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Understanding Specific Resistance and Resistivity in Conductors

In this lesson, we delve into the concepts of specific resistance and resistivity, crucial for understanding electrical conductors. We will explore the factors affecting conductor resistance, including length and cross-sectional area, alongside their relationship as expressed in Ohm's law. Students will learn about the units of resistivity and how to calculate resistance using the formula R = ρL/A, where ρ represents resistivity, L is length, and A is cross-sectional area. This foundational knowledge sets the stage for more complex electrical principles.

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Understanding Specific Resistance and Resistivity in Conductors

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  1. Basic Electricity (6711) G.M. JAHEDUL ISLAM Instructor Department of ELECTRICAL IDEAL INSTITUTE OF SCIENCE AND TECHNOLOGY (IIST)

  2. Previous Lesson In my firstclass I discuss about conductor and insulator Direction of current flow. • The factors on which the resistance of conductor depends and law of resistance.

  3. Learning outcomes • After finished the lesson. students will be able know: • SPECIFIC RESISTANCE. • Resistivity and its unit. • The problems relating to laws of resistance.

  4. SPECIFIC RESISTANCE • The resistivity of a material is based on the resistance of define volume of that material the letter symbol for resistivity is ϱ (the Greek letter Rho). • Resistivity may also be called specific resistance. Table list the resistivity in ohm- centimetre ( Ω.cm ). It also lists values in uints of circular mil-ohms per foot ( cmil -Ω/ft) for the conducotr group.

  5. Specific resistance can be derived from the following relations of the resistance dependence factors 1. LENGTH • Resistacne is directly proportional to the length of conductor. • R ∝ L ---------1 • 2. AREA • Resistance is inversily proportional to the area of conductor. • R ∝ 1/A ----------2 • by combining 1 and 2 • R ∝ L/A • R = ϱ L/A • Where ϱ (Rho) is constant for the material called its specific resistance or resistivity.

  6. Resistivity and its unit: • RESISTIVITY and CONDUCTIVITYResistivity is the resistance of a unit volume of a material. In the metric system, the unit of length is the meter, and area is the square meter. Thus, resistivity is measured in units of Ohm - meters squared per meter (Ohm-m2/m), often abbreviated as Ohm-m. Resistivity also equals the ratio of voltage to current, if the length and area are unity. Thus:    3: RES = V / I * L / A • WHERE:  A = area (square meters)  V = voltage (volts)  I = current (amperes)  L = length (meters)  RES = resistivity (ohm-m)

  7. Resistivity and its unit

  8. R=pL/A (Resistance = resistivity (ohms/m) x length / cross sectional area) • Resistance / resistivity =length/area [ Resistance X area ] / Resistivity =Length Length = [ Resistance X cross-sectional area ] / Resistivity [*Length is in meter Resistance is in ohms cross-sectional area is in meter^2 [ square meter] Resistivity is in ohms-meter. So, unit of length is, =ohms x meter^2 divided by ohms-meter =ohms x meter x meter / ohms-meter =meter

  9. R=pL /A (Resistance = resistivity (ohms/m) x length / cross sectional area) • Resistance of a wire R = ρL/A ρ is resistivity of the material L is length in meters A is cross-sectional area in m² R = ρL/A RA = ρL L = RA/ρ

  10. Feed back: • Distinguish SPECIFIC RESISTANCE • Resistivity and its unit: • The problems relating to laws of resistance

  11. Home work

  12. Next lecture: • Application of ohms Law

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