Engr 1100 Electrical Engineering Module
Charge
Recall from your high school science or physics that atoms are comprised of a nucleus (containing
neutral particles and positively charged particles or protons.) surrounded by a cloud of negatively
negatively charged particles or electrons.
The force acting on one charged particle due to another is defined as an electric force. This electric
force may be one of attraction, (opposite charges) or repulsion (same charges). This is shown in
Figure 1, where F represents the electric force.
Typically, an atom has the same number of protons and electrons, therefore it is electrically neutral (has
no charge). An atom may become positively charged by loosing electrons or negatively charged by
gaining electrons.
We use the unit Coulumbs (C) to measure electric charge. The Coulumb is defined as the charge
possessed by 6.242 x1018 electrons. In other words, a single electron will have a charge of 1.602 x 10‐19
C.
The symbol used for electric charge is typically Q or q. Charge may
F
F
F
F
F
F
Figure 1: Electric force of attraction or repulsion between charges
Current
The rate of flow of charge through a specific area is defined as current. Current is typically given the
symbol, I.
The average current that passes through an area in a given time can then be written as
1
where Q is the amount of charge passing through that area for a specified time interval t.
The SI unit for current is the ampere (A). From equation 1, we can write
Copyright©2010, Dr. Cameron Coates, Armstrong University. This article not be reproduced without permission of author.
Engr 1100 Electrical Engineering Module
1
1
1
2
Therefore 1 Ampere is by definition the amount of current generated when a charge of 1C passes
through a particular area for 1 second. A typical flashlight uses 3 mA, (milli‐Amperes), a toaster may use
around 8A while an electric dryer may use 15A or more.
Current is always through a given area, similar to a river flowing through a channel. For a Direct Current
(DC), the flow rate is always the same (one direction and not varying with time), for an Alternating
Current the flow of charge will alternate in direction over a given time (varying magnitude and direction
with time). Current may also flow as other functions of time eg. exponential, quadratic, random.
Voltage
Several electric charges will move in a single direction through a conductor only if there is a force
powerful enough to overcome the forces of attraction or repulsion between the moving electrons and
and electrons or protons in the atoms of the conductor. This force may be provided by creating a
substantial difference in the levels of opposite charge between the beginning and end points of the flow.
This difference is referred to as a potential difference of voltage. The magnitude of the voltage can be
thought of as a measure of the energy needed for the current to flow from one point to another. This is
similar to the difference in height needed for a river to flow from one point to another.
Voltage is therefore measured between two points (i.e. across an element), as shown in Figure 2.
Vab
A
Circuit Element
Iab
B
Figure 2: Voltage across and current through an element
Resistance
In 1826, Georg Simon Ohm, a german physicist discovered the fundamental relationship between
current and voltage. This now known as Ohm’s law. Ohm discovered that the current through different
materials will not be the same if the voltage is the same. So each material provided a different
“resistance” to the current flow. The symbol for resistance is R, and Ohm’s law tells us that the voltage
across a conducting element is proportional to the current through this element, and the constant of
proportionality is defined as the resistance of the material
3
Copyright©2010, Dr. Cameron Coates, Armstrong University. This article not be reproduced without permission of author.
Engr 1100 Electrical Engineering Module
Resistance is measured in Ohms, given the symbol Ω. Therefore 1 Ω is the magnitude of resistance
between two points on a conductor when a potential difference of one volt between them produces a
current of one ampere. Resistance is a property of the material.
1Ω
1
1
4
Circuits
An electric circuit is a closed loop in which a voltage or current source is connected to a conducting
element. This is shown below in Figure 3. A typical voltage source is a battery, the battery provides a
potential difference across its terminals. Note that the power source provides a gain in potential
(voltage) while the resistor causes a drop in potential (voltage).
Figure 3: Simple DC Circuit
Visit the following websites for examples and more information:
1. http://www.ndt‐ed.org/EducationResources/HighSchool/Electricity/atommodels.htm
2. http://www.school‐for‐champions.com/science/elect_ohms_law.htm
Copyright©2010, Dr. Cameron Coates, Armstrong University. This article not be reproduced without permission of author.