Download Chapter 17

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!
Current

Current = rate at which the
charge flows through a
surface in a material
Iav 


The SI unit of current is Ampere
(A); 1 A = 1 C/s
Instantaneous current:
I

Q
t
I 
lim
t  0 av
lim
t  0
Q
t
Direction (convention) - the
direction positive charge
would flow
• But, in most materials the electrons carry the
current
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?
Current and Drift Speed





n charge carriers per unit
volume with unit charge q
moving through a volume A
x
Total charge Q is
Q = n A x q
= n A vd t 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
Example Problem

17.8 An aluminum wire carrying a
current of 5.0 A has a crosssectional area of 4.0 x 10-6 m2.
Find the drift speed of the
electrons in the wire.
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
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
V
R
I

Units of resistance are ohms (Ω)


1Ω=1V/A
Resistance in a circuit arises due to collisions
between the electrons carrying the current
with the fixed atoms inside the conductor
Ohm’s Law


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
Ohmic
Non-ohmic
Resistivity

The resistance of an ohmic conductor is
proportional to its length, L, and
inversely proportional to its crosssectional area, A
R

A
ρ is the constant of proportionality and is
called the resistivity of the material
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?
Solution to 17.3
Solution to 17.8
Solution to 17.12