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# 10: Electromagnetic Induction

• 10.1: Introduction to Electromagnetic Induction
In 1820, Oersted had shown that an electric current generates a magnetic field. But can a magnetic field generate an electric current? This was answered almost simultaneously and independently in 1831 by Joseph Henry in the United States and Michael Faraday in Great Britain.
• 10.2: Electromagnetic Induction and the Lorentz Force
Lorentz force is underpining the observation that the movement of a meta rod through the magnetic field induces a potential difference across the ends of the rod.  This is electromagnetic induction, and, seen this way, there is nothing new: electromagnetic induction is nothing more than the Lorentz force on the conduction electrons within the metal.
• 10.3: Lenz's Law
Lenz's Law argues that when an EMF is induced in a circuit as a result of changing magnetic flux through the circuit, the direction of the induced EMF is such as to oppose the change of flux that causes it.
• 10.4: Ballistic Galvanometer and the Measurement of Magnetic Field
A galvanometer is similar to a sensitive ammeter, differing mainly in that when no current passes through the meter, the needle is in the middle of the dial rather than at the left hand end. A galvanometer is used not so much to measure a current, but rather to detect whether or not a current is flowing, and in which direction. In the ballistic galvanometer, the motion of the needle is undamped, or as close to undamped as can easily be achieved.
• 10.5: AC Generator
This and the following sections will be devoted to generators and motors. I shall not be concerned with – and indeed am not knowledgeable about – the engineering design or practical details of real generators or motors, but only with the scientific principles involved. The "generators" and "motors" of this chapter will be highly idealized abstract concepts bearing little obvious resemblance to the real things.
• 10.6: AC Power
When a current I flows through a resistance R , the rate of dissipation of electrical energy as heat is IR² . If an alternating potential difference V=V₀ sin ωt is applied across a resistance, then an alternating current I=I₀ sin ωt will flow through it, and the rate at which energy is dissipated as heat will also change periodically.
• 10.7: Linear Motors and Generators
In this section describes highly idealized and imaginary linear motors and generators, only because the geometry is simpler than for rotary motors, and it is easier to explain certain principles. Rotary motors will be discussed in the following section.
• 10.8: Rotary Motors
Most real motors, of course, are rotary motors, though all of the principles described for our highly idealized linear motor of the Section 10.7 still apply.
• 10.9: The Transformer
A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a varying EMF or "voltage" in a second coil. Power can be transferred between the two coils through the magnetic field, without a metallic connection between the two circuits.
• 10.10: Mutual Inductance
Consider two coils, not connected to one another, other than being close together in space. If the current changes in one of the coils, so will the magnetic field in the other, and consequently an EMF will be induced in the second coil. The ratio of the EMF induced in the second coil to the rate of change of current in the first is called the coefficient of mutual inductance.
• 10.11: Self Inductance
In this section we are dealing with the self inductance of a single coil rather than the mutual inductance between two coils. If the current through a single coil changes, the magnetic field inside that coil will change; consequently a back EMF will be induced in the coil that will oppose the change in the magnetic field and indeed will oppose the change of current.
• 10.12: Growth of Current in a Circuit Containing Inductance
• 10.13: Discharge of a Capacitor through an Inductance
• 10.14: Discharge of a Capacitor through an Inductance and a Resistance
• 10.15: Charging a Capacitor through and Inductance and a Resistance
• 10.16: Energy Stored in an Inductance
• 10.17: Energy Stored in a Magnetic Field
Energy can be stored per unit volume in a magnetic field n a vacuum.

Thumbnail: Animation showing operation of a brushed DC electric motor. (CC BY-SA 3.0; Abnormaal via Wikipedia)​​​​​​