5/22/2014 CT31-7http://www.colorado.edu/physics/phys1120/phys1120_fa01/Concept%20Tests/CT3100b.htm 1/4CT31-7.An electric motor consists of a coil, free to turn on an axis, and located in an magnetic field B created by anarrangement of permanent magnets. With the current and field directions as shown, which way will the coilrotate?Pink: CWYellow: CCWBlue: the coil won't rotate when at this particular orientation Answer: The loop will rotate CCW . Apply to the right and left sides of the square coil. On theright side, the direction of the force is up; on the left side the direction of the force is down.5/22/2014 CT31-7http://www.colorado.edu/physics/phys1120/phys1120_fa01/Concept%20Tests/CT3100b.htm 2/4This would not be a very useful motor, since after the coil had flipped 180o, the currents would be in directionssuch as to make the coil flip back CW. The coil would just oscillate back and forth. Real motors have somekind of clever switching arrangement that reverses the current direction after every 1800 turn so that themagnetic force always keeps it spinning in the same direction. CT31-8A solenoid has an increasing current causing an increasing B-field in its interior. An end-on view of thesolenoid is shown below. An electron is directly below the solenoid. While the current I in the solenoid isincreasing, what is the direction of the force on the electron?Hint: What is the direction of the E-field at the position of the electron? Think Lenz's Law. Answer: the force on the electron willbe left , the E-field will be right .5/22/2014 CT31-7http://www.colorado.edu/physics/phys1120/phys1120_fa01/Concept%20Tests/CT3100b.htm 3/4By Faraday's Law, the changing flux through any loopcentered on the coil will cause an emf. By Lenz's Law, the direction of the E-field willbe such as to tend to cause a current which opposes the change in flux. In this case, the E-field will circulateCCW to tend to create a CCW current, which will create an induced B-field out of the page, to fight the fluxchange. Since the electron has a negative charge, the forceis in the direction opposite the E-field. CT31-9.A long solenoid of radius R contains a uniform magnetic field B which is increasing at a steady rate [B(t) = A t,where A is a constant]. For distances r < R from the center of the solenoid, how does the magnitude of theElectric field depend on r? Hint: Pink: E = zeroYellow: E 1/rGreen: E rBlue: E r25/22/2014 CT31-7http://www.colorado.edu/physics/phys1120/phys1120_fa01/Concept%20Tests/CT3100b.htm 4/4Purple: E r3 Answer: E r, inside the solenoid. ( E 1/r outside the solenoid.)For any loop of radius r centered on the solenoid, we can argue that , due to symmetry. Forloops within the solenoid,, which leads to E r, inside.For loops outside the solenoid,= independent of r, which leads to E 1/r. Notice that the E-field here looks just like the B-field in and around a current-carrying wire of radius R.