Basic Electricity

DESCRIPTION:Covers basic circuits and theory of fundamental concepts of electricity. Presents a practical approach to discussion of components and devices.

REALATIONSHIP OF THE COURSE TO CURRICUUM OBJECTIVES:COURSE CONTENT:

Basic ConceptsThe Basic Electrical Circuit

A simple circuit:Electrical circuits involve 3 basic components:

 is the force created by the separation of charges. Kind of like when two opposite poles of a magnet are put close together, but are separated by a short distance. A force tries to pull them together. When there are more negative charges on the inside of the membrane of a cell, there is a force driving positive charges inward to neutralize them. The unit of voltage is the volt and it is represented by the symbol V. Voltage is also called "potential" or "potential difference".

is a measure of how hard it is for charges to move in the system. In a cell, the lipid portion of the membrane is impermiable to ions, so

the resistance to current across the membrane is determined by the opening and closing of ion channels. When the channels open, the resistance decreases. When they close, resistance increases (because ions can't move through the membrane). The units of resistance are ohms and it is represented by the symbol R. (Note - When talking about channels, "conductance" is usually used instead of resistance. Conductance is the inverse of resistance (1/R), or how easy it is to pass charges. Its units are seimans [S].)

is the movement of charges. In an electrical circuit, electrons move from the negative pole to the positive pole (although electrical current is

defined as the movement of positive charges, so current is said to go from the positive pole to negative pole - go figure). In cells, current is when ions move through the membrane (usually Na+, K+, Ca2+, or Cl-).

Electrical Quantities and Units

Basic Circuits, Laws and Measurements

Basic circuits, and Ohm's law

Imagine a simple circuit, which consists only of a wire

connected between the two poles of a 3V battery. The wire is made of metal, probably copper,

and like any metal it contains millions of electrons which are free to move if made to do so by a

voltage (remember that a voltage acts as an electrical driving force). Because all of the

electrons throughout the whole wire have to move together, if there is a break in the wire no

current will be able to flow, despite the existence of the potential difference produced by the

battery.

In the real world, electrons cannot pass through a wire unimpeded: the wire itself will

inevitably have a certainresistance (measured in Ohms, symbol Ω), which makes it more

difficult to push current through it. We usually ignore the resistance of wire as it is very

low, but if we try to drive our current through e.g. a light-bulb, we would have to take

account of the significant resistance of this device.

We could model this in a circuit diagram as a single resistor R, represented as a

rectangular symbol, connected to the battery via lines which represent (theoretical) "zero-

resistance wires". A battery is conventionally represented by two parallel vertical lines, one

shorter and thicker than the other.

Wh

at if we now measure voltages in the circuit? If we record from any two points on the same

length of wire on the left hand side, we will record zero volts. This is because there is no

potential difference along that bit of wire – there is no resistance between the two points which

would cause the potential to change, and so the potential of the wire at any point is the same

as the potential of the pole of the battery to which the wire is connected. You would also record

zero volts if you put both recording electrodes anywhere on the piece of wire on the right hand

side.

However, if you put one electrode on the wire

on the left, and one on the wire on the right, we now record +3V: the potential difference

between these two points must be the same as the potential difference between the two poles

of the battery. This must also be the potential difference across the resistor!

So the voltage across the resistor, which represents the "driving force" pushing current in

the form of electrons through it, is 3 volts. But how much current actually flows? We would

need to know what the value of the resistance was: for a given driving force, less current

will flow if you have a bigger resistor to push it through. This relationship is expressed

in Ohm's law, voltage = current x resistance, the most important law in electronics, and

perhaps in physiology too.

Ohm's law: V = IRFor example, if the voltage is 3V and the resistance is 6 Ω, we can calculate using Ohm's

law that the current must be 0.5 amps. But if the resistance were only 1 Ω, the current

would be 3 amps: a smaller resistance means that more current can flow, for the same

driving force (voltage) supplied by the battery.

Circuit Component

Multiple-Load Circuits Complex-Circuit Analysis Magnetism and Electromagnetics Power in AC Circuits Capacitance Inductance Transformers R C and L Circuits Electric Motors Instruments and Measurements

LEARNER OUTCOMESUpon successful completion of this course students should have:

A basic understanding of the use of meters and test equipment used to measure electrical quantities.

A basic understanding of voltage, current resistance and power in dc circuits and network analysis using Ohms Law, and Kirchoff’s Laws.

A basic understanding of magnetic field theory. A basic understanding of inductance, capacitance and impedance in ac circuits

and network analysis. A basic knowledge of motors and transformers

EVALUATION1. Attendance, homework and class-work, lab exercises, projects, quizzes and test