Note-A-Rific:
Lenz’s Law
A question still remains when it comes to generating electricity using a
changing magnetic field… what direction does the current flow?
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Just a
few years after Faraday discovered EM induction, the German physicist Heinrich
Lenz came up with a rule based on some logic.
Lenz started with a general idea and worked out his rule.
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If you
use a magnet to induce an electrical current to flow through a coil of wire,
you will actually be creating a brand new magnetic field in the coil.
o
This
magnetic field is called an induced field.
o
Remember,
any coil of wire with current flowing through it is a solenoid, a magnet.
·
Lenz
wondered if the induced field would attract or repel the magnet.
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He knew
that he would also have to look at a couple of different cases.
§
Magnets
being pushed into the coil.
§
Magnets
being pulled out of the coil.
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We’ll
look at the different possibilities here, and decide (based on our knowledge of
physics) which one is correct.
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For
now we will only look at situations of pushing the magnet into the coil.
First Possibility
·
As I
push the magnet into the coil, I measure the electron flow going in the
direction shown.
·
What
is the direction of the magnetic field induced in the coil…?
o
Using
the 2nd Left Hand Rule, we find that the end of the coil nearest to
the magnet is the south end…
·
Let’s
examine what would happen if this is correct…
o
I
start pushing the magnet to the right.
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This
induces a current in the wires as shown.
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The induced
current creates an induced magnetic field around the solenoid, with the south
end near the magnet I started pushing.
o
Since
south attracts north, the coil pulls the magnet into the magnetic field… I
don’t even have to push it anymore.
·
I now
have the electrical energy in the coils and the kinetic energy of a moving
magnet without having to do anything!!!
·
Sorry,
but this is wrong.
o
It
breaks the laws of thermodynamics… basically, “you can’t get something for
nothing.”
o
If this
situation was true, it would be like the universe was giving you energy for
free.
Second Possibility
·
As I
push the magnet into the coil, I measure the electron flow going in the
direction shown.
·
What
is the direction of the magnetic field induced in the coil…?
o
Using
the 2nd Left Hand Rule, we find that the end of the coil nearest to
the magnet is the north end (opposite to what it was before)…
·
Let’s
examine what would happen if this is correct…
o
I
start pushing the magnet to the right.
o
This
induces a current in the wires as shown.
o
The
induced current creates an induced magnetic field around the solenoid, with the
north end near the magnet I started pushing.
o
Since
north repels north, the coil pushes against the magnet … oh, man!
I’m going to have to keep pushing really hard to keep generating electricity!
·
Even
though you might not like doing work, this one is correct.
o
It
obeys the laws of thermodynamics… I am not “getting something for
nothing.”
o
The
energy of my muscles (or another device) doing work the whole time is being
transformed into electrical energy.
When I first learned about this, I had a way of remembering it.
“The universe is a tough place, and it will always make you do more
work than you want to!”
A similar situation happens when you are trying to pull the magnet out
of the coils…
·
Notice
that when I pull the magnet out of the coil, the direction of the current
reverses.
·
This
means that the induced magnetic field of the solenoid is also pointing in the
opposite direction.
o
Now the
north end of the magnet is near the south end of the solenoid… they’ll attract!
o
Oh no!
To keep on generating electricity I’ll have to do even more work to try to pull
these two apart!
o
See, I
told you the universe will always make you do more work than you want to…