1. Vern J. Ostdiek Donald J. Bord Chapter 8 Electromagnetism and EM Waves (Section 3)

2. 8.3 Principles of Electromagnetism The interactions between electricity and magnetism described in the previous section, along with other similar observations, suggest the following two general statements. We might call these the principles of electromagnetism: 1. An electric current or a changing electric field induces a magnetic field. 2. A changing magnetic field induces an electric field.

3. 8.3 Principles of Electromagnetism These two statements summarize the previous observations and also emphasize the symmetry that exists. In both cases, a “changing” field means that the strength or the direction (or both) of the field is changing.

4. 8.3 Principles of Electromagnetism The first principle can be used to explain the first observation: As a charge moves past a point in space, the strength of the electric field increases and then decreases. All the time the direction of the field is changing as well. The effect of this is to cause a magnetic field to be produced. Similarly, the second principle explains electromagnetic induction.

5. 8.3 Principles of Electromagnetism A transformer is a device used to step up or step down AC voltages. It represents one of the most elegant applications of electromagnetism. In essence, a transformer consists of two separate coils of wire in close proximity.

6. 8.3 Principles of Electromagnetism An AC voltage is applied to one of the coils called the input or primary coil, and an AC voltage appears at the other coil called the output or secondary coil.

7. 8.3 Principles of Electromagnetism The AC in the primary coil produces an oscillating magnetic field through both coils. Most transformers have both coils wrapped around a single ferromagnetic core to intensify the magnetic field and guide it from one coil to the other. This oscillating (and therefore changing) magnetic field induces an AC current in the output coil. Note that a DC input would produce a steady magnetic field that would not induce a current in the output coil. Transformers do not work with DC.

8. 8.3 Principles of Electromagnetism Now, how can the voltage of the output be different from the voltage of the input? Each “loop” or “turn” of the output coil has the same voltage induced in it. The voltages in all of the turns add together so that the more turns there are in the output coil, the higher the total voltage. The ratio of the number of turns in the two coils determines the ratio of the input and output voltages.

9. 8.3 Principles of Electromagnetism In particular,

10. 8.3 Principles of Electromagnetism If there are twice as many turns in the output coil as in the input coil, then the output voltage will be twice the input voltage. If there are one-third as many turns in the output coil, then the output voltage will be one-third the input voltage. Thus, the AC voltage can be stepped up or stepped down by any desired amount by adjusting the ratio of the number of turns in the two coils.

11. 8.3 Principles of Electromagnetism Example 8.1 A transformer is being designed to have a 600-volt output with a 120-volt input. If there are to be 800 turns of wire in the input coil, how many turns must there be in the output coil?

12. 8.3 Principles of Electromagnetism In addition to being used to change voltages in electrical distribution systems, transformers are found in a wide variety of electrical appliances. Most electrical components used in radios, calculators, and the like require voltages much smaller than 120 volts. Appliances designed to operate on household AC must include transformers to reduce the voltage accordingly.

13. 8.3 Principles of Electromagnetism High-intensity desk lamps also use transformers, which is what makes their bases so heavy.

14. 8.3 Principles of Electromagnetism The spark used to ignite gasoline in automobile engines is generated using a type of transformer called a “coil.” The number of turns in the output coil is many times the number of turns in the input coil. A spark is produced by first sending a brief current into the input. A magnetic field is produced that quickly disappears. This induces a very high voltage (around 25,000 volts) in the output, which is conducted to the spark plugs to ignite the fuel.

15. 8.3 Principles of Electromagnetism Understanding electromagnetism allows us to better appreciate how the metal detectors introduced at the start of the chapter work. The magnetic pulses are produced by sending an electric current through a coil of wire for a short period of time. When the current stops, the magnetic field that was created dies out quickly, and this decreasing field induces an electric current in the coil. This current is used to monitor how swiftly the magnetic pulse dies out.

16. 8.3 Principles of Electromagnetism Metals are detected because the rapidly changing magnetic field of each pulse induces electrons in the metal to move—as in the secondary coil in a transformer—and this current produces an opposite magnetic pulse. This change in the total magnetic field affects the current induced in the coil. The electronics are designed to detect any such change and signal an alarm.