Physics for Scientists and Engineers 8ed - ch32 - PowerPoint Slides

# Physics for Scientists and Engineers 8ed - ch32 - PowerPoint Slides

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Chapter 32 Inductance

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Joseph Henry 1797 – 1878 American physicist First director of the Smithsonian Improved design of electromagnet Constructed one of the first motors Discovered self-inductance Unit of inductance is named in his honor
Some Terminology Use emf and current when they are caused by batteries or other sources Use induced emf and induced current when they are caused by changing magnetic fields When dealing with problems in electromagnetism, it is important to distinguish between the two situations

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Self-Inductance When the switch is closed, the current does not immediately reach its maximum value Faraday’s law can be used to describe the effect
Self-Inductance, 2 As the current increases with time, the magnetic flux through the circuit loop due to this current also increases with time This increasing flux creates an induced emf in the circuit

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Self-Inductance, 3 The direction of the induced emf is such that it would cause an induced current in the loop which would establish a magnetic field opposing the change in the original magnetic field The direction of the induced emf is opposite the direction of the emf of the battery This results in a gradual increase in the current to its final equilibrium value
Self-Inductance, 4 This effect is called self-inductance Because the changing flux through the circuit and the resultant induced emf arise from the circuit itself The emf ε L is called a self-induced emf

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Self-Inductance, Equations An induced emf is always proportional to the time rate of change of the current The emf is proportional to the flux, which is proportional to the field and the field is proportional to the current L is a constant of proportionality called the inductance of the coil and it depends on the geometry of the coil and other physical characteristics L d I m L dt = -
Inductance of a Coil A closely spaced coil of N turns carrying current I has an inductance of The inductance is a measure of the opposition to a change in current B L N m L I d I dt Φ = = -

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Inductance Units The SI unit of inductance is the henry (H) Named for Joseph Henry A s V 1 H 1 =
Inductance of a Solenoid Assume a uniformly wound solenoid having N turns and length ℓ Assume ℓ is much greater than the radius of the solenoid The flux through each turn of area A is B o o N BA mnI A m I A Φ = = = l

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Inductance of a Solenoid, cont The inductance is This shows that L depends on the geometry of the object 2 o B mN A N L I Φ = = l
RL Circuit, Introduction A circuit element that has a large self- inductance is called an inductor The circuit symbol is We assume the self-inductance of the rest of the circuit is negligible compared to the inductor However, even without a coil, a circuit will have some self-inductance

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Effect of an Inductor in a Circuit The inductance results in a back emf Therefore, the inductor in a circuit opposes
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## This note was uploaded on 05/22/2010 for the course PHYS 2326 taught by Professor Staff during the Summer '08 term at HCCS.

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Physics for Scientists and Engineers 8ed - ch32 - PowerPoint Slides

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