ConcepTest #21: A particle has a net positive charge, but it experiences no net electric force. Which of the following statements must be true? Hold up as many cards as necessary. 1.There are no other charges nearby. 2.If there are other charges nearby, then they must all have a sign opposite that of the first particle. 3.If there are other charges nearby, then they must all have a sign the same as that of the first particle. 4.If there are other charges nearby, there must be precisely the same number of positive charges as there are negative charges. 5.None of the other statements is necessarily true. ConcepTest #22: A conducting sphere initially has no net charge. A positively charged rod is then brought close to the sphere. The sphere is then connected to ground. The rod is then removed, and then the connection to ground is broken. After these steps, what is the net charge on the sphere? 1. Positive 2. Negative 3. No net charge 4. Not enough info

ConcepTest #23: Consider the following directions for the deflection of an electron: 1. Up4. Left 2. Down5. Into the page 3. Right6. Out of the page (back of card) No Direction a) An electron is flying to the right through an electric field that points down as shown. What direction does it deflect as it passes through the electric field? b) An electron is flying to the right over the North end of a bar magnet as shown. What direction does it deflect as it passes over the bar magnet? NSNS

ConcepTest #24: The electric potential at point A is 30 V, while at point B it is 80 V. A +2 C charge moves from A to B, subjected only to electrostatic forces. If the charge has 120 J of kinetic energy at A, what is its kinetic energy at B? J 2.20 J J 3.70 J J

ConcepTest #25: Particle A has charge +Q and mass M. Particle B has the same charge +Q, but twice the mass, 2M. The particles begin at rest and are accelerated through the same potential difference  V. The particles then enter a region of uniform magnetic field B directed out of the page. Ignore the repulsive interaction between the two charged particles. Which particle has the largest radius of curvature in the region of the magnetic field? ●●●●●●●●●●●●●●●●●●●●●●●● VV 1. Particle A3. Same radius 2. Particle B4. Not enough info

ConcepTest #26: A long straight current carrying wire, length 2 L, is oriented along the y-axis as shown. A small piece of the wire at (0, y ) is indicated. A field point P is on the x-axis at a distance x P. Which of the following best represents ___, the magnitude of the contribution to the magnetic field due to the small piece of wire? P ( x P,0) ( 0,y ) L –L–L

ConcepTest #27: A metal ring drops downward into and then through a region of magnetic field pointing into the page as shown. The magnetic field is uniform and points into the page in the boxed region, and is zero outside the boxed region. For each of the following cases, consider the following choices for the direction of the current in the ring: 1. Clockwise 2. Counterclockwise 3. No current 4. Not enough info X X X v (a) X X X (b) X X X (c) X X X (d)

Try it Yourself: A uniform magnetic field points into the page as shown. A disk made out of a conducting material that lies in the plane of the page is rotated clockwise. x x x x x Calculate the magnitude of the induced  in the disk. Where on the disk is the potential higher? Explain how, even though the flux through the disk is constant,  is still induced in the disk.