Magnetism Everywhere!

Honors Physics completed their discussion of magnetism with an overview of back emf, mutual induction and self-induction. The evil finger of Lenz’s Law finds itself in every electromagnetic pot and must be taken into account when designing electrical devices. We also discussed the structure and function of transformers and their importance in the delivery of power over long distances. Because of AP exams next week, it was decided that the chapter test would fall on May 10 and we will move on in the interim.

Physical Science B conducted a lab investigation that allowed students to examine the properties of both permanent magnets and electromagnets. For the permanent magnet, a magnetic field sensor was used to determine the intensity and direction of the magnetic field of a bar magnet at distances from the magnet and when passed over the magnet’s length. Students were able to verify the normal pattern for field forces – force strength is strongest near the source and weaker at distance – and were able to document that both poles of the magnet generated magnetic fields. The fields, however, we opposite in direction and the pattern on the graph had a sine-curve appearance as students moved from one pole to the other. For the electromagnet, the number of windings directly correlated with magnetic field strength. The equation of the line could be used, then, to design an electromagnet at a specified strength, even if it had not been directly measured in lab. Also, the loss of magnetic strength of the nail after a sharp blow emphasized the role of domain alignment in promoting magnetic fields. Tomorrow, electromagnetic induction.

Physical Science E spent time with gadgets that rely on the ability of moving electric charges to generate magnetic fields. Students examined the structure and function of galvanometers, motors and speakers. Tomorrow, we take a look at the other end of the spectrum – using magnets to generate electric current.

Physics F explored the world of alternating current, generators and motors. The structure of a generator is such that the charges change direction of motion every half-turn of the coil, generating alternating current. And, is with an alternating direction of charge motion that electric motors are able to rotation continuously. Galvanometers cannot spin continuously due the fact that during the rotation there will be meeting of like poles between the permanent magnets and the electromagnet. However, alternating current ensures that at the time that like poles would meet, the charges change direction. This keeps the forces of attraction/repulsion correctly positioned to promote continued rotation. However, all is not perfect in motor land due to back emf. Lenz’s Law definitely applies to motors (and all electromagnetic devices) and the net voltage produced through induction equals the maximum emf minus the back emf. Something engineers have to design around when crafting devices.

Physics G spent time reviewing magnetism material and then dipped their toes in induction. The nature of electromagnetic induction was described and demonstrated using a bar magnet, galvanometer and solenoid. Moving the bar magnet in the core of the solenoid created an electric current that was registered by the galvanometer. When the magnet was held motionless, no current was produced. We will jump deeper into induction as we move through this chapter and also look at ways to put induction to work for our benefit.

Homework

Honors Physics A: Practice 22B #1,4; Practice 22C #1,3,4; Practice 22D #1,5,6
Physical Science B: Complete lab write up
Physical Science E: 17.3 Section Review
Physics F: 22.1 Section Review #1-4 and 22.2 Section Review
Physics G: 22.1 None