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Electrical Current and Resistance Chapter Announcements

Stay updated with Chapter 17 announcements covering current and resistance. WebAssign HW Set 3 due on 9/19. Exam 1 on 9/27, room assignments provided. Learn about current, resistance, Ohm's Law, and solving example problems.

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Electrical Current and Resistance Chapter Announcements

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  1. Announcements • Chapter 17 covers current and resistance • WebAssign HW Set 3 due on Monday 9/19 • Problems cover material from Chapters 16 • Exam 1 is coming! 8:20 – 10:10 pm Tuesday, 9/27 • Room assignments: • A-L:  NRN 137M-R: NPB 1001S-Z:  L007 • QUESTIONS? PLEASE ASK!

  2. Current • Current = rate at which the charge flows through a surface in a material • The SI unit of current is Ampere (A); 1 A = 1 C/s • Instantaneous current: • Direction (convention) - the direction positive charge would flow • But, in most materials the electrons carry the current

  3. Example Problem 17.3 In the Bohr model of the hydrogen atom, an electron in the lowest energy state moves at a speed of 2.19 x 106 m/s in a circular path having a radius of 5.29 x 10-11 m. What is the effective current associated with the orbiting electron?

  4. Solution to 17.3

  5. Current and Drift Speed • n charge carriers per unit volume with unit charge q moving through a volume A Δx • Total charge DQ is DQ = n A Dx q = n A vdDt q • Rewrite in terms of current as I = n A vd q • No E field present: charges move randomly • E field present: charges ‘drift’ preferentially in a direction producing current

  6. Example Problem • 17.8 An aluminum wire carrying a current of 5.0 A has a cross-sectional area of 4.0 x 10-6 m2. Find the drift speed of the electrons in the wire.

  7. Solution to 17.8

  8. Measuring current and voltage in a circuit • An ammeter is used to measure current • In series with the bulb • A voltmeter is used to measure voltage (potential difference) • In parallel with the bulb

  9. Resistance • The voltage applied across the ends of the conductor is proportional to the current through the conductor • The constant of proportionality is the resistance of the conductor • Units of resistance are ohms (Ω) • 1 Ω = 1 V / A • Resistance in a circuit arises due to collisions between the electrons carrying the current with the fixed atoms inside the conductor

  10. Ohm’s Law Ohmic • For many materials, including most metals, resistance remains constant over a wide range of applied voltages or currents • This statement has become known as Ohm’s Law ΔV = I R • Ohm’s Law is an empirical relationship that is valid only for certain materials Non-ohmic

  11. Resistivity • The resistance of an ohmic conductor is proportional to its length, L, and inversely proportional to its cross-sectional area, A • ρ is the constant of proportionality and is called the resistivity of the material

  12. Example Problem • 17.12 Suppose you wish to fabricate a uniform wire out of 1 g of copper. If the wire is to have a resistance R = 0.50 W, and if all of the copper wire is to be used,what will be the (a) length and (b) diameter of the wire?

  13. Solution to 17.12

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