Basic concepts of electric circuits

Electric current

Electric charges and electric current are analogous to the flow of water in a water hose or pipe. Water current is a flow of water through a water circuit (faucet, pipe or hose, etc.); electric current is a flow of electric charges through an electric circuit (wires, power supply, load, etc.). A current of 1 A (ampere) means that there is 1 C (coulomb) of electric charge passing through a given cross-sectional area of wire in 1 s (second). 1 A of current actually means there are about 6.25 x 10¹⁸ charges passing through a given cross-sectional area of wire in 1 s, because 1 C ≈ 6.25 x 10¹⁸ charges.

When early scientists started to work with electricity, the structure of atoms was not very clear, and they assumed at that time the current was a flow of positive charges (protons) from the positive terminal of a power supply (such as a battery) to its negative terminal. The concept of conventional current dates back to the early days of electrical theory. Benjamin Franklin, in the 18th century, assumed that electric current flowed from positive to negative. This idea was based on the notion of an invisible fluid moving through conductors. The conventional current flow model assumes that positive charge carriers move from the positive terminal to the negative terminal of a power source. Franklin's decision to assume current flowed from positive to negative was based on his theory that electrical charges involved a single type of "fluid" (what we now understand as charge) that could move through substances. He arbitrarily chose to call one type of charge positive and the other negative. Because this theory came long before the discovery of the electron and the understanding of subatomic particles, he had no way of knowing that the actual carriers of electric current in most materials (like metals) are negatively charged electrons moving from negative to positive. Later on, scientists discovered that electric current is in fact a flow of negative charges (electrons) from the negative terminal of a power supply to its positive terminal. In 1897, English physicist J.J. Thomson discovered the electron. He showed that cathode rays were composed of negatively charged particles much smaller than atoms. This discovery revealed that the actual flow of current in conductors was due to the movement of electrons from the negative terminal to the positive terminal. But by the time the real direction of current flow was discovered, a flow of positive charges (protons) from the positive terminal of a power supply to its negative terminal had already been well established and used commonly in electrical circuitry. This historical journey from Franklin's fluid theory to Thomson's electron discovery highlights the evolution of our understanding of electric current.

So, we now have two methods to express the direction of electric current:

1. Conventional current flow version: The current is defined as a flow of positive charges (protons) from the positive terminal of a power supply to its negative terminal.
2. Electrons flow version: The current is defined as a flow of negative charges (electrons) from the negative terminal of a power supply to its positive terminal.

It is important to emphasize that it will not affect the analysis, design, calculation, measurement, and applications of the electric circuits as long as one method is used consistently.
An ammeter is an instrument that can be used to measure current.

Electric voltage

Voltage is responsible for the pushing and pulling of electrons or current through an electric circuit. The higher the voltage, the greater the current will be. When the battery (voltage source) is connected to the load (lamp) by wires, it will produce electric current in the circuit. The positive electrode of the battery attracts the negative charges (electrons), and the negative electrode of the battery repels the electrons. This causes the electrons to flow in one direction and produce electric current. The battery is one example of a voltage source that produces electromotive force (EMF) between its two terminals. EMF moves electrons around the circuit or causes current to flow through the circuit since EMF is actually "the electron-moving force". EMF is the electric pressure or force that is supplied by a voltage source, which causes current to flow in a circuit. Voltage is symbolized by V (italic letter), and its unit is volts (non-italic letter V). EMF is symbolized by E, and its unit is also volts (V). Common sense tells us that "water flows to the lower end", so water will only flow when there is a water-level difference. It is the water-level difference that produces the potential energy for tank A, and work is done when water flows from tank A to B. Water will flow between two places in a water circuit only when there is a water-level difference. This concept can also be used in the electric circuit. Current will flow between two points in an electric circuit only when there is an electrical potential difference. The voltage or the potential difference always exists between two points.
V is the amount of energy or work required to move electrons between two points: V = W ⁄ Q [W - Joule, Q - Charge].
EMF is an electric pressure or force that is supplied by a voltage source, which causes electric current to flow in a circuit.
Although voltage and potential difference are not exactly the same, the two are used interchangeably.
Voltmeter is an instrument that can be used to measure voltage.

Resistance and Ohm’s law

Current resistance (R): the resistor (current resistance) "resists" the flow of electrical current.
– The higher the value of resistance, the smaller the current will be.
– The resistance of a conductor is a measure of how difficult it is to resist the current flow.
The lamp, electric stove, motor, and other such loads may be represented by the resistor R because once this kind of load is connected to an electric circuit, it will consume electrical energy, cause resistance, and reduce current in the circuit. There is no "perfect electrical conductor"; every conductor that makes up the wires has some level of resistance regardless of the material it is made from. There are four main factors affecting the resistance in a conductor: the cross-sectional area of the wire (A), length of the conductor (l), temperature (T), and resistivity of the material (ρ). R = ρ x l ⁄ A. Resistivity of a material is dependent upon the temperature surrounding the material. Resistivity increases with an increase in temperature for most materials, and and ρ is the resistivity (conducting ability of a material for a wire).
Ohmmeter is an instrument that can be used to measure the resistance. The resistor must be removed from the circuit to measure resistance.

Conductance G = 1⁄R [S] is the conductivity of the material. It is the opposite (the reciprocal) of resistance.

Ohm’s law states that current through a conductor in a circuit is directly proportional to the voltage across it and inversely proportional to the resistance in it, I = U⁄R. In an electric circuit, it is the voltage that causes current to flow, so current flow is the result or effect of voltage, and resistance is the opposition to the current flow.
Using a Cartesian coordinate system, voltage V (x-axis) is plotted against current I (y-axis); this graph of current versus voltage will be a straight line. The I–V characteristic of the straight line illustrates the behavior of a linear resistor, i.e. the resistance does not change with the voltage or current. When the relationship of voltage and current is not a straight line, the resultant resistor will be a non linear resistor.


Electric circuit is a closed loop of pathway with electric current flowing through it.
Requirements of a basic circuit:
● Power supply (power source): A device that supplies electrical energy to a load.
● Load: A device that is connected to the output terminal of a circuit, and consumes electrical energy.
● Wires: Wires connect the power supply unit and load together, and carry current flowing through the circuit.