too small (such as mΩ).
Basic Circuit Configurations
Circuits can have various electrical components, but when we are learning the basics we generally only start with cells and resistors. The way that we put these resistors together is called the association of resistors. This gives us three types of basic circuits:
- The series circuit
- The parallel circuit
- The series-parallel combination circuit