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# 23: Electromagnetic Induction, AC Circuits, and Electrical Technologies

Historically, it was very shortly after Oersted discovered currents cause magnetic fields that other scientists asked the following question: Can magnetic fields cause currents? The answer was soon found by experiment to be yes. In 1831, some 12 years after Oersted’s discovery, the English scientist Michael Faraday (1791–1862) and the American scientist Joseph Henry (1797–1878) independently demonstrated that magnetic fields can produce currents. The basic process of generating emfs (electromotive force) and, hence, currents with magnetic fields is known as induction; this process is also called magnetic induction to distinguish it from charging by induction, which utilizes the Coulomb force.

• 23.0: Prelude to Electromagnetic Induction, AC Circuits and Electrical Technologies
Today, currents induced by magnetic fields are essential to our technological society. The ubiquitous generator—found in automobiles, on bicycles, in nuclear power plants, and so on—uses magnetism to generate current. Other devices that use magnetism to induce currents include pickup coils in electric guitars, transformers of every size, certain microphones, airport security gates, and damping mechanisms on sensitive chemical balances.
• 23.1: RL Circuits
When the voltage applied to an inductor is changed, the current also changes, but the change in current lags the change in voltage in an RL circuit. In Reactance, Inductive and Capacitive, we explore how an RL circuit behaves when a sinusoidal AC voltage is applied.
• 23.2: Reactance, Inductive and Capacitive
• 23.3: RLC Series AC Circuits
• 23.4: Induced Emf and Magnetic Flux
Any change in magnetic flux Φ induces an emf—the process is defined to be electromagnetic induction.
• 23.5: Faraday’s Law of Induction- Lenz’s Law
Faraday’s experiments showed that the emf induced by a change in magnetic flux depends on only a few factors. First, emf is directly proportional to the change in flux ΔΦ. Second, emf is greatest when the change in time Δt is smallest—that is, emf is inversely proportional to Δt . Finally, if a coil has N turns, an emf will be produced that is Ntimes greater than for a single coil, so that emf is directly proportional to N.
• 23.6: Motional Emf
As we have seen, any change in magnetic flux induces an emf opposing that change—a process known as induction. Motion is one of the major causes of induction. For example, a magnet moved toward a coil induces an emf, and a coil moved toward a magnet produces a similar emf. In this section, we concentrate on motion in a magnetic field that is stationary relative to the Earth, producing what is loosely called motional emf.
• 23.7: Eddy Currents and Magnetic Damping
Motional emf is induced when a conductor moves in a magnetic field or when a magnetic field moves relative to a conductor. If motional emf can cause a current loop in the conductor, we refer to that current as an eddy current. Eddy currents can produce significant drag, called magnetic damping, on the motion involved.
• 23.8: Electric Generators
Electric generators induce an emf by rotating a coil in a magnetic field, as briefly discussed in "Induced Emf and Magnetic Flux." We will now explore generators in more detail. Consider the following example.
• 23.9: Back Emf
• 23.10: Transformers
Transformers do what their name implies—they transform voltages from one value to another. Transformers are also used at several points in the power distribution systema. . Power is sent long distances at high voltages, because less current is required for a given amount of power, and this means less line loss, as was discussed previously. But high voltages pose greater hazards, so that transformers are employed to produce lower voltage at the user’s location.
• 23.11: Electrical Safety - Systems and Devices
Electricity has two hazards. A thermal hazard occurs when there is electrical overheating. A shock hazard occurs when electric current passes through a person. Both hazards have already been discussed. Here we will concentrate on systems and devices that prevent electrical hazards.
• 23.12: Inductance
• 23.E: Electromagnetic Induction, AC Circuits, and Electrical Technologies (Exercise)

Thumbnail: Small cheap inductor. (CC-SA-BY 3.0; FDominec).