## F

Magnetic field: Magnetic fields, B, are due to moving charges or intrinsic spin (a property of particles just like charge is a property of particles). Some materials have a magnet field, and they are called permanent magnets. Permanent magnet alternators use rare earth atoms, which are more expensive than iron, nickel, and cobalt. If a moving charge feels a force at right angles to its motion, it is in a magnetic field. Also, changing electric fields create changing magnetic fields, and changing magnetic fields create changing electric fields. Maxwell formulated the theory of electromagnetism in all of its elegance of four equations, appropriately called Maxwell's equations. This is the theoretical basis for all of the electric power industry and communication by electromagnetic waves, which we accept as commonplace today.

FIGURE 7.2 Forces on the sides of a current-carrying loop in an external magnetic field. The resultant of the set of forces gives a torque, t, which makes the loop rotate, a motor.

If charged particles are placed in external electric fields, and if moving charged particles are placed in external magnetic fields, there is a force on the charged particles. The amount of force depends on the strength of the electric and magnetic fields, the amount of charge, and the velocity of the charge.

This equation is the basis for understanding the conversion of electric energy to mechanical energy, a motor, and the conversion of mechanical energy to electric energy, a generator.

Motor: A loop of wire has moving charges (current) in it due to a connection to an electric plug. The loop is in an external magnetic field; therefore, there is a force on the charges and a torque on the wire, a motor (Figure 7.2). The torque on the loop is given by the current in the wire, I, the area of the loop, A, and the strength of the magnetic field, B. Now in the motor there is a coil of wire, many loops. Check the links to see how an electric motor works.

Generator: A loop of wire is moved (rotated) by an external force (Figure 7.3). The shaft power, P = T * ffl, which in this case comes from the wind turbine rotor, either directly or through a gearbox. The charges (electrons in the wire) are moving in an external magnetic field, and there is a force on the charges, a generator. Of course, there is not just one loop of wire, but many loops in a coil. If there is just one coil, it is a single-phase generator. If there are three coils of wire, then it is three-phase generator.

The external magnetic field can be produced by permanent magnets or electromagnets. A current in a coil produces a magnetic field, and with an iron core in the coil, the magnetic field is stronger. The number of these coils is referred to as poles. So the current from the utility grid or a part of the generator current is used to produce the magnetic fields. Check the links to see how an electric generator works.

FIGURE 7.3 Rectangular loop rotated by outside force with angular velocity, (0, in a uniform external magnetic field, a generator.

FIGURE 7.3 Rectangular loop rotated by outside force with angular velocity, (0, in a uniform external magnetic field, a generator.

## Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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