Electrical Resistance

University High School

ConductorsPossess a great ability of conducting electricityContain free electrons that flow easily through materials when an electric field is appliedExamples of conductors: metals, some liquids, and plasma

InsulatorsConduct very small currents when a strong electric field is appliedElectrons are tightly bound and do not move freelyExamples of insulators: wood, plastic, glass, and rubber

SemiconductorsDepending on their form, they can be either better insulators or conductors. In pure form, they are better insulators, but

if an external substance is added, they become better conductors

Examples of semiconductors: Silicon, germanium, gallium, and arsenic

Equation for Electrical Resistance

Electrical Resistance = voltage drop current

R – Electrical ResistanceV – Voltage Drop I – Current

Unit of MeasurementUnit of measure for electrical resistance is the ohm.If:Potential difference is equal to 1, and;Flow of current is 1, then;Resistance is equal to 1.

Resistance ExampleA small stereo draws a current of 0.80 A when the power supply produces a potential difference of 110 V. What is the resistance of the stereo?R = ?V = 110 volts I = 0.80 amps

Resistivity DefinedMeasure of the capacity of a material to resist electrical charge

ResistivityFactors affecting resistance on a wire:Length

Longer wire, greater resistanceCross-sectional area

Smaller area, less resistanceMaterial

Higher resistivity, greater resistance

Calculating ResistivityR = p * L

A

R – Resistivityp – Rho (given constant for each material)L – LengthA – Cross-sectional area

Ohm’s LawThis law was devised to aid in simplifying electrical resistanceIs true when the following criteria are met:Resistance is constantResistance is independent of both potential

difference and current

Series CircuitsContain only one path for current flow.Charge flows from power supply into a switch, and then each light. Returns to power supply.Current is equal in all parts of the circuit.Any break will stop current throughout the entire circuit

Calculating Series CircuitsR total = R1 + R2 + ……

I total = I1 = I2 = ……

V total = V1 + V2 + …..

V1 = R1 * I1

V2 = R2 * I2

Series Circuit ExampleThere are two lamps in your home office that are supplied power through a series connection. The power supply produces 120 volts. One lamp has a resistance of 90 ohms, and the other a resistance of 70 ohms.Calculate: The current through the circuit. The voltage drop across each lamp.

Parallel CircuitsOnly partial current flows through each pathA positive lead and a negative leads starts at the power supply and ends at the last source.

Calculating Parallel Circuits

V total = V1 = V2 = …..

I total = I1 + I2 + …..I1 = (V1 / R1)I2 = (V2 / R2)

R total = R1 + R2

R1 * R2

Parallel Circuit ExampleYou have two lamps in your living room that are supplied power through a parallel connection. The power supply produces 120 volts. One lamp has a resistance of 90 ohms, and the other a resistance of 70 ohms. Calculate: The total current in the circuit. The voltage drop across each lamp. The current in each lamp

ResistorsAn electrical device that has a specific resistanceAdded into a circuit in order to provide additional resistance that is needed in a circuit.Value is shown on the outside of the resistor by a color coding system.

Resistor ValuesHas four separate colored bands; with each color representing a given value.

Band 1 – 1st significant digitBand 2 – 2nd significant digitBand 3 – multiplier; number of zeros addedBand 4 – tolerance of resistor

Determining Resistor Values

Band 1 – GreenBand 2 – RedBand 3 – Black Band 4 - Gold

Band 1 – BrownBand 2 – OrangeBand 3 – BlueBand 4 - Silver