College Physics

# Introduction to Electromagnetic Induction, AC Circuits and Electrical Technologies

College PhysicsIntroduction to Electromagnetic Induction, AC Circuits and Electrical Technologies
Figure 23.1 These wind turbines in the Thames Estuary in the UK are an example of induction at work. Wind pushes the blades of the turbine, spinning a shaft attached to magnets. The magnets spin around a conductive coil, inducing an electric current in the coil, and eventually feeding the electrical grid. (credit: modification of work by Petr Kratochvil)

### Chapter Outline

23.1 Induced Emf and Magnetic Flux
• Calculate the flux of a uniform magnetic field through a loop of arbitrary orientation.
• Describe methods to produce an electromotive force (emf) with a magnetic field or magnet and a loop of wire.
23.2 Faraday’s Law of Induction: Lenz’s Law
• Calculate emf, current, and magnetic fields using Faraday’s Law.
• Explain the physical results of Lenz’s Law
23.3 Motional Emf
• Calculate emf, force, magnetic field, and work due to the motion of an object in a magnetic field.
23.4 Eddy Currents and Magnetic Damping
• Explain the magnitude and direction of an induced eddy current, and the effect this will have on the object it is induced in.
• Describe several applications of magnetic damping.
23.5 Electric Generators
• Calculate the emf induced in a generator.
• Calculate the peak emf which can be induced in a particular generator system.
23.6 Back Emf
• Explain what back emf is and how it is induced.
23.7 Transformers
• Explain how a transformer works.
• Calculate voltage, current, and/or number of turns given the other quantities.
23.8 Electrical Safety: Systems and Devices
• Explain how various modern safety features in electric circuits work, with an emphasis on how induction is employed.
23.9 Inductance
• Calculate the inductance of an inductor.
• Calculate the energy stored in an inductor.
• Calculate the emf generated in an inductor.
23.10 RL Circuits
• Calculate the current in an RL circuit after a specified number of characteristic time steps.
• Calculate the characteristic time of an RL circuit.
• Sketch the current in an RL circuit over time.
23.11 Reactance, Inductive and Capacitive
• Sketch voltage and current versus time in simple inductive, capacitive, and resistive circuits.
• Calculate inductive and capacitive reactance.
• Calculate current and/or voltage in simple inductive, capacitive, and resistive circuits.
23.12 RLC Series AC Circuits
• Calculate the impedance, phase angle, resonant frequency, power, power factor, voltage, and/or current in a RLC series circuit.
• Draw the circuit diagram for an RLC series circuit.
• Explain the significance of the resonant frequency.

Nature’s displays of symmetry are beautiful and alluring. A butterfly’s wings exhibit an appealing symmetry in a complex system. (See Figure 23.2.) The laws of physics display symmetries at the most basic level—these symmetries are a source of wonder and imply deeper meaning. Since we place a high value on symmetry, we look for it when we explore nature. The remarkable thing is that we find it.

Figure 23.2 Physics, like this butterfly, has inherent symmetries. (credit: Thomas Bresson)

The hint of symmetry between electricity and magnetism found in the preceding chapter will be elaborated upon in this chapter. Specifically, we know that a current creates a magnetic field. If nature is symmetric here, then perhaps a magnetic field can create a current. The Hall effect is a voltage caused by a magnetic force. That voltage could drive a current. 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.

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. Not so familiar perhaps, but important nevertheless, is that the behavior of AC circuits depends strongly on the effect of magnetic fields on currents.