Voltage and Current - Key Concepts (ELET 2307, Chapter 2)

Objects become charged when they have a net charge, meaning an imbalance between the number of protons and electrons; this occurs when electrons are stripped from or added to parent atoms.
Methods of Charging:
- Friction (Triboelectric Effect): Rubbing two different materials together, causing electrons to transfer from one to the other (e.g., spinning clothes in a dryer, rubbing an ebonite rod with fur).
- Conduction: Direct contact between a charged object and a neutral object, allowing charge to transfer.
- Induction: Rearrangement of charge in a neutral object due to the proximity of a charged object, without direct contact.
Charge is a fundamental property of matter, is conserved, and is quantized.
The unit of charge: Coulomb (C).

Coulomb's Law quantifies the electrostatic force between charges. Like charges repel; unlike charges attract.
For two point sources, the magnitude of the electrostatic force $(F)$ is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them: $F = k rac{|Q1 Q2|}{d^2}$ where:
- $Q1$ and $Q2$ are the magnitudes of the point charges.
- $d$ is the distance between the centers of the two charges.
- $k$ is Coulomb's constant, approximately $8.987 imes 10^9 ext{ N} ext{m}^2/ ext{C}^2$ . This constant can also be written as $k = \frac{1}{4\pi\epsilon0}$ , where $\epsilon0$ is the permittivity of free space.
The force acts along the line connecting the two charges.

One coulomb (C) represents a very large amount of charge:
$1\ \text{C} = 6.24 \times 10^{18} \text{ electrons (or protons)}$
The elementary charge, which is the charge on a single electron (or proton) and the smallest observable unit of charge, is approximately:
$-1.602 \times 10^{-19}\ \text{C (for an electron)}$
$+1.602 \times 10^{-19}\ \text{C (for a proton)}$
Charges typically encountered in everyday static electricity are often in microcoulombs ( $\mu\text{C}$ ) or nanocoulombs ( $\text{nC}$ ).

When charges are transferred from one body to another, an electric potential difference, commonly known as voltage (V), is established; it provides the 'push' responsible for establishing current.
Voltage is defined as the energy (W) per unit charge (Q) and represents the potential energy difference between two points in an electric field:
$V = \frac{W}{Q}$
Its unit is the Volt (V), where $1\ \text{V} = 1\ \text{Joule/Coulomb (J/C)}$ .
When a charge moves in an electric field from one potential to another, work is done by or on the field.

A cell is an electrochemical device that converts stored chemical energy into electrical energy, creating a potential difference across its terminals.
A battery consists of one or more cells connected together (typically in series) to provide a desired voltage and current capacity.
Symbols are used to illustrate a cell and a battery in circuit diagrams (e.g., a 1.5 volt battery).
The concept is that the voltage source actively separates charges via internal chemical reactions, thereby maintaining an electric potential difference across its terminals to drive current through an external circuit.

Current (I) is defined as the rate of flow of electric charge; it is the ordered movement of charge carriers (e.g., electrons in metals, ions in solutions).
Direct Current (DC) is current that flows in only one direction, such as that produced by batteries or solar cells.
More electrons per second passing through a cross-section of a circuit means a higher current.
Definition (rate of flow):
$I = \frac{dQ}{dt}$
This means current is the amount of charge $(dQ)$ passing a point in a given amount of time $(dt)$ .
Example: If $Q = 2.0\ \text{C}$ passes a point in $t = 5.0\ \text{s}$ , then the current is:
$I = \frac{2.0\ \text{C}}{5.0\ \text{s}} = 0.40\ \text{A}$

One ampere (A), named after André-Marie Ampère, is the SI unit of electric current. It is defined as the current in a circuit when one coulomb of charge passes a given point in one second.
$1\ \text{A} = \frac{1\ \text{C}}{1\ \text{s}}$

Conventional current direction (I) is a historical convention, which assumes positive charge flows from the positive terminal to the negative terminal of a voltage source in an external circuit.
This convention was established before the discovery of the electron.
Electron flow direction is opposite to conventional current; electrons (which are negatively charged) flow from the negative terminal to the positive terminal.
For most circuit analysis, conventional current is used, and it yields the same results as long as consistently applied.