Wet Wednesday

Honors Physics spent the period going over homework and discussing the structure and function of generators and motors. As part of the discussion on generators, the concept of root-mean square (rms) values for current and voltage was introduced. Generators and motors are both energy-conversion devices. Generators convert mechanical energy to electrical energy and motors do the opposite. The basic motor and generator have very similar structures and build off of the galvanometer model. Make sure you can explain the structure and function of each and the type of energy conversions each exhibits. Tomorrow, we’ll continue on with electromagnetic induction and, on Friday, you will get to watch a film that brings together many of these ideas, while concentrating on Earth’s magnetic field.

Physical Science B reviewed the basics of electromagnetism and applied this knowledge towards understanding devices such as galvanometers, motors and speakers. The function of a galvanometer was demonstrated and students had a chance to see the inner workings of the device. With that model in mind, electric motors were introduced and the function of the brushes and commutators to promote continuous rotation was described. Then, we looked at speakers, which operate on a similar system, although the motion produced is back-and-forth, not rotational. It is this back and forth motion that produces the longitudinal waves associated with sound. Tomorrow, a lab is on deck that will allow you to explore both permanent magnets and their magnetic fields and the structure and properties of an electromagnet.

Physical Science E moved from a discussion of Earth’s magnetic field and the properties of permanent magnets to the properties of current-carrying wires. Moving electric charges generate both electric and magnetic fields and many moving charges (current) creates a noticeable net magnetic field. A straight wire produces a relatively weak field, a single coil of wire produces a stronger field, a series of coils (solenoid) generates an even bigger field and the iron-core solenoid (electromagnet) produces the strongest field of all (for the same current). For solenoids and electromagnets, the shape of the magnetic field is very similar to that of a bar magnet and there is a distinct north and south pole. Tomorrow, we take a look at devices that use the magnetic fields produced by current-carrying wires to do work.

Physics F started their examination of electromagnetic induction today after a review of last night’s homework problems. Moving a magnet through a coil of wire or moving a coil of wire through a magnetic field can generate an electric current in the wire. There are other things that can induce current, such as varying the magnetic field strength or fluctuating the size of the wire coil, but using relative motion of the coil of wire and magnetic field is the most common method. A bar magnet and a solenoid was used to demonstrate electromagnetic induction and the direction change of the current caused by changing the direction of the relative motion. Students examined the use of Faraday’s Law to calculate the size of the induced current and used Lenz’s Law to predict the current’s direction. Tomorrow – devices that rely on electromagnetic induction.

Physics G took time to discuss the forces that act on charged particles and current-carrying wires when placed in a magnetic field. Both the magnitude and the direction of the force can be determined, for a prediction of the change in motion of the particle or wire. Students practiced using the right hand rule to determine magnetic field and force direction and time was given for students to practice working problems using formulas to calculate force on a particle and force on a wire. Tomorrow we shift gears a bit and take up the case where magnetism is used to generate electric current.

Homework

Honors Physics A: None
Physical Science B: 17.3 Section Review and Chapter Review items #28, 30, 32
Physical Science E: 17.2 Section Review and Chapter Review items #17, 18, 28, 30, 32
Physics F: None
Physics G: Practice 21A #2,5; Practice 21B #3,4; Chapter Review #19-24, 26