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Part 1
Current & Energy Transfer
Electric Current
Objectives
Define an electric current, and the ampere Describe conditions that create current in an
electric circuit Define power in electric circuits Draw circuits and recognize that they are
closed loops Define resistance and state Ohm’s Law Use equations to solve problems
Electric Circuit
o Closed path along which charged particles move
Current
Flow of charged particles Unit is ampere (A) or “amp”
Rate at which charge passes a point in a circuit 1 ampere = 1 coulomb per second
Symbol is “I”
Iq
t
Electric Current
Conducting path Closed circuit
Supply of energy Potential difference (voltage)
Conventional current Flows from positive to negative Electrons actually move from
negative to positive
Conductivity
Ability of a material to conduct electricity Depends on number of free charges per unit
volume Charges move under the influence of an electric
field Potential difference created between two points
Pure metals are good conductors Many electrons are not bound or are loosely bound to
a particular atom
Resistivity
Reciprocal of conductivity Characteristic of a material that depends on its
electronic structure & temperature Increases with temperature Conductors
Low resistivity Insulators
High resistivity Electrons are tightly bound; few are free to move
Energy Transfer
Power Rate at which
energy (work) is transferred
Units are watts (W) Joules per second
Multiply the potential difference by the current
P I V
PW
t
Power
PW
t
EF
q
VW
q
VW
qW q V
Iq
ttq
I
PW
tP
q VqI
I V
Example
Textbook, Page 509 Current through the
motor is 3.0 coulombs per second
Potential difference is 120 volts 1 volt = 1 joule per
coulomb
What is the power? 1 watt = 1 joule per sec
P I V
P Cs
JC
P Js W
30 120
360 360
.e je j
Practice Problem
A 6.0 V battery delivers a 0.50 A current to an electric motor that is connected across its terminals What power is consumed by the motor? If the motor runs for 5.0 minutes, how much
electric energy is used? If the motor does 600 J of work, what is the
efficiency of the motor?
Practice Problem
Draw a diagram Show the conventional current
List the knowns and unknowns
Known:
min
V V
I A
t
6 0
050
50
.
.
.
Unknown:
P W
W J
eff
?( )
?( )
?(%)
Practice Problem
Known:
min
Unknown:
V V
I A
t
P W
W J
eff
6 0
050
50
.
.
.
?( )
?( )
?(%)
Do Textbook, Pg. 511 #1 – 4
P I V
A V
W
PW
tW P t
W J s
x J
effJ
J
050 6 0
30
30 50 60 1
9 0 10
600
900100% 67%
2
. .
.
. / . min sec/ min
.
b gb g
b gb gb g
Part 2
Resistance & Ohm’s Law
Electric Current
Resistance
Property that determines how much current will flow between two points at a given potential difference Measured by placing a potential difference across
two points on a conductor, and measuring the current
Resistance (R) is the ratio between the voltage (V) and the current (I)
Resistance
Opposition to the flow of electric current Ratio of potential difference to current flow Unit is “ohm” (Ω)
Ohm’s law Device which obeys this relationship is “ohmic” Not a general physical “law”
RV
I
Resistance Unit of resistance is the ohm ()
One ohm is the resistance that will allow one ampere of current to flow when the potential difference is one volt
Ohm’s law Describes the behavior of a “pure” resistor Devices that behave this way are called, “ohmic”
Ohm’s Law
CurrentR = slope of the VI curve
RV
I
Ohm’s Law
RV
IV R I
IV
R
Application
Circuit diagrams Pg. 512
Potentiometer (variable resistor) Pg. 513
See Pg. 514, Table 22-1
Example
A 30.0 V battery is connected to a 10.0 ohm resistor What is the current in the circuit?
Draw a diagram List the knowns and unknowns Calculate your answer Check your answer
Example
Known:
Unknown:
V V
R
I A
30 0
10 0
.
.
?( )
IV
R
IV
I A
30 0
10 0300
.
..
Resistance of a Wire
Depends on its physical properties Length Cross-sectional area (thickness) Resistivity of material (ρ)
RL
A FHG
IKJ
Units of Resistivity
What are the units of resistivity?
Ohm·meter (Ω·m)
RL
A
RA
L
m
m
2
Example
What is the resistance of 500 yards of 12AWG copper wire?
If one amp of current is flowing in the wire, what is the potential difference (voltage)?
SolutionL m
A r
rD x m
x m
457 2
2
2 0525 10
2172 10
2
3
8
.
.
.
RL
A
Rx m m
x m
x m m
x m
FHG
IKJ
172 10 457 2
2 0525 102
4 172 10 457 2
2 0525 10
8
3 2
8
3 2
. .
.
. .
.
c hb g c hb gc h
Check
Rft
Rft
ft
yd
yd
15883
1000
15883
1000
3
1
500
1
.
.
Temperature Coefficient of ResistanceTemperature Coefficient of Resistance
The The temperature coefficient of resistancetemperature coefficient of resistance is the is the change in change in resistanceresistance per unit resistance per degree change in per unit resistance per degree change in temperature. temperature.
R
R t0
R
R t0
Where:
= temperature coefficient of resistance
R = change in resistance
R0 = initial resistance
t = change in time
Where:
= temperature coefficient of resistance
R = change in resistance
R0 = initial resistance
t = change in time
SuperconductivitySuperconductivity
Superconductivity Superconductivity is a condition of is a condition of zero resistancezero resistance encountered at relatively low temperatures for certain encountered at relatively low temperatures for certain materials.materials.
The The transition temperaturetransition temperature is is the temperature at which the the temperature at which the resistivity of a resistivity of a superconductor material superconductor material rapidly approaches zerorapidly approaches zero..
Extra Credit
What animal is capable of producing nearly 1 amp of current? Genus & species Where found
Practice Problems
Drawing schematic diagrams Pg. 516
Textbook, Pg. 515 #5 – 10 Textbook, Pg. 517 #11 – 13