Physics 102 - Induction

Chapter 25

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Electromagnetic induction


In this section, we will discuss the induction of voltage by changing the magnetic field in loops of wire. We have seen that moving a magnet through a coil of wire induces a current to flow in the wire.

The more loops of wire there are in the coil, the more current is induced. Faraday's law states that the voltage induced in a coil is directly proportional to the number of loops in the wire, as well as the cross-sectional area of each loop and the rate of change of the magnetic field in the loops.



A changing magnetic flux induces a current in a metal ring. In order to conserve energy, the induced magnetic field is in the opposite direction from the original field.

This is the mechanism involved with the Ring Flinger demo. The opposing magnetic fields repel each other, flinging the ring into the air. If the ring is not complete, a current cannot flow and no opposing magnetic field is created.





When two coils are placed near each other, alternating current in one (the primary, which is the one connected to the power source) induces a changing magnetic field. The second coil is within that field, so it has a current induced in it. If the first coil has fewer loops than the second, the second will experience a higher induced voltage than the first. This is called a step-up transformer. If the first coil has more loops than the second, the second will experience a lower induced voltage than the first and it is called a step-down transformer. Placing an iron core in the coils will intensify the magnetic field in each.

The relationship between the voltages of the primary and secondary coils is given by:




The transformer transfers energy from one coil to the other. The rate at which the energy is transferred is the power. Since energy is conserved, the power of the first coil equals the power of the second coil.




Rewriting this in terms of voltage and current yields:



Heat and energy loss in wires is much higher when the current is high. For power transmitted through wire, using P = IV, we can see that higher voltage results in lower current for the same power. Thus, we step up the voltage for transmission and then step it down again where we want to use the electricity. Since alternating current is more easily stepped up and stepped down than is direct current, electricity is transported over long distances via AC rather than DC.