# Lenz's Law

Discussion in 'Homework Help' started by extremeads1, Mar 26, 2007.

Feb 10, 2007
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Lenz's Law states that the direction of induced emf is such that it oppses the vary cause which produces, What is meant by " VARY CAUSE WHICH OPPOSES IT ".

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
A change in current(di/dt) produces a voltage(V) that reduces(opposes) the change in current(di/dt).

http://en.wikipedia.org/wiki/Lenz's_law

That this happens can be seen from the first order differential equation for the voltage across an inductor.
Code ( (Unknown Language)):
1.
2. L*(di/dt) + R*i = V*h(t)
3. subject to i(0) = 0
4.
5. where L is the inductance and R is the winding resistance, and
6. i(t) is the instantaneous current.
7. V is the magnitude of the power source, and
8. h(t) is the unit step function
9.
The only function for which the derivative plus the function can add upt to a constant is the exponential function. As we know when the circuit is completed at time t=0 the instantaneous current i(t) starts at 0 and rises toward the constant V. as the voltage acorss the inductor increases the rate of change slow down. This is why we say that the induced voltage or back emf opposes the orriginal change in current.

3. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
A better expression of Lenz's Law is:

"The direction of current flow and hence induced current flow is such that it opposes the change to which it's due"

Still a touch confusing maybe, but then it reads better than the above expression IMO.

Dave

4. ### Blue_turnip New Member

Mar 5, 2007
8
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Perhaps you've learn't what an electromagnet is?
It's a coiled bit of wire. When electricity flows through this wire, a magnetic field is created and the coil becomes a magnet.

Some d00d discovered that when a conductor moves in a magnetic field, a current is induced in that conductor. Don't ask why - it just is. However the conductor only gets a current running through it when it is MOVED within the magnetic field.

Eddy currents are basically just a combination of these facts i've told you, When a magnet moves relative to a conductor, a current is induced in that conductor. The current swirls in circles(learn about the right hand rule and you'll get me) and as a result, it makes a magnetic field. Now if the magnetic field it created didn't oppose the movement of the magnet, it would attract it and we'd get free energy - the world would be saved! Life's tough and the laws of physics dictate that that sorta shizzle can't happen. The magnetic field which is created as a result of the swirling electricity opposes the movement of the magnet that created the swirling electricity.

That's Lenz's law. He basically took the idea that you can't get energy for free, then said it in complex terms relating to magnetism so that he became recognised globally.

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
"Some d00d?!?!!"

Lad, you refer to one of my heroes!

He twice declined presidency of the Royal Society and once declined Knighthood. Despite being born to poverty in a rigidly socially stratified society, he earned an education among the Europen scientific elite. He became not only a chemist (conducting early experiments in diffusion and studying early alloys of steel) but was also a pioneer in electricity. The things he discovered are required reading in every course of study in electronics on this planet!

He built the world's first dynamo. He demonstrated that there was only one kind of electricity at a time when several were thought to exist. He discovered diamagnetism (which is NOT known as "the d00d effect") and surface effects (which are NOT called "d00d cages").

Furthermore, the unit of capacitance is NOT called the "d00d."

"Some d00d" was in a shoe commercial. "Some d00d" recorded a pop song. History will forget both of them. History will NOT forget the giants of human endeavors - even if "some d00ds" do forget them.

Michael Faraday was not just "some d00d."

6. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Doesn't d00d = dude?

Which if I'm not mistaken is a good thing by todays standards

Dave

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Possibly. I just spent some time Googling "d00d" and "some d00d" and my head hurts now.

If Blue Turnip's intent was "K3w1 d00d" then I'll need to draft a retraction. My interpretation of "some d00d" held (and still holds, based on what I dregded up from Google) the connotation of annonymity. I assumed (perhaps unjustly) our Cerulean Crucifer simply didn't care who made the discovery. If my assumption is false, then I offer apology.

8. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
While it is always correct to use proper grammar and spelling, most attempts at clever abreviation come off as pretentious affectations. I don't regard them as either clever, or particularly meaningful.

9. ### Dave Retired Moderator

Nov 17, 2003
6,960
145
Sometimes these abbreviations lead to a misunderstanding, I am only assuming d00d = dude.

Either way, you drafted a nice little biography of Mr Faraday!

Dave