# Ohm s Law and Power Today s Agenda Resistance

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Today’s Agenda

Potentiometers
Ohm’s Law Continued
Power & Energy
Review from Last Week
• How is voltage related to charge and
energy?
• What is the formula for resistance?
• What is Ohm’s Law?
• What does it mean?
Potentiometers
• A potentiometer is a variable resistor

• The total resistance is fixed between terminals
A and B
A
• A portion of the resistance
is between A and C
C
• The remainder is between
B and C                            B

• C can be physically moved between A and B
2 Basic Ways to Use
Potentiometers
• As a variable resistor:      A

– The center tap (C) is
connected to one end
(B)                       B/C
– The total resistance is
only from A to C

• As a voltage divider
(to be covered in a later
lecture)
In-Class Activity
If you have a 1k Ω potentiometer and the
center tap, C, is set ¼ of the way between
A and B (closer to A),
• What is the resistance between A and C
and between B and C?
• What is the resistance R if the
potentiometer is connected as below
(assume C has not been moved):
R
A       B/C
Relationship between Current and
Voltage
• Current through a FIXED resistance
– Increases when the voltage increases
– Decreases when the voltage decreases
• The current changes as a result of the change
in voltage!

+                         +

_                         _

What is the value of the resistance?
Relationship between Current and
Resistance
• For a FIXED voltage,
– The current decreases proportionally to an increase in
resistance
– The current increases proportionally to a decrease in resistance
• The current changes as a result of the change in
resistance

+                                   +

_                                   _
In-Class Activities
1. What is the effective resistance of each
potentiometer in these circuits?

+                          +
A                         A
5V            C     R1    10 V          C    R2
_                          _
B                         B

2. If R1 and R2 actually were the same
potentiometer set to different values and R2
corresponds to C adjusted all the way to the B
end (i.e. total resistance value), what
percentage of the total resistance is R1?
Energy
• Think of a battery like sand in an hour glass
– Sand = charge
• Voltage is the force that moves charge
– Think of being on the moon vs the Earth
• Energy = V.Q
– You use much more energy to move sand on Earth than on the
moon where gravity is 1/6th the Earth’s
Power & Energy
• The Instantaneous Power, P, is the
Change of Energy, E, per unit time.
– In our sand analogy, power       E
is a measure of how quickly   P
the hourglass is emptying        t
• Units:   [E] = Joules (J).
[t] = seconds (s).
J
 P   Watt W 
s
Power & Energy
E
P
t
The change in energy can be written as:

E  P  t
We often assume initial energy is zero
Power in terms of Voltage and
Current
Energy
Previously you learned that Voltage 
Charge
E
or V             Using this and E    P  t
Q
Q
yields V    Q  P  t   or V    P
t
Q
Since   I             then   P  VI
t
Power
- The amount of energy used per unit time
- The battery shown below uses 1 J/s to
generate current – it has used 1 W of
power.
Determining Power

P  VI 
Other Power Equations

P VI
V  IR
In this example,
P I R2

P=
Other Power Equations (continued)
P V I
V
I 
R          In this example,
V    2

P             P=
R
In-Class Activity for Power and
Ohms Law
• In pairs, complete the following chart
ITEM #   CURRENT   VOLTAGE   RESISTANCE   POWER

1       10 mA                            4W

2                 32 V                  16 mW

3                            3.3 kΩ     231 mW

4       15 mA     45 V

5       24 mA                1.2 kΩ
In-Class Activity
Practice Problem 3.11 (p 86)
• Calculate the total energy used by a
1500W dishwasher, a 3600W clothes
dryer, and a 750W air conditioner that are
all being used for 2 hours.