As we all know electrical circuits consist of interconnected electrical elements. Current is measured in amperes (A), represents the flow of electric charge around a closed loop.
This flow is driven by a potential difference (electromotive force, EMF) measured in volts (V).
We know that, all materials are composed of atoms, however these atoms also consist of protons, neutrons, and electrons.
Protons have a positive electrical charge, while neutrons are neutral. Conversely.. electrons possess a negative electrical charge.
The attractive forces between the atomic nucleus and the outer shell electrons bind the atoms together.
Within an atom, protons, neutrons, and electrons remain stable together. But, if separated, they feel a pull and want to get back together.
This pull creates a potential difference.
On the other hand, when a closed circuit is formed, free electrons are attracted to protons and start to move, flowing back towards them.
This flow of electrons is what we call electric current. However the material across which the electrons travel is never a perfect path which slows them down.
This slowdown is called resistance.
So, all basic electrical and electronic circuits rely on three important and relevant electrical quantities: voltage (V), current (I), and resistance (Ω).
What is Electrical Voltage
In DC circuits theory we define voltage (V) as the potential energy stored within an electrical supply as an electric charge. This voltage acts as the force that pushes electrons through a conductor. The greater the voltage the greater its ability to “push” these electrons through the circuit.
Since energy allows work to be done, we can describe this potential energy as the work (measured in joules) required to move electrons as current around a circuit, from one point (node) to another.
The difference in voltage between any two points, connections, or junctions (nodes) in a circuit is known as the potential difference (pd), commonly called the voltage drop.
In a DC circuit theory we measure the potential difference (pd) between two points in volts, with the circuit symbol V or v. While energy (E) can sometimes be used to indicate a generated electromotive force (emf), “voltage” is the more common term. The greater the voltage, the greater the pressure (pushing force) on the electrons and their capacity to do work.
We can categorize voltage sources based on their variation over time.
A constant and unchanging source is called a DC voltage. Conversely, a voltage source that varies periodically in amplitude is called an AC voltage.
Regardless of whether it’s AC or DC, we measure voltage in volts.
One volt is defined as the electrical pressure required to push a current of one ampere through a resistance of one ohm.
We normally express voltages in volts but for very small or large values, we use prefixes. For example microvolts (μV = 10-6 V) denote very small voltages while kilovolts (kV = 103 V) indicate much larger ones.
It’s important to note that voltage can be either positive or negative in amplitude. On the source side, batteries, power supplies, or solar cells produce a DC (direct current) voltage with a fixed value and polarity.
These come in various voltages such as 5V, 12V, or -9V. In contrast, AC (alternating current) voltage sources like those powering homes and industries have a value related to the power they supply.
The voltage and frequency of AC electricity used in homes varies by region. For instance the UK uses 230 volts AC (230V), while the USA uses 110 volts AC (110V).
In electronics, we typically rely on low voltage DC battery supplies to power our circuits. These DC voltages usually range between 1.5V and 24V.
In order to indicate these constant voltage sources in our circuits, we use a symbol that looks like a battery with positive (+) and negative (-) signs.
These signs help us to understand the polarity direction.
For alternating voltage sources, on the other hand, we use a circle with a sine wave inside as its circuit symbol.
Symbols of Voltage
In electronics, we can make an analogy between a water tank and a voltage source.
The higher the water tank is placed above the outlet, the greater the water pressure, because more potential energy is released.
Also, the higher the voltage, the greater the potential energy available because more electrons are ready to flow. We always measure voltage as the difference between two specific points in a circuit. That difference is often called the “voltage drop”.
It is important to remember that a circuit can have voltage without current. However there can be no current without voltage. Therefore, voltage sources, whether DC or AC, prefer open or semi-open circuit conditions where they can maintain their potential.
On the contrary, they do not like short circuits because they can overload and possibly destroy them..
What is Electrical Current
In DC circuit theory, we make use of electric current, denoted by I. This current represents the movement or flow of electric charge, measured in amperes (A).
Think of it as a constant stream of electrons (tiny negatively charged particles inside atoms) drifting around a circuit and being “pushed” by a voltage source.
Although electrons actually move from negative (-) to positive (+) terminals, to facilitate circuit analysis we use the conventional concept of current, which assumes that current flows from positive to negative.
In circuit diagrams we often see an arrow next to the current symbol (I or i) to indicate its normal flow direction, not necessarily the actual flow of electrons.
Understanding Conventional Current flow
Early ideas about electricity were a bit off the mark. Scientists back then thought current flowed like positive charges moving from positive to negative in a circuit.
This imagined flow, though not entirely accurate, became the standard way to depict current, we call it conventional current flow.
Think of it as a simplified way to understand circuits, eventhough its opposite the actual movement of electrons. In reality the current is carried by tiny negatively charged particles called electrons.
These electrons actually flow from negative to positive in a circuit.
Circuit diagrams use arrows on components to show the direction of conventional current flow, eventhough it might seem backwards compared to electron flow.
This consistency helps us analyze circuits more easily. So, while the real action is with the electrons moving negative to positive, we often use conventional current flow (positive to negative) for circuit analysis.
Understanding the flow of Electrons
Early ideas about current flow differed from reality.
Scientists believed current flowed as positive charges moving from positive to negative in a circuit.
This conceptual flow, though not entirely accurate became the standard way to depict current, we call it conventional current flow.
Think of it as a simplified way to understand circuits, eventhough it’s opposite the actual movement of electrons.
In reality the current is carried by tiny negatively charged particles called electrons.
These electrons actually flow in the opposite direction of conventional current, moving from the negative terminal (cathode) of the battery to the positive terminal (anode).
This is because electrons have a negative charge and are attracted to the positive terminal. This flow of electrons is called electron current flow.
Circuit diagrams use arrows on components to show the direction of conventional current flow.
This might seem counterintuitive compared to the actual flow of electrons but it helps maintain consistency for circuit analysis.
Both conventional current flow and electron flow are used in many textbooks. As long as the direction is used consistently, it doesnt affect the behavior of the current within the circuit.
Generally conventional current flow (positive to negative) is considered easier to grasp initially.
Current sources are circuit elements that provide a specified amount of current measured in amperes (Amps).
One Amp (or ampere) is defined as the number of electrons (charge denoted as Q in Coulombs) passing a certain point in the circuit in one second (t in Seconds).
Electrical current is generally expressed in Amps, with prefixes like microamps (µA = 10-6 A) or milliamps (mA = 10-3 A) used to denote smaller currents.
It’s important to note that electrical current can be either positive or negative in value depending on its direction of flow around the circuit.
Current that flows in a single direction is called Direct Current (DC).
Current that alternates back and forth through the circuit is known as Alternating Current (AC)
Whether AC or DC, current only flows through a circuit when a voltage source is connected, and its “flow” is limited by both the circuit’s resistance and the voltage source pushing it.
Alternating currents (AC) and voltages are periodic, meaning they vary over time.
For AC circuits, we use the concept of RMS (Root Mean Square) current, denoted as Irms.
This value represents the effective current that produces the same average power loss as an equivalent DC current (direct current), denoted as Iaverage.
Current sources behave oppositely to voltage sources. They prefer short circuit or closed circuit conditions where they can deliver their specified current.
In contrast, they dislike open circuit conditions because no current can flow in this scenario.
Heres another way to think about it using our water tank analogy:
Current is like the water flow rate through a pipe. The flow remains consistent throughout the pipe.
A higher flow rate translates to a higher current. Importantly, current cannot exist without voltage.
Just like a current source any source (DC or AC), prefers a short circuit or semi short circuit condition where it can deliver current. Open circuits act as roadblocks, preventing current flow.
Understanding Resistance in DC Circuits
In circuits certain materials resist the flow of electrical current (more precisely the movement of electric charge Q).
This opposition is termed as resistance (R) measured in ohms (Ω) denoted by the Greek symbol Omega.
We use prefixes for larger resistances, kilohms (kΩ) equal to 103 Ω and megaohms (MΩ) equal to 106 Ω.
Importantly resistance can never be negative, it’s always a positive value.
Understanding Resistor Symbols in DC Circuits
The resistance value of a resistor depends on the relationship between the current (I) flowing through it, and the voltage (V) across it.
This relationship can be expressed through Ohms Law.
Good conductors like copper, aluminum, or carbon offer low resistance (typically 1Ω or less). This allows easy current flow through them with minimal voltage drop.
Oppositely, poor conductors like glass, porcelain, or plastic have high resistance (mostly 1MΩ or more). This greatly hinders current flow through them and causes larger voltage drops.
We can categorize materials based on their electrical conductivity.
Conductors like metals allow electricity to flow easily.
Insulators like plastic, block electricity.
Semiconductors such as silicon or germanium fall in between. Their conductivity is halfway between a conductor and an insulator, hence the name. We use semiconductors to make diodes and transistors.
Now lets talk about resistance. It can be either linear or nonlinear but never negative. Linear resistance follows Ohm’s Law, meaning the voltage across the resistor is directly proportional to the current.
Nonlinear resistance don’t follow Ohm’s Law, but it still has a voltage drop related to the current, just not in a simple linear way.
Resistance is a constant value. It opposes current flow and doesnt care how often the current flips (frequency).
In AC circuits, a resistor’s resistance is the same as its DC resistance, so it’s always positive.
Think of resistance as the slope in Ohm’s Law, slopes can never be negative.
Resistors are passive components. They can not give out power or store it. Instead they absorb power, which usually becomes heat or light.
No matter which way the electricity flows, the power a resistor uses is always positive.
For tiny resistances like those in milliohms (mΩ), it is often easier to work with conductance (G) instead.
Conductance just tells us “how well something conducts electricity,” and it is the opposite of resistance (1/R).
Easy current flow simply refers to high conductance (G). It is just another way of saying how good something is at conducting electricity.
This is mathematically the inverse of resistance (G = 1/R). This relationship allows us to express current flow (I) using conductance and voltage (V) with the formula: I = V * G.
So high conductance (like copper) means good conductor. This allows for high current flow, and low conductance (like wood) means bad conductor, resulting in low current flow.
The standard unit of measurement for conductance is the Siemens (S).
There is also an older unit called the mho (℧) which is simply “ohm” spelled backward. It’s symbolized by an upside-down ohm sign. Although both units are valid, Siemens (S) is the preferred unit.
We can even express power using conductance with the following formulas: P = I² / G or P = V² * G. Here, P represents power, I represents current, and V represents voltage.
Imagine a circuit as shown above, with constant resistance (R). If we plot the voltage (V) across the resistance on the vertical axis and the current (I) flowing through it on the horizontal axis, we will get a straight line, as shown below.
The amazing thing is, the slope of this straight line relationship between current and voltage is exactly equal to the value of the resistance (R).
Conclusions
We have been learning about DC circuits and you might have noticed the connection between voltage (V), current (I), and resistance (R).
This relationship is the foundation of Ohm’s Law, which is expressed by the formula: I = V / R.
How it works in a fixed-resistance circuit:
- Voltage and Current: If we increase the voltage (V), the current (I) will also increase according to Ohm’s Law (I ∝ V). This means high voltage leads to high current, and low voltage leads to low current.
- Resistance and Current: With a set voltage, increasing the resistance (R) reduces the current (I) (I 1/∝ R). So, high resistance means low current, and low resistance means high current.
Current Flow Summary: We can see that current flow (I) is directly proportional to voltage (V↑ causes I↑) but inversely proportional to resistance (R↑ causes I↓).
Units:
- Voltage (V): Measures potential energy difference between two points in a circuit (often called “volt-drop”). Unit: Volts (V) or electrical energy (E).
- Current (I): It is the continuous flow of charge. Unit: Amperes (A) or Amps. It’s directly proportional to voltage (I ∝ V).
- Resistance (R): It is the Opposition to current flow. Unit: Ohms (Ω). It is inversely proportional to current (I 1/∝ R). Low resistance indicates a conductor, and high resistance indicates an insulator.
The following table provides quick reference guide for the above explanation:
| Quantity | Symbol | Unit of Measure | Short-Form |
| Voltage | V or E | Volt | V |
| Current | I | Ampere | A |
| Resistance | R | Ohms | Ω |
References: Direction of Electricity
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