Basic Electrical Circuit Principles: Current, Voltage, and Power

Basic operations of circuits or electronic devices require a stable power source to provide voltage and current, which together constitute the circuit’s power. Current, voltage, and power are key concepts in circuit design and performance analysis. In this article, TechSparks delves into these fundamental circuit concepts to help readers better understand the foundational knowledge in the field of electronics. Let’s get started!

Table of Contents

Current

Current, represented by the symbol “I,” is a physical quantity used to describe how charge particles move and their speed within an electrical circuit. It is measured in Amperes (A). Current is typically defined as the amount of charge passing through the cross-section of a conductor per unit time, with the formula:

                                                                            I = q/t

Where

  • I: Current intensity, Amperes (A)
  • q: Amount of charge passing through the cross-section of the conductor, Coulombs (C)
  • t: Time elapsed, seconds (s)

In circuit analysis, there are generally two ways to represent the direction of current: electron flow and conventional current flow. These two directions are relative and can sometimes lead to confusion.

  • Electron flow direction: Describes the actual movement direction of electrons (negative charge carriers). According to this direction, current is considered to flow from the negative terminal of the power source to the positive terminal.
  • Conventional current direction: This is an abstract concept established historically during the early study of electricity. According to this direction, current is considered to flow from the positive terminal of the power source to the negative terminal, opposite to the actual movement of electrons.

Externally in a power source, current is regarded as flowing from the positive terminal (typically labeled as “+” on the source) to the negative terminal (usually labeled as “-“). However, internally within the power source, current actually flows from the negative terminal (“-“) back to the positive terminal (“+”).

Voltage

Voltage, defined as the work done by an electric field in moving a unit positive charge from one point to another, is typically represented by the symbol “V.” Its unit of measurement is the Volt (V). Voltage is a scalar quantity that signifies the difference in electric potential energy associated with the movement of charge between two points.

                                                                            V = W/Q

Where

  • V: Voltage, measured in Volts (V)
  • W: Work done, measured in Joules (J)
  • Q: Charge, measured in Coulombs (C)

In electrical circuit analysis, it’s important to establish which end of a component has the higher voltage (high potential) and which has the lower voltage (low potential). This choice is arbitrary, but once made, it remains consistent throughout the analysis. Typically, the high potential end is denoted as “+,” while the low potential end is denoted as “-.” To clearly indicate the reference polarity of voltage, arrows can sometimes be used, with the arrow pointing from the “+” end to the “-” end. This helps provide a clear visual indication of which end is at high potential and which is at low potential.

Another way to represent voltage is by using double subscripts, such as “Vab.” In this example, point a is designated as the “+” end, while point b is designated as the “-” end, defining the reference polarity of voltage.

uab example

Power

Power, represented by the symbol “P,” is the rate at which energy is transferred or converted per unit of time. It is used to indicate the speed of energy consumption or generation in an electrical circuit. Power is typically measured in Watts (W), with the international unit symbol “W.” The formula for calculating power is as follows:

                                                                            P = I × V

Where

  • P: Power, measured in Watts (W)
  • I: Current, measured in Amperes (A)
  • V: Voltage, measured in Volts (V)

However, for certain circuit components, the directions of voltage and current may not align with a predefined reference direction. Therefore, when calculating the power absorbed by these components, the relationship between voltage and current directions needs to be considered. If the directions are not aligned with the predefined reference direction, the power absorbed by the circuit component is calculated based on the actual directions of voltage and current, rather than the predefined reference direction.

If p > 0, it indicates that the component is absorbing power; if p < 0, it means the component is absorbing negative power, essentially delivering power.

Assuming that the directions of current and voltage align with the associated reference directions, the electrical energy absorbed by the two-terminal component over the time interval t1 to t2 is given by:

Electric energy absorbed by two-terminal components

Where

  • W: Represents the absorbed electrical energy
  • p(t): Represents the power of the component at time t

A two-terminal component (or circuit) is said to satisfy the following condition for all time intervals t:

another formula

This means that over the entire operating cycle of the component, the total energy it absorbs is zero. In other words, the energy absorbed by the component at one point in time is released at other points in time, ensuring the conservation of total energy.

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