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: 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: wood, plastic, glass, and rubber

Semiconductors Depending 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 Resistance V – Voltage Drop I – Current

Unit of Measurement Unit 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 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 = ? V = 110 volts I = 0.80 amps

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

Resistivity Factors affecting resistance on a wire: Length Longer wire, greater resistance Cross-sectional area Smaller area, less resistance Material 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 Resistance is independent of both potential difference and current

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 + ….. V 1 = R 1 * I 1 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: 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 + ….. I 1 = ( V 1 / R 1 ) 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: 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