After discussing our test results, we returned to our current subject of interest–electromagnetic waves. The fact that they can be polarized is consistent with the theoretical result (courtesy of Faraday and Maxwell) that they are transverse waves.
We used polarizing filters to explore light around the classroom. We saw that light from computer displays, phone displays, and calculator displays is polarized. Then we also saw that light reflected from our tabletops and other surfaces is also polarized. We need to note the orientation of the plane of polarization of light reflected from surfaces. On Monday we will take our filters outside to see if we can find some polarized light there, too.
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Electromagnetic waves turn out to be a consequence of Faraday's Law and Maxwell's insight that changing electric fields could produce magnetic fields surrounding them (like magnetic fields surround electric currents). So we now have electric fields produced without electric charges (Faraday's Law) and magnetic fields produced without electric currents (Maxwell's contribution). That's a brand new part of reality that seems a bit novel--except for the fact that that's how our universe has been for quite a while (many, many weeks now!).
Today is our Celebration of Knowledge of Faraday's Law!
With everyone back in the saddle today we worked out solutions to the Practice Test for Chapter 21.
It was strangely quiet today....See you Tuesday!
We worked through some of the problems and the use of Lenz's Rule. And one group in Period 2 got their motor to run like mad today!
Announcement:
Chapter 21 Test will be on Wednesday. Study the Ch 21 Objectives, the practice test on the Chapter 21 Web page, and your homework solutions. We had pretty good success overall with our motors today. Second Period will get some time tomorrow, since we had only 50% success. Our problems from Ch 21 will be collected tomorrow after we wrap them up. I will return them on Friday. Tomorrow we will build electric motors. Not only do electric motors turn due to the forces of magnetic fields on the currents in the motors, they also act as generators, generators having an emf that oppose to some degree the current from the power supply. If you force an operating motor to stop while it is receiving current from a power supply, there is no longer an opposing emf caused by the changing flux due to rotation. Therefore, there is less opposition (or resistance) to current, and so the current surges through the wires. Motors can burn out this way, because the wires can get too hot due to all the electrons banging through them.
Anyway, tomorrow we build. The main features of today's lesson were phenomena that arise from Faraday's Law and Lenz's Rule. Eddy currents were one of the major phenomena that occur due to the induced emf's created by changing magnetic fields around and inside conductors like copper. We also discussed the voltage supplied to our homes by Arizona Public Service and Salt River Project, namely 120 V. It is alternating current (AC), not direct current (DC). Transformers allow electric energy to flow either at higher voltage and lower current or at lower voltage and higher current. Why is it that transformers work with AC current but not with DC current? Step-up transformers raise the voltage (technically the emf), and step-down transformers lower voltage. Is the transformer shown below a step-up or a step-down transformer? In modern transformers, the rate of energy flowing into the primary coil of a transformer is virtually identical to the rate of energy flowing out from the secondary coil. Engineers have learned how to make them very efficient. Of course, the rate of flow of energy is called 'power,' and it is measured in Joules/second, which is Watts. Therefore, We also dealt with Vrms, the "root-mean-squared" voltage. Nearly all your household appliances are designed for an emf of 120 V. What does 120 V mean in a situation like your home in which the voltage varies as shown in the plot below: The average voltage in the plot above is 0 V. But you can bet if you got shocked by a loose wire, you wouldn't feel like it's zero! The way to properly evaluate the effective voltage of AC voltage is by
Assignment:
On Monday we will finish discussion about the solutions to the problems from Giancoli. We'll turn them in on Tuesday. They are: Ch 21 P: 2, 4, 5, 6, 9, 10, 11, 12, 30, 31, 32, 33, 34 We presented our solutions to the problems in the recent handout, Assignment 1.
Assignment: On Monday we will finish discussion about the solutions to the problems from Giancoli. They are: Ch 21 P: 2, 4, 5, 6, 9, 10, 11, 12, 30, 31, 32, 33, 34 |
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