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Section 1 Electrical Theory

Section 2 Concepts

Section 3 Simple AC Networks
 

Description: The purpose of this unit is to enable you to understand basic principles of electrical theory and its applications. The unit will particularly develop your knowledge regarding AC electrical networks

Author: Gates MacBain Associates


Section 1 Electrical Theory



Aims and Objectives

At the end of this section you should be able to:
  • Understand and apply basic principles of electrical theory.


Introduction to Electricity 

Before you can start exploring the basic principles of electrical theory, it is important that you understand the structure of matter. 

All materials are made up of atoms which are further divided into protons, neutrons and electrons. Protons and neutrons are found in the centre of the atom known as nucleus. Electrons surround the nucleus. Protons carry a positive charge; electrons a negative charge; and neutrons are ‘neutral’.  

Some useful websites are listed which will help you understand how an atomic structure works. A video resource is also listed which contain a simple-to-understand presentation. These resources have been given self explanatory titles and you can find these in the relevant section.    


Electric Charge and Current 

An electric charge defines the quantity of electricity. When this charge moves or flows through a material, it becomes an electrical current. This is usually due to movement of electrons.

Symbol for Electric Charge is Q while unit of measurement is Coulomb ( C )
Symbol for Electric Current is I while unit of measurement is Ampere (A) 

Relationship between the two is defined by the formula: I = Q/t where t is the time taken in seconds. 

The publications listed contain a more detailed explanation of electric charge and current and you are strongly advised to read these before attempting the tasks.  

The video resources listed under Principles of Electricity provides a good overview of the concepts. You should watch the video repeatedly as you go through the unit.    


Conductors and Insulators 

You have learnt that each atom is made up of protons, neutrons and electrons. Electrons move in orbits around the nucleus. In some materials, the freedom of movement is more the others and this relative mobility is called electrical conductivity. Materials where electrons can move freely will allow flow of electric current through them. Such a material is called an electrical conductor. A material through which electric current can not flow freely is called an insulator. 

Metals are generally considered better conductors of electricity. 

Some useful websites and video resources are listed under Conductors and Insulators which will help you understand the properties of conductors and insulators. These also contain some interesting examples of materials and, of how these are used in electrical circuits.   




Videos / DVDs



Websites



Publications

  • McMullan, R. (2007) Environmental Science in Buildings. 6th edition: Palgrave Macmillan, New York (Chapter 11 and Part II Resource 4)
  • Burberry, P. (1997). Environment and Services (Mitchell’s Building Series). 8th edition: Addison Wesley Longman Limited, England  (Chapter 15)
  • Chadderton D V. (2007). Building Services Engineering. 5th edition: Taylor & Francis, England (Chapter 13)



Self-Assessment Task

  • Illustrate the structure of matter.
  • Describe how current flows in an electrical cable.
  • Describe properties of conductors and insulators.
  • Identify 5 conductors and 5 insulators found in everyday use.





Section 2 Concepts



Aims and Objectives

At the end of this section you should be able to:
  • Be able to explain a number of the important concepts related to electricity.


Electrical Potential 

Consider water flowing in a pipe: it normally flows from high to the area of low pressure. Same can be said about flow of electrical current: there should be a difference in Charge (called Potential Difference) and there should be a force or pressure causing this difference (called Electromotive Force). Both are measured in Volts (V).   


Resistance 

It is property of a material measuring its resistance to flow of current. The symbol is R and the unit of measurement is ohm. Resistance depends upon a number of factors such as material, length, etc.  


Ohm’s Law 

According to this law, the current flowing in a metal conductor is directly proportional to the Potential Difference.  

V = I x R 

Where               
V = Potential Difference (V)
I = Current (A)
R = Resistance (ohm)   

Some useful websites and video resources are listed with self explanatory titles which will help you understand one of the most important and fundamental law. The videos have some interesting application of this law as well while the websites deal with mathematical applications.    


Cells 

Cells produce electromotive force to create Potential Difference so that Electric current can flow. Cells convert chemical energy into electrical energy. 

These can be Primary Cells (cannot be re-charged) or Secondary Cells (can be re-charged).  

Some useful websites and video resources are listed with self explanatory titles which will help to understand how cells work and their practical applications in a jargon free manner.    


Electromagnetism  

When electric current passes through a conductor, a magnetic field is produced converting the conductor into a bar magnet with North and South poles. This phenomenon is called electromagnetism and has a number of applications: from electric buzzers to motors.  

Some useful websites and video resources, listed with self explanatory titles, contain some interesting illustrations and animations. These explain the concepts as well as practical applications of electromagnetism.    


Induction 

It is a special form of electromagnetism and has number of applications such as generators and transformers. The effect of one circuit is ‘induced’ into the other through the magnetic field and without any apparent contact.   

Some useful websites and video resources, listed with self explanatory titles, will help you understand the scientific principles. The resources listed under Generators and Transformers explain the application of induction in generators and transformers.      



Videos / DVDs






Publications

  • McMullan, R. (2007) Environmental Science in Buildings. 6th edition: Palgrave Macmillan, New York (Chapter 11 and Part II Resource 4)
  • Burberry, P. (1997). Environment and Services (Mitchell’s Building Series). 8th edition: Addison Wesley Longman Limited, England  (Chapter 15)
  • Chadderton D V. (2007). Building Services Engineering. 5th edition: Taylor & Francis, England (Chapter 13)



Self-Assessment Task

Describe and explain the following:
  • Ohm’s Law and give examples to show its application.
  • The structure of a simple cell with the help of well-annotated sketches.
  • At least two common applications of electromagnetism.
  • The principles of induction.
  • The construction and operation of a typical generator and transformer.







Section 3 Simple AC Networks



Aims and Objectives

At the end of this section you should be able to:
  • Investigate simple single phase resonant & non-resonant ac electrical networks.


Introduction to Circuits 

A circuit is complete path on which the electric current can flow. A circuit, in its basic form, comprises of conductors connecting an electrical device to the supply. The layout of a circuit is shown in a geometrical manner using standard symbols. 

There are two basic layouts: Series where the same current flows through each resistor; and Parallel where potential difference or voltage across each resistor is the same. 

Components used in an ac circuit are:            

Capacitors: These consist of a pair of conductors separated by an insulator. The current flowing through a capacitor build up the charge, and the voltage. These are used in electronic circuits to allow the alternating current to pass. Capacitance (C) is measured in Farads.             

Resistors: These produce voltage proportional to the current passing through them in accordance with Ohm’s Law.             

Inductors: These are used to store magnetic energy called inductance (L) and measured in Henries  Single-phase AC Electrical Networks In an Alternating Current or AC, the movement of electric charge continuously reverses and hence the name ‘alternating’. This takes the shape of a sine or sinusoidal wave. One such example is output from a simple ac generator.   


To understand such circuits, you must learn some additional terms. 

Frequency (f): It is the number of cycles per second measured in Hertz (Hz) 

Peak Value: It is the maximum value of ac voltage or current. 

RMS value: Root Mean Square (RMS)  value is the value of effective voltage or current and is ~70% of the peak value. The voltage value of 230 Volts quoted for domestic supply is actually a RMS value. 

Power Factor: When voltage and current in an AC supply reach their maximum and zero value at the same time, they are said to be ‘in phase’. When this is not the case, some of the energy is lost thereby reducing the actual power available. Power Factor expresses the ratio between these two states. 

Power Factor (PF) = (Active Power in Watts) / Apparent Power in volt-amperes.   


Some useful websites and video resources, listed with self explanatory titles, cover important concepts about circuits from a basic to an advanced level.     


Non-resonant and resonant circuits  

According to Ohm’s Law, current (I) and voltage (V) in a circuit are proportional to each other.  Such circuits are called linear circuits. The ratio between voltage and current is the resistance (R), the value of which does not depend upon Frequency (f). 

However, the value of this ratio does depend upon frequency. Impedance (Z) is the term used to represent ratio of voltage and current when these are not ‘in phase’. Resistance can be considered as a special case of Impedance. 

If the voltage and current are out of phase by 90°, the ratio of voltage to current is called the Reactance (X).  

Electrical resonance can occur in an electrical circuit when the impedance between the input and output of a circuit is minimum (almost a zero). Such circuits can generate higher voltage than their input. The frequency at which resonance occurs is called the resonance frequency. Value of this frequency is determined by inductance, capacitance and impedance of the AC circuit.  

Resonant circuits are widely used in wireless transmission networks.     


RLC Circuits   

An RLC circuit comprises a resistor (R), Inductor (L) and a capacitor (C) and hence the name RLC. Similarly, other combinations are RL and RC. These circuits are used as oscillators, filters and tuners.   

In an RLC circuit, there is large voltage across the inductor and the capacitor while the resistance is low. However, these large voltage values almost cancel each other giving a small total voltage. In this case, it is therefore possible to produce a large voltage with a small voltage source.   

Some useful websites and video resources, listed with self explanatory titles, cover important concepts about resonance in AC circuits from a basic to an advanced level. The video resources include practical construction of a RLC circuit.



Videos / DVDs



Websites


Publications

  • McMullan, R. (2007) Environmental Science in Buildings. 6th edition: Palgrave Macmillan, New York (Chapter 11 and Part II Resource 4)
  • Burberry, P. (1997). Environment and Services (Mitchell’s Building Series). 8th edition: Addison Wesley Longman Limited, England  (Chapter 15)
  • Chadderton D V. (2007). Building Services Engineering. 5th edition: Taylor & Francis, England (Chapter 13)


Self-Assessment Task

With the aid of a drawing illustrate:
  • a typical series and a parallel circuit using standard symbols
  • the working of an RLC circuit
Describe the following:
  • the significance of Power Factor in electricity distribution
  • the phenomenon of electrical resonance.
  • at least three applications of resonant circuits






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