Electrical Resistance University High School Conductors Possess a

Electrical Resistance University High School

Conductors Possess a great ability of conducting electricity Contain free electrons that flow easily through materials when an electric field is applied Examples of conductors: l metals, some liquids, and plasma

Insulators Conduct very small currents when a strong electric field is applied Electrons are tightly bound and do not move freely Examples of insulators: l wood, plastic, glass, and rubber

Semiconductors Depending on their form, they can be either better insulators or conductors. l In pure form, they are better insulators, but if an external substance is added, they become better conductors Examples of semiconductors: l Silicon, germanium, gallium, and arsenic

Equation for Electrical Resistance = voltage drop current R – Electrical Resistance l V – Voltage Drop l I – Current l

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

Resistance Example A 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=? l V = 110 volts l I = 0. 80 amps l

Resistivity Defined Measure of the capacity of a material to resist electrical charge

Resistivity Factors affecting resistance on a wire: l Length l l Cross-sectional area l l Longer wire, greater resistance Smaller area, less resistance Material l Higher resistivity, greater resistance

Calculating Resistivity R=p*L A R – Resistivity p – Rho (given constant for each material) L – Length A – Cross-sectional area

Ohm’s Law This law was devised to aid in simplifying electrical resistance Is true when the following criteria are met: Resistance is constant l Resistance is independent of both potential difference and current l

Series Circuits Contain 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 Circuits R total = R 1 + R 2 + …… I total = I 1 = I 2 = …… V total = V 1 + V 2 + …. . l V 1 = R 1 * I 1 l V 2 = R 2 * I 2

Series Circuit Example There 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: l l The current through the circuit. The voltage drop across each lamp.

Parallel Circuits Only partial current flows through each path A positive lead and a negative leads starts at the power supply and ends at the last source.

Calculating Parallel Circuits V total = V 1 = V 2 = …. . I total = I 1 + I 2 + …. . l I 1 = ( V 1 / R 1) l I 2 = ( V 2 / R 2) R total = R 1 + R 2 R 1 * R 2

Parallel Circuit Example You 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: l l l The total current in the circuit. The voltage drop across each lamp. The current in each lamp

Resistors An electrical device that has a specific resistance Added 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 Values Has four separate colored bands; with each color representing a given value. Band 1 – 1 st significant digit Band 2 – 2 nd significant digit Band 3 – multiplier; number of zeros added Band 4 – tolerance of resistor

Determining Resistor Values Band 1 – Green Band 2 – Red Band 3 – Black Band 4 - Gold Band 1 – Brown Band 2 – Orange Band 3 – Blue Band 4 - Silver