Fundamentals of Electric Circuits

Introduction

An electric circuit is formed by interconnecting components having different electric properties. It is therefore important, in the analysis of electric circuits, to know the properties of the involved components as well as the way the components are connected to form the circuit. In this introductory chapter some ideal electric components and simple connection styles are introduced. Without resort to advanced analysis techniques, we will attempt to solve simple problems involving circuits that contain a relatively small number of components connected in some relatively simple fashions. In particular we will derive a set of useful formulae for dealing with circuits that involve such simple connections as ``series'', ``parallel'', and ``delta''.

SI base units

an internationally accepted system of physical units, using the meter, kilogram, second, ampere, kelvin, mole, and candela as the basic units of length, mass, time, electric current, temperature, amount of substance, and luminous intensity

SI Prefixes

SI prefixes are used to form decimal multiples and submultiples of SI units.

They should be used to avoid very large or very small numeric values.

The prefix attaches directly to the name of a unit, and a prefix symbol attaches directly to the symbol for a unit.

CHARGE, Q   &  CURRENT,  I

                              

This is measured in units called coulombs, C, after Charles Coulomb who made the first precise measurements of the effects of charges on each other way back in 1785. In chemistry thus far, charge is merely referred to as 1+ or 2- or +3, it is enough to merely refer to the number of electrons lost or gained.

A man by the name of Robert Millikan actually made a precise measurement of the electronic charge value early this century.

It turned out to be extremely small, -1.6x10-19 coulombs. From what we know of the atom and the number of them in a crystal, we should not be so surprised.

Current (I ) and charge are linked through time. Effectively, to measure current we count the passing electrons in one second and ask ourselves, "how much charge has passed?"

The formula which applies linking these elements is

Current = Amount of charge passing 

             time in seconds

The unit of current is coulombs per second or ampere ( amp, A).

In symbol terms

I = ΔQ 

    t

POTENTIAL DIFFERENCE OR VOLTAGE

 We have all heard of "voltage" V, more correctly potential difference, PD. Most small modern batteries are supposed to be 1.5 or 9 volts. A car battery is slightly over 12 volts. The mains power is supposed to be 240 volts. So, what is this value, how different is it to current?

Remember the rafts - they had to be lifted to the top of the run by a continuously moving ramp or conveyer belt - they were given gravitational potential energy- which they then lost mostly in the rapid. The amount of energy would depend on the mass of the raft as well as the height ( Ep = mgh ). In fact the height alone, all other things being standardised (m,g) is a measure of the energy lost.

Electrons however, move because of the rules of charges, not gravity. They move because electrical potential energy is gained or lost. This is not easy to visualise because of our lack of experience with this energy so -we will translate them into terms of rafts and rivers.

The electrons are given electrical potential energy by the battery. They lose a tiny amount in the wiring because no wiring you will come across is perfect ( perfect wiring does exist - it is called a "superconductor") but loses the vast bulk of the energy in the player or lamp or refrigerator to make that device do the work you want - play music or whatever.

Potential Difference = change in energy between two points in the circuit 

                                    amount of charge involved in measuring energy

PD (V) = ΔEnergy of Q 

              Q 

Power and Energy

What is power?

Power is the rate of using or supplying energy:

There are three ways of writing an equation for power, current and voltage:

where:

P = power in watts (W) V = voltage in volts (V) I  = current in amps (A)

or:

P = power in milliwatts (mW) V = voltage in volts (V) I  = current in milliamps (mA)

Using Ohm's Law V = I × R   we can convert P = I × V to:

What is Energy?And by the way you can use the PIV triangle to help you remember the three versions of the power equations. Use it in the same way as the Ohm's Law triangle. For most electronic circuits the amp is too large, so we often measure current in milliamps (mA) and power in milliwatts (mW). 1mA = 0.001A and 1mW = 0.001W. 

The amount of energy used (or supplied) depends on the power and the time for which it is used: