Lenz's Law (1 Viewer)
- Thread starter melllyy
- Start date
Lenz's law simply states that any induced EMF will be in a direction so that it opposes whatever change induced the EMF, and simply is an application of conservation of energy. To explain this, one must understand what induces an emf, and from your second question it seems you do not understand this.
EMF = delta flux/ delta time
Flux refers to the amount of perpendicular components of magnetic field lines that are going through an area at any point in time, and is given by BAcos theta where theta is the angle between the magnetic field and the area for a plane area and uniform magnetic field.
Knowing this, and understanding what flux is, I am sure that you can appreciate that a change in flux is, simply, the change in BAcos theta. So, if the magnetic field reduces in size from B2 to B1, then the change in flux is (B1-B2)Acos theta...
Now, this principle shows that a change in flux will produce an EMF, and this EMF in a loop will produce a current. This current causes the creation of a magnetic field, and this magnetic field should oppose the change that caused the initial change in flux.
Now, an example
___________
|xxxxxxxxxx |
Let us say that the magnetic field in this loop is decreasing. The change in flux is given by (delta B)A / delta t, which equals the EMF
This EMF is a voltage, and as V=IR, I=V/R, and a current will be created, but the question remains... in what direction will this current be?
Case 1: Anticlockwise current
Consider the direction that the magnetic field caused by an anticlockwise current in the centre of this current loop would be using the right hand screw rule (That is, place thumb of right hand in direction of current, fingers curl in direction of the magnetic field) and you shall see that this would cause a current out of the page. THIS IS NOT GOOD, as the magnetic field into the page is decreasing, and an induced magnetic field that went out of the page would simply add to this effect, not counteract it is per lenz's law, so this is incorrect
Case 2: Clockwise current
The magnetic field would increase into the page, counteracting the change in magnetic field causing the change in flux (That is a decreasing magnetic field into the page). Hence this is correct.
And that, my friend, is lenz's law
I hope I have been of assistance
EMF = delta flux/ delta time
Flux refers to the amount of perpendicular components of magnetic field lines that are going through an area at any point in time, and is given by BAcos theta where theta is the angle between the magnetic field and the area for a plane area and uniform magnetic field.
Knowing this, and understanding what flux is, I am sure that you can appreciate that a change in flux is, simply, the change in BAcos theta. So, if the magnetic field reduces in size from B2 to B1, then the change in flux is (B1-B2)Acos theta...
Now, this principle shows that a change in flux will produce an EMF, and this EMF in a loop will produce a current. This current causes the creation of a magnetic field, and this magnetic field should oppose the change that caused the initial change in flux.
Now, an example
___________
|xxxxxxxxxx |
Let us say that the magnetic field in this loop is decreasing. The change in flux is given by (delta B)A / delta t, which equals the EMF
This EMF is a voltage, and as V=IR, I=V/R, and a current will be created, but the question remains... in what direction will this current be?
Case 1: Anticlockwise current
Consider the direction that the magnetic field caused by an anticlockwise current in the centre of this current loop would be using the right hand screw rule (That is, place thumb of right hand in direction of current, fingers curl in direction of the magnetic field) and you shall see that this would cause a current out of the page. THIS IS NOT GOOD, as the magnetic field into the page is decreasing, and an induced magnetic field that went out of the page would simply add to this effect, not counteract it is per lenz's law, so this is incorrect
Case 2: Clockwise current
The magnetic field would increase into the page, counteracting the change in magnetic field causing the change in flux (That is a decreasing magnetic field into the page). Hence this is correct.
And that, my friend, is lenz's law
I hope I have been of assistance
conman
Member
You wrote that flux=BAcosthetaGowr said:
But according to definition, magnetic flux shows number of magnetic field lines passing through surface area (of course we know at 90 degree).
All right flux=BA cos 90
flux = 0
But maxium flux when at 90 therefore, I think something wrong with the formula!!!
jed.hamers
Member
Nope just means it's parallel.
If the angle is taken from where the plane intercepts the magnetic lines of flux then the formula is BAsin @ not cos@. This is because as @ approaches 90 degrees, cos @ approaches 0, contradicting the statement that magnetic flux density is greatest when it is perpendicular to the lines of flux.