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Lenz's Law

Lenz’s law states that:-
"The direction of induced current is always such as to oppose the cause which produces it".

Consider a bar magnet and a coil of wire.

When the N-pole of magnet is approaching the face of the coil, it becomes a north face by the induction of current in anticlockwise direction to oppose forward motion of the magnet.

When the N-pole of the magnet is receding the face of the coil becomes a south pole due to a clockwise induced current to oppose the backward motion.

SOME TIPS...

SELAMAT MENEMPUH EXAM...

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Something To Be Remembered ♥

For the first equation, the induced emf tends to oppose the flux changes AND the minus sign indicating the opposition BUT we often neglect the minus sign, seeking only magnitude of induce emf.

If we change the magnetic flux through a coil of N turns, an induced emf appears in every turn and the total induced in the coil is sum of  these individual induced emfs. If the coil is tightly wound (closely packed), so that the same magnetic flux passes through all the turns, the total emf induced in the coil is shown as second equation.

To determine the direction of current using the "Right-Hand Rule":

Thumb- points in the direction of motion of the field relative to the conductor

Forefinger- points in the direction of that field

Middle finger- the direction of induced current

Mutual Induction

In the previous tutorial we saw that an inductor generates an induced emf within itself as a result of the changing magnetic field around its own turns, and when this emf is induced in the same circuit in which the current is changing this effect is called Self-induction, ( L ). However, when the emf is induced into an adjacent coil situated within the same magnetic field, the emf is said to be induced magnetically, inductively or by Mutual induction, symbol ( M ). Then when two or more coils are magnetically linked together by a common magnetic flux they are said to have the property of Mutual Inductance. Mutual Inductance is the basic operating principal of transformers, motors, generators and any other electrical component that interacts with another magnetic field. Then we can define mutual induction as the current flowing in one coil induces an emf in an adjacent coil. But mutual inductance can also be a bad thing as "stray" or "leakage" inductance from a coil can interfere with the operation of another adjacent component by means of electromagnetic induction, so some form of electrical screening to a ground potential may be required.

The amount of mutual inductance that links one coil to another depends very much on the relative positioning of the two coils. If one coil is positioned next to the other coil so that their physical distance apart is small, then nearly all of the magnetic flux generated by the first coil will interact with the coil turns of the second coil inducing a relatively large emf and therefore producing a large mutual inductance value. Likewise, if the two coils are farther apart from each other or at different angles, the amount of induced magnetic flux from the first coil into the second will be weaker producing a much smaller induced emf and therefore a much smaller mutual inductance value. So the effect of mutual inductance is very much dependant upon the relative positions or spacing, ( S ) of the two coils. Some example of mutual inductance:

Here the current flowing in coil one, L1 sets up a magnetic field around itself with some of these magnetic field lines passing through coil two, L2 giving us mutual inductance. Coil one has a current of I1 and N1 turns while, coil two has N2 turns. Therefore, the mutual inductance, M12 of coil two that exists with respect to coil one depends on their position with respect to each other and is given as:

Likewise, the flux linking coil one, L1 when a current flows around coil two, L2 is exactly the same as the flux linking coil two when the same current flows around coil one above, then the mutual inductance of coil one with respect of coil two is defined as M21. This mutual inductance is true irrespective of the size, number of turns, relative position or orientation of the two coils. Because of this, we can write the mutual inductance between the two coils as: M12 = M21 = M. We remember from our tutorials on Electromagnets that the self inductance of each individual coil is given as:

Then by cross-multiplying the two equations above, the mutual inductance that exists between the two coils can be expressed in terms of the self inductance of each coil.

giving us a final and more common expression for the mutual inductance between two coils as: Mutual Inductance Between Coils

However, the above equation assumes zero flux leakage and 100% magnetic coupling between the two coils, L1 and L2. In reality there will always be some loss due to leakage and position, so the magnetic coupling between the two coils can never reach or exceed 100%, but can become very close to this value in some special inductive coils. If some of the total magnetic flux links with the two coils, this amount of flux linkage can be defined as a fraction of the total possible flux linkage between the coils. This fractional value is called the coefficient of coupling and is given the letter k.

nabihah, akasyah, faezah, diyanah

Discovery of Mr. Faraday

Faraday realized that when E.M.F. is setup in a coil placed in a magnetic field whenever the flux through the coil changes. This effect is called Electromagnetic Induction. If the coil forms a part of a close circuit, the E.M.F. causes a current to flow in the circuit.

E.M.F. setup in the coil is called "induced E.M.F" and the current thus produced is termed as "Induced Current".

Experiments show that the magnitude of E.M.F. depends on the rate at which the flux through
the coil changes. It also depends on the number of turns on the coil.

There are various ways to change magnetic flux of a coil such as;

(1) By changing the relative position of the coil with respect to a magnet.

(2) By changing current in the coil itself.

(3) By changing current in the neighboring coil.

(4) By changing area of a coil placed in the magnetic field etc.

The explanation of the diagram above:

At upper left in the illustration, two coils are penetrated by a changing magnetic field. Magnetic flux F is defined by F=BA where B is the magnetic field or average magnetic field and A is the area perpendicular to the magnetic field. Note that for a given rate of change of the flux through the coil, the voltage generated is proportional to the number of turns N which the flux penetrates. This example is relevant to the operation of transformers, where the magnetic flux typically follows an iron core from the primary coil to the secondary coil and generates a secondary voltage proportional to the number of turns in the secondary coil.

Proceeding clockwise, the second example shows the voltage generated when a coil is moved into a magnetic field. This is sometimes called "motional emf", and is proportional to the speed with which the coil is moved into the magnetic field. That speed can be expressed in terms of the rate of change of the area which is in the magnetic field.

The next example is the standard AC generator geometry where a coil of wire is rotated in a magnetic field. The rotation changes the perpendicular area of the coil with respect to the magnetic field and generates a voltage proportional to the instantaneous rate of change of the magnetic flux. For a constant rotational speed, the voltage generated is sinusoidal.

The final example shows that voltage can be generated by moving a magnet toward or away from a coil of wire. With the area constant, the changing magnetic field causes a voltage to the generated. The direction or "sense" of the voltage generated is such that any resulting current produces a magnetic field opposing the change in magnetic field which created it. This is the meaning of the minus sign in Faraday's Law, and it is called Lenz's Law.