1. Announcements
Prof. Reitze will be giving this week’s lectures on Ch. 17
Chapter 17 covers current and resistance
WebAssign HW Set 4 due this Friday
Problems cover material from Chapters 17
Exam 1 is coming! 8:20 – 10:10 pm Wednesday, February 16
Room assignments:
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 m. What is the effective current associated with the orbiting electron?

4. 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 vd Dt 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

5. 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.

6. 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

7. 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

8. Ohm’s Law Ohmic Non-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

9. 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

11. 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?

12. Solution to 17.3

13. Solution to 17.8

14. Solution to 17.12