Electricity Basics

4.2. Electricity Basics#

By the end of this section you will be able to:

The smallest amount of electric charge that can exist is that of a single electron which has a charge of \(1.602 \times 10^{-19}\) C

A constant charge is usually denoted by Q, while a charge that changes with time is written as q or q(t).

Charge can be either positive or negative.

Dissimilar charges attract

opposite charges attract

Similar charges repel

like charges repel

By the Electrostatic Force

When a particle has equal parts positive and negative charge it is electrically neutral.
When there is an imbalance, the particle is electrically charged.
In conductors, a significant number of charged particles are free to move.
When charge moves through a material, an electric current exists in the material.
However, unless an external force is applied, the electric charges in a conductor move about at random in an electric field.

Once an electromotive force is applied, charges move in a unified manner.
The electric potential difference, or voltage V, between two points is work done in moving a charge from one point to the other.
The instantaneous voltage \(v\) is described as a derivative

\[ v = \frac{dw}{dq} \]

\[ 1 \ V = \frac{1 \ J}{1 \ C} \]

Electric current, \(I\), is the rate at which charge flows through an area.

\[ I_{avg} = \frac{\delta q}{\delta t} \]

The instantaneous current is written as a derivative

\[ i = \frac{dq}{dt} \]

The SI unit for electric current is the ampere (A) and it is defined as

\[ 1 \ A = \frac{1 \ C}{1 \ s} \]

current through a wire

Conventional Current – based on the flow of positive charges, flows from positive to negative.

Conventional current is generally used in circuit analysis.

Electron Current – the movement of free electrons from negative to positive.

Direct Current (DC) - direction of charge flow is always the same

Alternating Current (AC) - direction of charge flow alternates in direction, often sinusoidal.