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Making connections: conservation of energy

Lenz’s law is a manifestation of the conservation of energy. The induced emf produces a current that opposes the change in flux, because a change in flux means a change in energy. Energy can enter or leave, but not instantaneously. Lenz’s law is a consequence. As the change begins, the law says induction opposes and, thus, slows the change. In fact, if the induced emf were in the same direction as the change in flux, there would be a positive feedback that would give us free energy from no apparent source—conservation of energy would be violated.

Calculating emf: how great is the induced emf?

Calculate the magnitude of the induced emf when the magnet in [link] (a) is thrust into the coil, given the following information: the single loop coil has a radius of 6.00 cm and the average value of B cos θ size 12{B"cos"θ} {} (this is given, since the bar magnet’s field is complex) increases from 0.0500 T to 0.250 T in 0.100 s.

Strategy

To find the magnitude of emf, we use Faraday’s law of induction as stated by emf = N Δ Φ Δ t , but without the minus sign that indicates direction:

emf = N Δ Φ Δ t .

Solution

We are given that N = 1 size 12{N=1} {} and Δ t = 0 . 100 s , but we must determine the change in flux Δ Φ size 12{ΔΦ} {} before we can find emf. Since the area of the loop is fixed, we see that

Δ Φ = Δ ( BA cos θ ) = A Δ ( B cos θ ). size 12{ΔΦ=Δ \( BA"cos"θ \) =AΔ \( B"cos"θ \) } {}

Now Δ ( B cos θ ) = 0 . 200 T size 12{Δ \( B"cos"θ \) =0 "." "200"`T} {} , since it was given that B cos θ size 12{B"cos"θ} {} changes from 0.0500 to 0.250 T. The area of the loop is A = πr 2 = ( 3 . 14 . . . ) ( 0 . 060 m ) 2 = 1 . 13 × 10 2 m 2 size 12{A=πr rSup { size 8{2} } = \( 3 "." "14" "." "." "." \) \( 0 "." "060"`m \) rSup { size 8{2} } =1 "." "13" times "10" rSup { size 8{ - 2} } `m rSup { size 8{2} } } {} . Thus,

Δ Φ = ( 1.13 × 10 2 m 2 ) ( 0.200 T ). size 12{ΔΦ= \( 1 "." "13" times "10" rSup { size 8{ - 2} } " m" rSup { size 8{2} } \) \( 0 "." "200"" T" \) } {}

Entering the determined values into the expression for emf gives

Emf = N Δ Φ Δ t = ( 1.13 × 10 2 m 2 ) ( 0 . 200 T ) 0 . 100 s = 22 . 6 mV. size 12{E=N { {ΔΦ} over {Δt} } = { { \( 1 "." "13" times "10" rSup { size 8{ - 2} } " m" rSup { size 8{2} } \) \( 0 "." "200"" T" \) } over {0 "." "100"" s"} } ="22" "." 6" mV"} {}

Discussion

While this is an easily measured voltage, it is certainly not large enough for most practical applications. More loops in the coil, a stronger magnet, and faster movement make induction the practical source of voltages that it is.

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Phet explorations: faraday's electromagnetic lab

Play with a bar magnet and coils to learn about Faraday's law. Move a bar magnet near one or two coils to make a light bulb glow. View the magnetic field lines. A meter shows the direction and magnitude of the current. View the magnetic field lines or use a meter to show the direction and magnitude of the current. You can also play with electromagnets, generators and transformers!

Faraday's Electromagnetic Lab

Section summary

  • Faraday’s law of induction states that the emf induced by a change in magnetic flux is
    emf = N Δ Φ Δ t size 12{"emf"= - N { {ΔΦ} over {Δt} } } {}

    when flux changes by Δ Φ size 12{ΔΦ} {} in a time Δ t size 12{Δt} {} .

  • If emf is induced in a coil, N is its number of turns.
  • The minus sign means that the emf creates a current I size 12{I} {} and magnetic field B size 12{B} {} that oppose the change in flux Δ Φ size 12{ΔΦ} {} —this opposition is known as Lenz’s law.

Conceptual questions

A person who works with large magnets sometimes places her head inside a strong field. She reports feeling dizzy as she quickly turns her head. How might this be associated with induction?

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A particle accelerator sends high-velocity charged particles down an evacuated pipe. Explain how a coil of wire wrapped around the pipe could detect the passage of individual particles. Sketch a graph of the voltage output of the coil as a single particle passes through it.

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Questions & Answers

explain the three laws of isaac Newton with the reference
glory Reply
1st law ; a body will continue to stay at a state of rest or continue to move at a uniform motion on a straight line unless an external force is been acted upon
Austine
3rd law; in every action there is an equal or opposite reaction
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2nd law: F=ma
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newtons law of motion
hasiya
First law:In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
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is the ability to do work
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Energy
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energy is ability of the capacity to doing work
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A quantity that has both magnitude and direction
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Is the quantity that has both magnitude and direction
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Yes it can float in space,e.g.polyethene has no mass that's why it can float in space
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A charge of 1.6*10^-6C is placed in a uniform electric field in a density 2*5^10Nc^-1, what is the magnitude of the electric force exerted on the charge?
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Phenomena is an observable fact or event.
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the time taken for a radioactive element to decay by half of its original mass
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Half of the total time required by a radioactive nuclear atom to totally disintegrate
Justice
radioactive elements are those with unstable nuclei(ie have protons more than neutrons, or neutrons more than protons
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in other words, the radioactive atom or elements have unequal number of protons to neutrons.
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Velocity is the rate of change of displacement
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An atom is the smallest indivisible particular of an element
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Practice Key Terms 2

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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