SECTION 8.3 – Resistance and ohm’s Law

SECTION 8.3 – Resistance and ohm’s Law As electrons flow through a circuit, they do not do so unobstructed. As we see with insulators and conductors, some objects transfer electrons more efficiently than others. In circuits, we use the term resistance (R) to measure how easily electrons flow in a circuit.

LIGHT BULBS For example, the filament tungsten (W) in a light bulb resists the movement of electrons and in the process creates a large amount of heat that makes the filament glow to create light.

A simple hydraulic analogy The water flowing through pipes is like the electrons flowing in a circuit. Voltage can be thought of as a pump, pushing water through the system (i.e. more pressure); the larger the pump (i.e.: battery) the more water (electrons) goes through the system. The sizes of the pipes restrict the flow of water in the system: the larger the pipe (lower resistance) the easier it is for the water to flow. The smaller the pip (i.e. higher resistance) the harder it is for water to flow.

In 1827, Georg Ohm, a German physicist found a relationship between voltage (V), current (I) and resistance (R). Now referred to as Ohm’s Law, it states that electrical resistance is directly proportional to voltage and inversely proportional to current (or the ratio of voltage to current). The unit for resistance is the ohm (Ω).

FORMULA FOR OHM’S LAW In words: 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒= 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 As a mathematical equation. 𝑅= 𝑉 𝐼 How can we manipulate these equations?

EXERCISES: SOME USEFUL CONVERSIONS “milli” means thousandth (0.001) “cent” means hundredth (0.01) “deci” means tenth (0.1) BASE UNIT (e.g. meter) (1) “hecto” means hundred (100) “kilo” means thousand (1000) “mega” means million (1 000 000) “giga” means billion (1 000 000 000) “tera” means trillion (1 000 000 000 000)

EXERCISES Try the exercises in your notes package. Work with a neighbour if you like!

When designing circuits it is important to have a precise amount of current or voltage at a particular point in order for some electrical components to work properly. In order to regulate the current and voltage at a particular point a resistor is used. A resistor is an electrical component that has specific resistance. Resistance can be measured with a special device call an ohmmeter.

Resistors serve a very important function in circuits and as a result there are many different sizes. Since resistors are so small, colour bands are used to identify its size. Each colour corresponds to a particular number and its position on a resistor is the placeholder of that digit.

READING RESISTORS To read the resistor correctly, the first band represents this first digit, the second band the second digit, the third band is the power of ten (X10) multiplier (i.e. the number of zeros that follow the second digit). There is also a fourth band on the resistor that represents how accurate the resistor is to its indicated value.

RESISTOR READING – COLOUR TABLE Resistor Colour Coding Numerical Value Black Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9

Gold = 5% Silver = 10% No 4th band = 20% RESISTOR ACCURACY Gold = 5% Silver = 10% No 4th band = 20%