Download Lecture 10 Monday February 9

Lecture 9: Currents
Friday—February 6
Batteries and EMF
Lecture 10 Monday February 9
Properties of a Current
Slide 22-5
Reading Quiz
1. The charge carriers in metals are
A. electrons
B. positrons
C. protons
D. a mix of protons and electrons
Slide 22-2
The charge carriers in metals are
A
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nd
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Po
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Pr
ot
on
s
s
25% 25% 25% 25%
s
Electrons
Protons
Positrons
A mixture of
protons and
electrons
El
ec
tr
on
1.
2.
3.
4.
Rank the bulbs in the following
circuit according to their brightness,
from brightest to dimmest
25%
25%
D
25%
C
25%
B
A
B
C
D
A
1.
2.
3.
4.
Problems due Monday
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•
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•
Problems: 22: CQ6, 1, 2, 6, 11
CQ6: ranking currents using sum in = sum out
1: I =ΔQ/Δt in coulombs and electrons
6: I =ΔQ/Δt lightning strike
11: sum in = sum out
QUIZ #4
• In a typical lightning stroke, 2.5 C flows from a
cloud to ground in 0.20 ms. What is the
current in the strike?
Circuits
• Current needs a complete circuit to flow.
• What makes the current flow around the circuit?
We know energy is being lost, for example as
heat and light in a light bulb.
• What is flowing?
– Electrons in metals
– Average velocity very low because so many
– Collisions with ions cause energy loss so velocity on
the average is constant even though E field is always
accelerating the electrons.
A battery is connected to a resistor. As charge flows,
the chemical energy of the battery is dissipated as
25% 25% 25% 25%
en
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m
al
Th
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Current
Voltage
Charge
Thermal energy
C
1.
2.
3.
4.
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
∆Vbat =
W
chem
____
q= 
Slide 22-9
Simple Circuits
The current is determined by
the potential difference and
the resistance of the wire:
I =
∆Vchem
_____
R
Slide 22-10
Resistivity
The resistance of a wire
depends on its dimensions
and the resistivity of its
material:
Slide 22-11
Measuring Body Fat
The resistivity of the body is a good measure of its overall composition. A
measure of the resistance of the upper arm is a good way to estimate the percent
fat in a person’s body. Let’s model a person’s upper arm as a cylinder of diameter
8.0 cm and length 20 cm. We can model the composition of the arm by assuming
that the muscle, far, and nonconductive portions (the bone) form simple regions.
This simple model actually works quite well. For a typical adult, the bone has a
cross-sectional area of 1.0 cm2; to a good approximation, the balance of the arm
is fatty tissue or muscle.
A. Assume that the arm has 30% fat and 70% muscle. What is the resistance of
the arm?
B. Now, assume that the arm has only 10% muscle. What is the resistance
now?
The measurement of the resistance of the arm is made by applying a voltage and
measuring a current. Too much current can be uncomfortable and, as we will see,
can be dangerous. Suppose we wish to limit the current to 1.0 mA. For each of
the above cases, what is the maximum voltage that could be employed?
Slide 22-14
Wednesday
• 22: 5-6 Ohm’s Law and Power
Wednesday’s Homework
• 22: 13, 14, 16, 17, 18, 19, 21, 27, 40, 41, WB
22: 5-7