Some Electrical Notes On Resistance

(Disclaimer: these are not teaching notes and the examples I give  are more detailed. They are taken from John Birds Books Fundamentals of Electrical and Electronic Engineering.)

These notes do not explain everything, (for that you will have to see me), but show some of the less than obvious things in the resistance section, of a basic electrical and electronic engineering course.


The Unit of charge is the coulomb with the symbol (C) this is equivalent to 1 Ampere second.


1 Ampere is a charge of 1 C flowing in one second.


1 volt is 1 joule per coulomb. One volt is defined as the difference in potential between two points in a conductor which when carrying a current of one ampere, dissipates a power of 1 watt.



This is measured in Siemens the unit is  (G) 1G=1/R

Kilowatt Hours

When dealing with large amounts of energy the unit used to measure it is Kilowatt hours (Kwh).


Current Flow

Potential difference between two points in a circuit is necessary for current to flow.A complete conducting  path is necessary to and from t5he sources of the electrical energy for a continuous current to flow between two points in a circuit.

Potential difference is measured in volts by the equaltion V=IR where i is the current in Amperes.

This gives rises to the equations Power(Watts)=(I^2)*R  = (V^2)/R and I=V/R

Ohms Law

Ohms Law States that the current flowing through a circuit is directly proportional to the resistance R and Applied Voltage V, provided the temperature remains the same.

So a filament lamp which is brighter at different voltages is non linear.


Where Φ = Temperature in degrees.


The resistance of a conductor depends on 4 factors, these being.

(a) The length of the conductor

(b) the temperature of the conductor

(c) The type of material

(d) the temperature of the material

Resistors in Parallel

For two resistors in parallel and in parallel with the voltage source.The total resistance is given by R=(R1*R2)/(R1+R2)= (1/ R1+1/R2)-1

I1 = I*(R2/(R1+R2)) where (I) is the total current and I1 is the current flowing through Resistor R1.

Loading Effect

This is the current drawn by measuring instrument such as a voltmeter or oscilloscope and upsets the current under test. To mitigate this it is necessary to use a high resistance probe.


If the arrow is moved to the right the voltage across the lamp is reduced, because the resistance across the potentiometer load is increased. The potentiometer if for controlling voltage.


This is for controlling current across the load. The resistance of the rheostat should be greater than that of the load.