 # Electromagnetic Induction

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Electromagnetic Induction
Chapter 22

Expectations After this chapter, students will:
Calculate the EMF resulting from the motion of conductors in a magnetic field Understand the concept of magnetic flux, and calculate the value of a magnetic flux Understand and apply Faraday’s Law of electromagnetic induction Understand and apply Lenz’s Law

Expectations After this chapter, students will:
Apply Faraday’s and Lenz’s Laws to some particular devices: Electric generators Electrical transformers Calculate the mutual inductance of two conducting coils Calculate the self-inductance of a conducting coil

Motional EMF A wire passes through a uniform magnetic field. The length of the wire, the magnetic field, and the velocity of the wire are all perpendicular to one another:

Motional EMF A positive charge in the wire experiences a magnetic force, directed upward:

Motional EMF A negative charge in the wire experiences the same magnetic force, but directed downward: These forces tend to separate the charges.

Motional EMF The separation of the charges produces an electric field, E. It exerts an attractive force on the charges: E

Motional EMF In the steady state (at equilibrium), the magnitudes of the magnetic force – separating the charges – and the Coulomb force – attracting them – are equal. E

Motional EMF Rewrite the electric field as a potential gradient:
Substitute this result back into our earlier equation: E

Motional EMF Substitute this result back into our earlier equation: E

Motional EMF This is called motional EMF. It results from the constant velocity of the wire through the magnetic field, B. E

Motional EMF Now, our moving wire slides over two other wires, forming a circuit. A current will flow, and power is dissipated in the resistive load:

Motional EMF Where does this power come from? Consider the magnetic
force acting on the current in the sliding wire:

Motional EMF Right-hand rule #1 shows that this force opposes the motion of the wire. To move the wire at constant velocity requires an equal and opposite force. That force does work: The power:

Motional EMF The force’s magnitude was calculated as: Substituting:
which is the same as the power dissipated electrically.

Motional EMF Suppose that, instead of being perpendicular to the plane of the sliding-wire circuit, the magnetic field had made an angle f with the perpendicular to that plane. The perpendicular component of B: B cos f

Motional EMF The motional EMF: Rewrite the velocity: Substitute:

Motional EMF L Dx is simply the change in the loop area.

Motional EMF Define a quantity F : Then: F is called magnetic flux.
SI units: T·m2 = Wb (Weber)

Magnetic Flux Wilhelm Eduard Weber 1804 – 1891
German physicist and mathematician

Faraday’s Law In our previous result, we said that the induced EMF was equal to the time rate of change of magnetic flux through a conducting loop. This, rewritten slightly, is called Faraday’s Law: Why the minus sign?

and mathematician

Faraday’s Law To make Faraday’s Law complete, consider adding N conducting loops (a coil): What can change the magnetic flux? B could change, in magnitude or direction A could change f could change (the coil could rotate)

Lenz’s Law says that the direction of the induced current is always such as to oppose the change in magnetic flux that produced it. The minus sign in Faraday’s Law reminds us of that.

Lenz’s Law Heinrich Friedrich Emil Lenz 1804 – 1865 Russian physicist

Lenz’s Law Lenz’s Law says that the direction of the induced current is always such as to oppose the change in magnetic flux that produced it. What does that mean? How can an induced current “oppose” a change in magnetic flux?

Lenz’s Law How can an induced current “oppose” a change in magnetic flux? A changing magnetic flux induces a current. The induced current produces a magnetic field. The direction of the induced current determines the direction of the magnetic field it produces. The current will flow in the direction (remember right-hand rule #2) that produces a magnetic field that works against the original change in magnetic flux.

A coil rotates with a constant angular speed in a magnetic field. but f changes with time:

So the flux also changes with time: Get the time rate of change (a calculus problem): Substitute into Faraday’s Law:

The maximum voltage occurs when : What makes the voltage larger? more turns in the coil a larger coil area a stronger magnetic field a larger angular speed

Back EMF in Electric Motors
An electric motor also contains a coil rotating in a magnetic field. In accordance with Lenz’s Law, it generates a voltage, called the back EMF, that acts to oppose its motion.

Back EMF in Electric Motors
Apply Kirchhoff’s loop rule:

Mutual Inductance A current in a coil produces a magnetic field.
If the current changes, the magnetic field changes. Suppose another coil is nearby. Part of the magnetic field produced by the first coil occupies the inside of the second coil.

Mutual Inductance Faraday’s Law says that the changing magnetic flux in the second coil produces a voltage in that coil. The net flux in the secondary:

Mutual Inductance Convert to an equation, using a constant of proportionality:

Mutual Inductance The constant of proportionality is called the mutual inductance:

Mutual Inductance Substitute this into Faraday’s Law:
SI units of mutual inductance: V·s / A = henry (H)

Mutual Inductance Joseph Henry 1797 – 1878 American physicist

Self-Inductance Changing current in a primary coil induces a voltage in a secondary coil. Changing current in a coil also induces a voltage in that same coil. This is called self-inductance.

Self-Inductance The self-induced voltage is calculated from Faraday’s Law, just as we did the mutual inductance. The result: The self-inductance, L, of a coil is also measured in henries. It is usually just called the inductance.

Mutual Inductance: Transformers
A transformer is two coils wound around a common iron core.

Mutual Inductance: Transformers
The self-induced voltage in the primary is: Through mutual induction, and EMF appears in the secondary: Their ratio:

Mutual Inductance: Transformers
This transformer equation is normally written: The principle of energy conservation requires that the power in both coils be equal (neglecting heating losses in the core).

Inductors and Stored Energy
When current flows in an inductor, work has been done to create the magnetic field in the coil. As long as the current flows, energy is stored in that field, according to

Inductors and Stored Energy
In general, a volume in which a magnetic field exists has an energy density (energy per unit volume) stored in the field: