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CONCLUSION – CHAPTER CONCLUSION – CHAPTER 20 &20 &
CHAPTER 21 – CHAPTER 21 – ELECTROMAGNETIC ELECTROMAGNETIC INDUCTIONINDUCTION
Chapter 21
Quick Review IB
Magnetism2
r(exact) 104
2
70
0
A
Tm
r
IB
Magnetism3
Force Between Two Current Carrying Conductors
First wire produces a magnetic field at the second wire position.
The second wire therefore feels a force = Bil
Solenoid
Magnetism4
Length
Turns of
number Total0
L
Nn
nIB
B=~0 outside
The Toroid–
Magnetism5
r
NIB
20
B=0 outside
Magnetism6
A rectangular loop has sides of length 0.06 m and 0.08 m. The wire carries a current of 10 A in the direction shown. The loop is in a uniform magnetic field of magnitude 0.2 T and directed in the positive x direction. What is the magnitude of the torque on the loop?
8 × 10–3 N × m
)sin( NIAB
Magnetism7
A solenoid of length 0.250 m and radius 0.0250 m is comprised of 440 turns of wire. Determine the magnitude of the magnetic field at the center of the solenoid when it carries a current of 12.0 A.
2.21 × 10–3 T
Magnetism8
The drawing shows two long, thin wires that carry currents in the positive z direction. Both wires are parallel to the z axis. The 50-A wire is in the x-z plane and is 5 m from the z axis. The 40-A wire is in the y-z plane and is 4 m from the z axis. What is the magnitude of the magnetic field at the origin?
3 × 10–6 T
(exact) 104
2
70
0
A
Tm
r
IB
Magnetism9
Moving On to the next chapter……………….
I
I
INTRODUCTION TO INDUCTION
04/18/23
10
Induction
Important Definition – Magnetic Flux
04/18/23Induction
11
AREA
Magnetic Field
04/18/23Induction12
The Essence of this Topic
04/18/23Induction
13
Consider a conductor that is shaped in a loop but is continuous.
The conductor has a magnetic field through the loop that is not necessarily uniform.
There is a MAGNETIC FLUX through this loop. If the FLUX CHANGES, an “emf” will be
induced around the loop. This emf can cause a current to flow around
the loop.
How Can You Change the Magnetic Flux Going Through The Loop? Huh?
04/18/23Induction
14
Divide the area of the loop into a very large number of small areas A.
Find the Magnetic Field through each area as well as the angle that it makes with the normal to the area.
Compute the total flux through the loop.
The Magnetic Flux Going Through The Loop:
04/18/23Induction
15
iii
i AB )cos(
Add up all of these piecesthat are INSIDE the loop.
Changing
04/18/23Induction
16
Change any or all of the Bi
i
Change the SHAPE of the loop Change the ANGLE that the loop
makes with the magnetic field (subset of above)
And the Flux will change!ii
ii AB )cos(
WAIT A SECOND …….
04/18/23Induction
17
You said that there is a conducting loop. You said that there is therefore a
VOLTAGE or emf around the loop if the flux through the loop changes.
But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop?
‘tis a puzzlement!
04/18/23Induction18
REMEMBER when I said E Fields start and end on CHARGES???
04/18/23Induction
19
DID I LIE??
The truth
04/18/23Induction
20
Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously.
Electric Fields created by changing magnetic fields can actually be shaped in loops.
Why do you STILL think I am a liar?
04/18/23Induction
21
Because you said that an emf is a
voltage so if I put a voltmeter from one point on the loop
around to the same point, I will get
ZERO volts, won’t I
04/18/23Induction22
The POTENTIAL between two points
04/18/23Induction
23
Is the WORK that an external agent has to do to move a unit charge from one point to another.
But we also have (neglecting the sign): sEV
s
So, consider the following:
04/18/23Induction
24
E
Conductor
x x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x x
zeroREemf
sEsEemf
2
THEREFORE WHAT WILL A VOLTMETER READ FROM A to A?
04/18/23Induction
25
E
Conductor
x x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x x
A
A The emfB ZeroC Can’t tell
04/18/23Induction
26
temf loopthe
Through
Michael Faraday (1791-1867)
MINUS????
A: The way that you don’t want it to point! (Lenz’s Law).
Lenz’s Law Explains the (-) sign!
Q: Which way does E point?
04/18/23
27
Induction
OK. LET’S DO THE PHYSICS NOW
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28
Induction
04/18/23Induction29
Is there an induced current???
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30
Induction Effects
Faraday’s Experiments
??
Insert Magnet into Coil
Remove Coil from Field Region
Summary
Does the Flux Change?
04/18/23
36
In the Previous Example, if there are N coils rather than a single coil,
04/18/23Induction
37
A The current is increased by a factor of NB The current is decreased by a factor of NC The current stays the same.
Push a magnet into a coil of two wires and a current is produced via an emf.
04/18/23Induction
38
In this case, 2 coils, each has the SAME emf.
Ohm’s Law still works, so
coilR
emfi
tNemf
If we go from 2 to 4 coils, the current
04/18/23Induction
39
A Stays the sameB DoublesC Is halvedD Is four times larger
A rectangular circuit containing a resistor is perpendicular to a uniform magnetic field that starts out at 2.65 T and steadily decreases at 0.25 T/s. While this field is changing, what does the ammeter read?
04/18/23Induction
40
04/18/23Induction41
••The conducting rod ab shown makes frictionless contact with metal rails ca and db. The apparatus is in a uniform magnetic field of 0.800 T, perpendicular to the plane of the figure. (a) Find the magnitude of the emf induced in the rod when it is moving toward the right with a speed 7.50 m/s.
04/18/23Induction42
tABemf sin
Almost DC
04/18/23Induction
43
THE STRANGE WORLD OF DR. LENTZ
LENZ’S LAWInduced Magnetic Fields always FIGHT to stop what you are trying to do!i.e... Murphy’s Law for Magnets
Example of Nasty Lenz
The induced magnetic field opposes thefield that does the inducing!
Don’t Hurt Yourself!
The current i induced in the loop has the directionsuch that the current’s magnetic field Bi opposes thechange in the magnetic field B inducing the current.
Let’s do theLentz Warp
again !
Again: Lenz’s Law
An induced current has a directionsuch that the magnetic field due tothe current opposes the change in the magnetic flux that induces thecurrent. (The result of the negative sign that we always leave out!) …
OR
The toast will always fall buttered side down!
What Happens Here?
Begin to move handle as shown. Assume a resistance R in the loop.
Flux through the loop decreases.
Current is induced which opposed this decrease – current tries to re-establish the B field.
What about a SOLID loop??
METAL Pull
Energy is LOSTBRAKING SYSTEM
04/18/23Induction53
A cardboard tube is wrapped with two windings of insulated wire, as shown. Is the induced current in the resistor R directed from left to right or from right to left in the following circumstances?
The current in winding A is directed
(a) from a to b and is increasing,
(b) from b to a and is decreasing,
(c) from b to a and is increasing, and
(d) from b to a and is constant.
left right
Mutual inductance –
Circulation of currents in one coil can generate a field in the coil that will extend to a second, close by device.
Suppose i1 CHANGES
Flux Changes
Current (emf) isinduced in 2nd
coil.
The two coils
04/18/23Induction
55
Remember – the magneticfield outside of the solenoidis pretty much zero.
Two fluxes (fluxi?) are the same!Two fluxes (fluxi?) are the same!
Self-inductance –
Any circuit which carries a varying current self-induced from it’s own magnetic field is said to
have INDUCTANCE (L).
An inductor resists CHANGESCHANGES in the current going through it.
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57
An inductor resists CHANGESCHANGES in the current going through it.
04/18/23Induction
58
An inductor resists CHANGESCHANGES in the current going through it.
04/18/23Induction
59
Inductance Defined
04/18/23Induction
60
i
NL B
If the FLUX changes a but during a short time t, then the current will change by a small amount i.
t
iL
tN
NLi
B
B
This is actually acalculus equation
Faraday says this is the emf!
So …
04/18/23Induction
61
t
iLemf
E=
The UNIT of “Inductance – L” of a coil is the henry.
SYMBOL:
There should bea (-) sign but weuse Lenz’s Lawinstead!
04/18/23Induction62
Consider “AC” voltage
04/18/23Induction
63
V1
Maximum Change/t
Minimum Change/t
The transformer
04/18/23Induction
64
FLUX is the same throughboth coils (windings). 2
2
1
1
222
111
N
V
N
V
tNVemf
tNVemf
04/18/23Induction65
04/18/23Induction66
Input/Output Impedance (Resistance)
04/18/23Induction
67
!resistance
inputan like looks
)/(
(Lossless)
2121
1
2211
1
2
1
2
NN
R
I
V
So
VIVI
PowerPower
N
N
V
V
outin
Read in the textbook section 21.10:
04/18/23Induction
68
0
2
2B
VolumeUnit
Energyu
Energy associated with an induced current.
As energy is introduced at induces a field, energy is stored in an electronic device. Refer to worked example 21.12 in your text.
The R-L circuit – Figure 21.29 When an inductor is part of a wired circuit, - voltages, currents and capacitor charges are a function of time, not constants. Refer to worked example 21.13 in your text.
The L-C circuit – Figure 21.34 When an inductor is part of a wired circuit with a capacitor, the capacitor charges over time. Commonly used in radio as a tuner for the induced current from an antenna.