Download 01-02MurraysOhmsLaw

Ohm’s Law
•Definition
•Whiteboards
•An Elaborate Analogy
Ohm’s Law
V = Voltage (like pressure)
I = Current (like flow rate)
+
-
V
A
R
R=V
I
R = Resistance in ohms ()
V = Voltage in Volts (V) (pressure)
I = Current in Amps (A) (flowrate)
Ohm’s Law - The way I think of it
V = Voltage (like pressure)
I = Current (like flowrate)
R = Resistance to flow
+
- V
I=V
R
R = Resistance in ohms ()
V = Voltage in Volts (V) (pressure)
I = Current in Amps (A) (flowrate)
A
R
Ohm’s Law
I=
+
-
V
R
R = Resistance in ohms ()
V = Voltage in Volts (V) (pressure)
I = Current in Amps (A) (flowrate)
Demo - pass around resistors
(two properties - power and resistance)
Demo - power supply, resistor, light bulb
(show increase/decrease V, and R)
A
V
R
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Gold
Silver
No color
0
1
2
3
4
5
6
7
8
9
5%
10%
20%
Ohm’s Law
+
-
12.0 V
A
R=V
I
R = Resistance in ohms ()
V = Voltage in Volts (V) (pressure)
I = Current in Amps (A) (flowrate)
120. mA
R= ??
Example 1: What is the resistance in the circuit above?
R = V/I = (12.0 V)/(.120 A) = 100. 
Example 2: If there is a 220  resistor in the circuit above, what is
the current?
R = V/I, I = V/R = (12.0 V)/(220 ) = .05454 = .055 A or 55 mA
Whiteboards
1|2|3|4|5|6
What is the resistance of a light bulb if it draws
250 mA of current from a 6.15 V battery? (1000
mA = 1 A)
Given:
R=V
I
R = ??
V = 6.15 V
I = .250 A
R = 24.6 
24.6 
What current will flow if you plug a paperclip
with a resistance of .065  into a 12 V source?
Given:
R=V
I
R = .065 
V = 12 V
I = ??
I = 184.6 A
180 A
What voltage do you need to push 1.5 Amps of
current through a 210 ohm resistor?
Given:
R=V
I
R = 210 
V = ??
I = 1.5 A
V = 315 V
315 V
What current flows through an 8.1 ohm speaker
when there is a voltage of 3.45 V?
Given:
R=V
I
R = 8.1 
V = 3.45 V
I = ??
I = 0.425925926
I = .43 A
.43 A
An unknown resistor draws .0128 Amps of
current from a 23.9 V source. What is the
resistance?
Given:
R=V
I
R = ??
V = 23.9 V
I = .0128
R = 1867.1875
R = 1870 
1870 
A 1200 ohm resistor is hooked up to an unknown
voltage source, and it draws 87.5 mA of current.
What is the voltage?
Given:
R=V
I
R = 1200 
V = ??
I = .0875 A
V = 105 V
105 V
A 670 ohm resistor is connected to a 1.5 V
battery. What time will it take for 3.0 C of
charge to flow from the battery?
Given:
R=V
I
I = q
t
R = 670 
V = 1.5 V
I = ??
q = 3.0 C
t = ??
t = 1340 s
1340 s
A 3.0 V battery with a capacity of 350 mAh
(350 milli amp hours) is drained in 4 hours and
27 minutes. What is the resistance of the circuit
that it is attached to?
Given:
R=V
I
I = q
t
R = ?? 
V = 3.0 V
I = ??
q = (.350e-3)(3600) = 1260 C
t = 4*3600 + 27*60 = 16020 s
R = 38.14286  = 38 
38 
Symbols
+
-
A Cell. + and -, Infinite current, R = 0, what’s really
inside
A Battery of cells
A resistor
V3
A voltmeter, R = infinity
A3
An ammeter, R = 0
An analogy
Water in pipes:
Flow rate (GPM)
Pressure
Constriction
Constant P Pump
Electrons in wires:
Current (Amps = C/s)
Voltage
Resistance
Batteries/Cells
R1
+
-
A3
R2
Current convention
Restriction
Wires are superconductors