Electrical Resistance

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 • Rtotal = R1 + R2 + …… • Itotal = I1 = I2 = …… • Vtotal = V1 + V2 + ….. • V1 = R1 * I1 • V2 = R2 * I2

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 • Vtotal = V1 = V2 = ….. • I total = I1 + I2 + ….. • I1 = (V1 / R1) • I2 = (V2 / R2) • R total = R1 + R2 R1 * R2

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 – 1st significant digit • Band 2 – 2nd significant digit • Band 3 – multiplier; number of zeros added • Band 4 – tolerance of resistor

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

Electrical Resistance