what we’ve observed an increasing magnetic field induces a negative emf a decreasing magnetic...
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Lenz’s Law Heinrich Lenz Image obtained from:TRANSCRIPT
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What We’ve Observed• An increasing magnetic field
induces a negative emf• A decreasing magnetic field
induces a positive emf• A magnetic field that alternates
by increasing and decreasing causes current to move back and forth
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Lenz’s Law• An emf (voltage) means there is
current flowing in the wire• How to determine direction of
current?• Heinrich Lenz: studied currents
moving in induced circuits• emf = - NA
Lenz’s LawHeinrich Lenz
Image obtained from: http://upload.wikimedia.org/wikipedia/commons/c/cc/Heinrich_Friedrich_Emil_Lenz.jpg
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field
B
⨡
B
⨡
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field
Binc.
⨡
Binduced
⨡
Binduced
⨡
I
⨡
×××
××× I
⨡
Binc.
⨡
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field
Binitial
⨡
Binduced
⨡
Binc.
⨡
Bnet
⨡
B still increases, but was opposed by B from induced current Negative feedback
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field• If B is decreasing, current will
flow to try and increase B field
B
⨡
B
⨡
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field• If B is decreasing, current will
flow to try and increase B field
Bdec.
⨡
Binduced
⨡
Binduced
⨡
I
⨡
I
⨡
Bdec.
⨡
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Lenz’s Law• A current induced by a changing B
field opposes the change in the B field• If B is increasing, current will
flow to try and decrease B field• If B is decreasing, current will
flow to try and increase B field
Binitial
⨡
Binduced
⨡
Bdec.
⨡
Bnet
⨡
B still decreases, but was opposed by B from induced current Negative feedback
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Lenz’s Law• A helpful analogy:• Inertia: mass resists
changes to its velocity• If velocity is = 0 m/s, wants
to remain at 0 m/s• If velocity is ≠ 0 m/s, wants
to keep moving with that velocity
B
⨡
B
⨡
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Lenz’s Law• A helpful analogy:• Inertia: mass resists
changes to its velocity• If velocity is = 0 m/s, wants
to remain at 0 m/s• If velocity is ≠ 0 m/s, wants
to keep moving with that velocity
B
⨡
B
⨡
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Lenz’s Law• A helpful analogy:• Inertia(?): charge resists
changes to its current• If velocity is = 0 m/s, wants
to remain at 0 m/s• If velocity is ≠ 0 m/s, wants
to keep moving with that velocity
B
⨡
B
⨡
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Lenz’s Law• A helpful analogy:• Inertia(?): charge resists
changes to its current• If current is = 0 A, wants
to remain at 0 A• If velocity is ≠ 0 m/s, wants
to keep moving with that velocity
B
⨡
B
⨡
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Lenz’s Law• A helpful analogy:• Inertia(?): charge resists
changes to its current• If current is = 0 A, wants
to remain at 0 A• If current is ≠ 0 A, wants
to keep flowing with that current
• Lenz’s Law describes how current in wires do this
B
⨡
B
⨡
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Inductance• Suppose you have the following
circuit:• Inductor- resists changes in
current• If connected to source,
keeps current from flowing for a while• If disconnected from
source, keeps current flowing for a while
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Inductance• Assume switch has just been
closed• Current flowing through
inductor was 0 A• Current now increasing
through inductor• Lenz’s Law: inductor opposes
change by inducing a current in opposite direction of increasing current• Acts like a temporary battery
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Inductance• Assume switch has just been
closed• Current flowing through
inductor was 0 A• Current now increasing
through inductor• Lenz’s Law: inductor opposes
change by inducing a current in opposite direction of increasing current• Acts like a temporary battery
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Inductance• After letting this run for a while,
inductor operates like a normal wire
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Inductance• After letting this run for a while,
inductor operates like a normal wire• But what happens to a solenoid
with a current flowing through it?• Strong B field inside inductor
B
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Inductance• Now suppose switch is opened B
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Inductance• Now suppose switch is opened• Was current flowing through
inductor• Current now decreasing
through inductor• Lenz’s law: inductor opposes
change by inducing a current in same direction as decreasing current• Acts like a temporary battery
B
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Inductance• Now suppose switch is opened• Was current flowing through
inductor• Current now decreasing
through inductor• Lenz’s law: inductor opposes
change by inducing a current in same direction as decreasing current• Acts like a temporary battery
B
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Inductance• After letting this run for a while,
inductor operates like a normal wire
B
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Inductance• After letting this run for a while,
inductor operates like a normal wire• Where did the current come
from?• Strong B field inside inductor
is no longer thereHmm…
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Inductance• Inductors store energy in a B field• When current first flows into
inductor, some current gets stored in B field• When current is cut off, current
stored in B field released• Capacitors & Inductors• Capacitors store charge in E
field• Inductors store current in B
field
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Inductance• Inductance:
L = Alternatively,V = L
• Units of inductance: Henry (H)
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Inductance• Inductance:
L = Alternatively,V = L
• Units of inductance: Henry (H)
Image obtained from: http://en.wikipedia.org/wiki/Joseph_Henry#mediaviewer/File:Joseph_Henry_-_Brady-Handy.jpg
Joseph Henry
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Inductance• Inductors in series:
Leqv = L1 + L2 + L3 + …• Inductors in parallel:
= + + + …
Image obtained from: http://en.wikipedia.org/wiki/Joseph_Henry#mediaviewer/File:Joseph_Henry_-_Brady-Handy.jpg
Joseph Henry