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G.M . JAHEDUL ISLAM

Basic Electricity (6711). G.M . JAHEDUL ISLAM. Instructor Department of ELECTRICAL IDEAL INSTITUTE OF SCIENCE AND TECHNOLOGY (IIST). Previous Lesson. In my first class I discuss about conductor and insulator. Direction of current flow.

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G.M . JAHEDUL ISLAM

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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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