Some of the concepts in this unit
- a 3-dimensional magnetic field is produced when any electrically charged object moves relative to an observer, even a single charged object
- all magnetic fields make complete loops, including the magnetic fields produced by a single moving charged object
- magnetic fields start, by convention, on north magnetic poles and are directed towards south magnetic poles; since magnetic fields make complete loops, magnetic field lines don’t “end”
- there are no magnetic monopoles; even a single moving charged object’s magnetic field has a north and a south pole
- like magnetic poles repel; unlike magnetic poles attract
- the magnetic field around a current-carrying wire makes concentric circles around the wire; the same is true for that around individual moving charged objects
- Earth has a magnetic field similar in shape to a bar magnet; the south magnetic pole is at the north geographic pole and vice versa
- Earth’s magnetic field is parallel to the surface only near the equator
- right-hand-rules can be used to determine the direction of magnetic fields and forces; they are a memory aid to help us reason in three dimensions
- a moving charged object in a uniform magnetic field moves in a circular path at constant speed, in the absence of other forces; Newton’s second law is nonzero because the velocity is constantly changing direction
- Since the force of an external magnetic field is always perpendicular to the velocity of a moving charged object, no work is done on a moving charged object in a magnetic field and the object will maintain a constant speed/kinetic energy in the absence of other forces
- a “velocity selector” can be constructed by placing a uniform electric field at right angles to a uniform magnetic field; the velocity selector “selects” for moving charged objects with v= E/B (this is sometimes called “crossed fields”)
- when a charged object moves relative to an external magnetic field, the moving charged object and the charged objects producing the external magnetic field both feel a magnetic force; the force depends on the directions the charged objects are moving relative to each other and can be deduced by drawing the fields
- two parallel wires carrying current in the same direction are attracted; if the currents are in the opposite directions the wires are repelled
- an external magnetic field that exerts a torque on a current carrying-coil causes the coil to attempt to align itself with the magnetic field
- a torque on a current-carrying coil that causes the coil to rotate can be used to create a meter or a motor
- a motor transfers electrical energy to kinetic energy; a generator transfers kinetic energy to electrical energy
- magnetic flux is proportional to the product of an area and a magnetic field strength that has a component that is perpendicular (“normal”) to the area
- magnetic flux has a direction (which may be depicted as “into the page” or “out of the page” for the sake of clarity)
- a changing magnetic flux induces a potential difference (emf) in conductors which induces a current to flow
- a constant change in magnetic flux produces a constant emf
- the emf can be deduced from a graph of flux vs. time; the change in magnetic flux can be deduced from a graph of emf vs. time
- when a changing external flux induces charge carriers to move in a conductor, the induced current flows in a way that produces an induced flux that opposes the external flux that is causing the current
marcreif
