ELECTRICITY & MAGNETISM - Gavilan...
Transcript of ELECTRICITY & MAGNETISM - Gavilan...
ELECTRICITY & MAGNETISMCHAPTER 8
E & M - Focus
Electric Charge & Force
Current, Voltage& Power
Magnetism
Simple Electrical Circuits
Voltage & Current Transformation
Electromagnetism
Electric Charge & Force
Fg = Gm1m2
r2Fe = k
q1q2r2
Gravitational Case Electrical Case
Only One Kind of Mass
Unlike Charges
Like Charges+
+
-
+
Electric Current, IVehicle “current” Electric current
current = kg (mass of vehicles)time
current = chargetime
I = qt
amperes = coulombssecond
Electric Potential Energy - VGravitational Case Electrical Case
h
w
Fg is assumed to be constant Fe is not constantm
dFe
Fe
q1
q2
PEg = wh = mgh V = Wq
volts = joulescoulomb
Batteries - Sources of Voltage
1.5V+ -
q
Move charge q from one battery pole to another. How much work did you do?
-
W =Vq
Note that is does not matter what path the charge goes through!
Electrical Resistance - R• Conductors support high
electric currents becauseof mobile electrons.
• Semiconductors have fewmobile electrons.
• Insulators have no mobileelectrons.
• Conductors have low (but not zero) resistance.
• Semiconductors have highresistance.
• Insulators have infinite resistance.
Resistance - Gravity CaseThese guys are doingwork against gravity,but their also doing work against friction.
h
Electrical Resistance - R
Circuit Elements
Electrical Resistance - RR = V
Iohms = volts
amp
The vulture is above the rabbit.
The vulture is above the iguana.
The iguana and the rabbit are on the same level.
R = VI
I = VR
V = IR
Electrical Power - PPower = Work
Timewatts = joules
second
I = qt
, V = Wq
Recall the equations for I and V
P = Wt
…and so, rearranging things IV = qtWq
= Wt= P
P = IV
Electrical Power - P
P = IVFrom
And V = IR
We get P = I 2R
VR= I
P = IV
P = V2
R
Simple Electrical Circuits
Series
Parallel
But First - Gravity!
h
h1
h2
h3
Key Points:• The flow of water through each wheel is the same.• The total potential energy drop ∝ h = h1 + h2 + h3
Series waterwheels
But First - Gravity
h
Key Points:• Water flow is split unevenly over 3 wheels. Cw = C1 + C2 + C3• The total potential energy drop, h is the same for each wheel.
Parallel waterwheels
C1 C2 C3Cw
C1 C2 C3
Series Electrical Circuit
V = IR V1 = IR1V2 = IR2V3 = IR3
V1 +V2 +V3 =V = IR1 + IR2 + IR3V = I R1 + R2 + R3( )
Req = R1 + R2 + R3 + ...
We know and so,
Equivalent series resistance:
Parallel Electrical Circuit
I = VR
I1 =VR1
I2 =VR2
I3 =VR3
1Req
= 1R1
+ 1R2
+ 1R3
+ ...
We know
and
so,Equivalent parallelresistance:I1 + I2 + I 3 = I =
VR1
+ VR2
+ VR3
I =V 1R1
+ 1R2
+ 1R3
⎛⎝⎜
⎞⎠⎟
Batteries in Series
1.5V+ - 1.5V+ - 1.5V+ -
Total = 4.5 V
Batteries in series can supply 3 times the voltage at the same current.
Batteries In Parallel1.5V+ -
1.5V+ -
1.5V+ -
Total = 1.5 V
Batteries in parallel can supply3 times the current at the samevoltage.
MAGNETISM
Magnetite, or Loadstone
Early MagnetiteCompass
Gravity, Electric AnalogiesGravity Electricity Magnetism
• Only one kind of mass
• Attractive only
• Two kinds of charge, + and -
• Opposites attract, Like charges repel.
• There are no magnetic charges, only fields!
• Opposite poles attract, Like poles repel.
Background - Earth Magnetism
• Geomagnetism:Magnetite Compasswas used for navigation.
• Paleomagnetism:Most rocks have residualmagnetism from the timeof their solidification.
The Magnetic Field
• Tiny “test” compasses could be used to “map” a magnetic field.
Also:• You could do this
with a test mass to map a gravitational field.
• You could do this with a test charge to map out and electric field.
Thought Experiments:
Electric Current & Magnetic Field
R (wire)
V+-
I
MagneticField
A flow of electric current comes with a magnetic field.
Moving charge creates a magnetic field.
Electric Current & Magnetic Field
R (wire)
V+-
I
MagneticField
In a loop, the magnetic field becomes more concentrated at the center.
Electric Current & Magnetic FieldIn a coil, the magnetic field becomes even more concentrated at the center.
R (wire)V+-
I
MagneticField
N
S
Right Hand Rule for IIf your right thumb points in the direction of conventional current, then your fingers point in the direction of the magnetic field (B-field).
Magnetic Force On Moving Charges
• Relative motion between the magnetic field and the charge is what counts.
• For negative charges (electrons) use you left hand.
• If a conductor is passed though a magnetic field, a current can flow!
Electrical Generation
Changing Magnetic FieldA changing magnetic field can also
induce electric currents.
I
Alternating Magnetic
Field
R, load
IOutput from AC generator on previous slide
This induced current can drive a useful electrical device.
The Transformer
V
V2 =N2
N1
⎛⎝⎜
⎞⎠⎟V1
I2 =N1N2
⎛⎝⎜
⎞⎠⎟V1
Transformers are used to change voltage and current levels in practical power distribution networks.
P2 = I2V2 =N1N2
⎛⎝⎜
⎞⎠⎟I1
N2
N1
⎛⎝⎜
⎞⎠⎟V2 = I1V1
P2 = P1
Note: Power is conserved
The Power Grid