Transcript Ohm`s Law and Power

This Hour
• Potentiometers
• Ohm’s Law Continued
• Power & Energy
EGR 101
1
Review
• How is voltage related to charge and
energy?
• What is the formula for resistance?
• What is Ohm’s Law?
• What does it mean?
EGR 101
2
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
EGR 101
3
2 Basic Ways to Use
Potentiometers
• As a variable resistor:
– The center tap (C) is
connected to one end
(B)
– The total resistance is
only from A to C
A
B/C
• As a voltage divider
(to be covered in a later
lecture)
EGR 101
4
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):
EGR 101
5
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?
EGR 101
6
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
+
+
_
_
EGR 101
7
In-Class Activities
1. What is the effective resistance of each
potentiometer in these circuits?
+
+
A
5V
C
R1
10 V
_
A
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?
EGR 101
8
Energy
• Think of a battery like sand in an hour glass
– Sand = charge
• Voltage is the force that moves charge
– Think of gravity 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
EGR 101
9
Power & Energy
• The Instantaneous Power, P, is the
Change of Energy, E, per unit time.
– In our sand analogy, power
is a measure of how quickly
the hourglass is emptying
• Units:
E
P
t
[E] = Joules (J).
[t] = seconds (s).
J
 P    Watt W 
s
EGR 101
10
Power & Energy
E
P
t
The change in energy can be written as:
E  P  t
We often assume initial energy is zero
EGR 101
11
Power in terms of Voltage and
Current
Energy
Previously you learned that Voltage 
Charge
E
or V 
Q
Using this and E
yields V  Q  P  t
Q
Since I 
t

Q
or V
P
t
then
EGR 101
 P  t
P V I
12
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.
EGR 101
13
Determining Power
P VI 
EGR 101
14
Other Power Equations
P V I
V  IR
P I R
In this example,
2
P=
EGR 101
15
Other Power Equations (continued)
P V I
V
I 
R
V
P 
R
2
In this example,
P=
EGR 101
16