Write a good paragraph or two in your own words about how a motor works. In your account you should include the power supply, brushes (contacts), armature (which is the spinning loop of wire), conventional current, magnetic forces on the current, and how the magnets are arranged.
You may use this Google Form, DC Motors, to submit your paragraph if you wish. Welcome to our online version of Physics II! It will be an adventure, for sure. I decided to begin where I had hoped to pick up after Spring Break--namely, with motors. I am sorry that we can't build them, but we can at least figure out how they work, especially since motors are all around us. So that's what we are going to try to do. Here is what we would have built: We would take some copper wire coated with enamel for insulation, and wrap it around and around a piece of cork on an axle. Then we would take a match and burn off the enamel from the two ends of our piece of copper wire. That's the bare wire shown above. The other end would be on the other side of the axle. The windings of copper wire are called the "armature" of the motor. We would put screws into each end of the axle in order to support the axle on a small frame that is not shown. Then we would take two strips of metal (brass) and nail them to the frame, and then connect each brass strip to one of the two terminals on our power supply, one strip to the positive terminal and one to the negative. When the two bare ends of the armature simultaneously touch the brass strips, we will have a complete circuit and current will surge through the armature. If we have two magnets, one with its North pole facing the armature and the other its South pole, then the sides of the armature will feel a magnetic force according to F = il x B. The two sides of the armature will experience forces, one side an upward force and the other a downward force. That will cause the armature to spin, which, of course, breaks the circuit. But inertia keeps the armature spinning until contact is again achieved when the armature has spun 180˚. The current is established again, and the wires experience a force again, repeating the process. Do you get it? The right-hand-rule will help you figure out how the forces on the sides of the armature are upward on one side and downward on the other. The resources listed in today's assignment will help you figure out what's going on, too. Let's see how it goes, and let me know via email if there are questions or problems. All the best to you all! Assignment:
Freeman Dyson has died at 96 years old. He has been a creative and productive physicist, and I always enjoyed reading his work. In his early career he worked on the nature of fields like electric fields, for example. That branch of physics is called Quantum Field Theory, and it is amazing.
Mia Dyson says her father, at the age of 96 still regularly went to his office at Princeton University. "You could tell that the world was a beautiful place through his eyes, and somehow understanding all the formulas and the natural laws and all the mysteries he had plumbed through the study of physics, that it only grew more and more beautiful, the more he understood." RIP We worked on understanding the Right Hand Rule for forces on moving charged particles and electric currents today. I'm hoping that you are fairly comfortable with them now. Of course, we will be using them in our work, so we can really become adept at using the RHR! Can you figure out the force experienced by the negative ion at the moment it enters the magnetic field shown in the diagram above and on the left? How about the direction of the force felt by the conventional current in the rightward-directed magnetic field shown above on the right?
The two right hand rules were featured today as we tried our best to get a grip on what nature is like. The first right hand rule we encountered was the right hand rule for magnetic fields created by electric currents. As an exercise, you should be able to figure out the direction of the magnetic field created by the current inside the loop shown below. Can you? Can you figure out the direction of the magnetic force on the particle at the moment that it enters the magnetic field in the diagram below? You should be able to use magnetic field lines to sketch a representation of a bar magnet's magnetic field by now. You also should be able to use the right hand rule for currents to sketch the magnetic fields produced around electric currents. Today we dealt with forces that magnetic fields exert on moving electric charge. It's a weird force, one that we wouldn't have predicted, and it is described by a mathematical tool, the cross product: Can you figure out which of the particles that trace out the paths shown below are positively charged? The magnetic field that made the charged particles curve as they traveled along is directed into the picture. The initial particle that caused all the havoc you see entered the picture traveling from bottom to top, and that's the general flow of things here (as a result of momentum conservation).
We saw where the mathematical relationships for charging and discharging come from. We used our calculus to work it all out. You should be able to determine the time constant for each plot. Can you?
Today we did an experiment in which we charged up a capacitor, and then discharged it. Our goal was to see how the voltage across the capacitor depends on time.
Capacitors are on the front burner now. We've waited until now to deal with them all at once.
A good, practical resource for learning about capacitors is:
Assignment:
|
Physics II
Mr. Swackhamer Scottsdale Preparatory Academy Archives
March 2020
Categories |