Transcript Lecture 11 Wednesday February 11x

Lecture 11—Ohm’s Law, Power
February 11
Batteries
The potential difference
between the terminals of a
battery, often called the
terminal voltage, is the
battery’s emf.
∆Vbat = VT = 
Slide 22-9
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
Power in Circuits
Slide 22-15
A resistor is connected to a 3.0 V battery; the power
dissipated in the resistor is 1.0 W. The battery is now
traded for a 6.0 V battery. The power dissipated by the
resistor is now
W
25%
4
W
25%
3
25%
W
W
25%
2
1W
2W
3W
4W
1
1.
2.
3.
4.
A set of lightbulbs have different rated voltage and power,
as in the table below. Which one has the highest
resistance?
E
D
Rated
power20%
20% 20%
1W
1W
2W
2W
3W
C
20%
B
A
B
C
D
E
Rated voltage
20%
10 V
8V
12 V
6V
3V
A
1.
2.
3.
4.
5.
Bulb
A
B
C
D
E
Resistances
•
•
•
•
•
RA = 100Ω
RB= 64Ω
RC = 72Ω
RD= 18Ω
RE= 3 Ω