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?
• 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.
• 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.
o 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.
o We’ll look at the different possibilities here, and decide (based on our
knowledge of physics) which one is correct.
o 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.
o This induces a current in the wires as shown.
o 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…