<< Chapter < Page Chapter >> Page >

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.


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 .


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"} {}


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.

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!

Questions & Answers

what's the period of velocity 4cm/s at displacement 10cm
Andrew Reply
What is physics
LordRalph Reply
the branch of science concerned with the nature and properties of matter and energy. The subject matter of physics includes mechanics, heat, light and other radiation, sound, electricity, magnetism, and the structure of atoms.
and the word of matter is anything that have mass and occupied space
what is phyices
Aurang Reply
Whats the formula
Okiri Reply
what aspect of black body spectrum forced plank to purpose quantization of energy level in its atoms and molicules
Shoaib Reply
a man has created by who?
Angel Reply
What type of experimental evidence indicates that light is a wave
Edeh Reply
double slit experiment
The S. L. Unit of sound energy is
Chukwuemeka Reply
what's the conversation like?
some sort of blatherring or mambo jambo you may say
I still don't understand what this group is all about oo
ufff....this associated with physics ..so u can ask questions related to all topics of physics..
what is sound?
what is upthrust
Mercy Reply
what is upthrust
Up thrust is a force
upthrust is a upward force that acts vertical in the ground surface.
yes rodney's answer z correct
what is centre of gravity?
you think the human body could produce such Force
what is wave
Bryan Reply
what is specific latent heat
Omosebi Reply
the total amount of heat energy required to change the physical state of a unit mass of matter without a corresponding change in temperature.
is there any difference between specific heat and heat capacity.....
what wave
why medical physics even.we have a specoal branch of science biology for this.
Sahrrr Reply
what is physics
AbleGod Reply
what is the s.i unit for work
Betty Reply
Practice Key Terms 2

Get the best College physics course in your pocket!

Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?