RevIII-1 A charged particle is orbiting in auniform B-field in the sense shown. Is theparticle positive or negative?A) positiveB) negativeC) impossible to tellRevIII-2Two particles with the same charge but different masses are moving in circular orbits in a magnetic field. They have the same speed. Which one will have the larger radius orbit?A) Neither, the orbits have the same size.B) The larger mass particle will have a larger orbit.C) The smaller mass particle will have a larger orbit. Answers: The direction of acceleration and net force must be toward the center of the circle. For CW rotation and B(in) and a negative particle, the force on particle is toward the center of the circle.The larger mass particle will have a larger orbit. You can see this from a physical argument or from the formula for the radius of an orbit: R = mv/(qB). The physical argument is this: both particles feel the same (sideways) size force F = qvB. The more massive particle has more inertia and is therefore harder to turn, so it turns through a larger orbit.B(in)RevIII-3 A coil of wire carrying current I can rotate freely about an axis in a magnetic field. If released from rest in the position shown, which way does it rotate?A) right side will move out of pageB) left side will move out of page.C) loop will not rotate at allRevIII-4 Two bar magnets are brought near each other as shown. The magnets...A) attract B) repel C) exert no net force on each other.Hint:B axis I SNSNAnswers: Right side will move out of page. Use Fwire = I L BAnswer: Anti-parallel currents repel. Think about the current directions on the sides of the magnets. Repel.atomic current on rim,like solenoidNSB-field comesout of "North" endB-field enters"South" endRevIII-5 A coil of wire attached to a resistoris in an oscillating magnetic field, as shown.The B-field vs. time is shown. At what pointin time is the magnitude of the currentthrough the resistor a maximum?RevIII-6 A circuit with a battery and a variable resistor is near a loop of wire as shown. When the resistance R is decreased, the induced current in the loop is ..A) CWB) CCWC) ZEROAnswers:. At point D, the rate of change of B is maximum. So the rate of change of flux is maximum, so maximum emf by Faraday's law.Answer: A tough one! The B-field at the location of the circular loop due to the current in the circuit is out of the page. As R decreases, current I in the circuit increases, so the B-field at the location of the loop is increasing out of the page. To fight this increase in the externally applied B, we want a Binduced into the page. The induced current is CW.R B (oscilating) time 0 B-field (T) (B) (C) (D) (E)RevIII-7 A uniform solid sphere of copper rotates about a stationary axis in a uniform magnetic field B. Are there eddy currents?A) Yes, there are eddy currents.B) No, no eddy currents.RevIII-8 You have a transformer with Np=6 primary windings, andNs=3 secondary windings, as shown.If Vp=120 V AC, what is the currentmeasured by the ammeter "A" in thesecondary circuit?A) 120 AB) 60 AC) 240 AD) Nothing is measured because thefuse in the ammeter blows!ANSWER: The flux through any loop rotating with the sphere does not change. Therefore no emf, and no eddy currents. Answer: the fuse in the ammeter blows! The secondary voltage is 60 VAC (it's a step-down transformer). The internal resistance of the ammeter is zero. So the ammeter current is I = V/R = 60 V/(0 ohms) = infinite current. The fuse will blow. Vp Vs(in) A(in) Np=6 Ns=3BRevIII-9 What happens to the speed of an electromagnetic wave when you double the wavelength?A) speed decrease by 2B) speed increases by 2C) speed stays the sameRevIII-10 If you double the wavelength of an electromagnetic wave, what happens to the period, T?A) period is the same, it is independent of wavelength.B) period decreases by 2C) period decreases by 4.D) period increases by 2.E) period increases by 4RevIII-11 The planet Jupiter is 5 times further from the sun than the earth. How does the intensity of sunlight (W/m2) seen from theearth compare to the intensity as seen from Jupiter?A) SameB) Intensity is 5 times greater as seen from EarthC) Intensity is 25 times greater as seen from the Earth D) None of theseAnswer: the speed of light is a constant = c = 3E8 m/s.Answer: period increases by 2.Answer: The intensity at the Earth is 25 times greater than the intensity at Jupiter.RevIII-12 Suppose we turn out all sources of light in the lecture hall. It is pitch black, you cannot see a thing. What kind of electromagnetic radiation are you being exposed to, for the most part?A) Essentially none (except perhaps for an occasional cosmic ray.)B) Quite a bit - mostly UV and higher frequencies.C) Quite a bit - mostly IR and lower frequencies D) Quite a bit at all frequencies throughout the entire spectrum.RevIII-13 Two light rays are heading toward the center of semicircular piece of glass from two different directions as shown. Which ray (if any) might experience total internal reflection on its way through the glass?A) Only ray 1 B) Only ray 2C) Both rays can -- depending on the angle of incidence and the index of refraction of the glassD) Neither ray can, under any circumstances.Answer: Quite a bit - mostly IR and lower frequenciesair glass air ray 1 ray 2 Answer: Only ray 2 can undergo total internal reflection. Total internal reflection can only occur if a ray is in a medium of higher index n (glass), heading toward a medium of lower index (air). When ray 2 is in the glass, it can then undergo total internal reflection at the glass/air interface.RevIII-14 Do your eyes form real or virtual images?A) Real B) virtualIs the image on a movie screen real or virtual? A) real B) virtualIs the image seen with a virtual reality headset real or virtual? A) real B) virtualAnswers: Real images are formed on your retina. The movie screen image is real. When the rays actually converge at a point in space (where you can put a screen to see the image) then the image is real.Virtual reality image is virtual. The image must be further than about 25 cm from the viewer's eye for the viewer to be able to focus on the image.imageRevIII-15 If the film-lens distance in a camera is smaller than the focal length, is there some object distance at which images are in focus?f film lens object do =? A) No, there is no object distance which will give a sharp image.B) Yes, there is one object distance which will give a sharp
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