Physics 180B Fall 2008 Test Points

Transcription

1 Physics 180B Fall 2008 Test Points Name You can cross off questions or problems worth up to15 points. Circle your answers or pu them in the box provided. 1) The diagram represents a one loop coil in a magnetic field. Draw the direction of the real current in the loop that creates the forces shown? Show your work. Prove the direction you choose. (5pts) 2) In each diagram show the direction of the current that will produce a force that causes the compass needle to rotate from the North Pole as shown. Show your work. Prove the direction of the current you choose. (5 pts) 3) Is this situation in stable equilibrium or unstable equilibrium? (5pts) a) Unstable, the force on the top of the loop points up and the force on the bottom points down. One little bump and the loop will return to this orientation. b) Stable, the force on the top of the loop points down and the force on the bottom of the loop points up. One little bump and the loop will return to this orientation. c) Unstable, the force on the top of the loop points down and the force on the bottom of the loop points up. One little bump and the loop will be forced to make a half-turn. d) Stable, because all the forces on the loop point away from the loop. One little bump and the loop will return to this orientation. 4) A solenoid is connected to a battery as shown in the figure, and a bar magnet is placed nearby. What is the direction of the mag force that the solenoid exerts upon the bar magnet? Prove your answer Show the fields and currents. (5pts) a) downward b) left c) upward d) right

2 5) The Mass Spectrometer. Draw the Acclerator, the Velocity Selector, the Mass Spectrometer and the Detector. Draw the Electric and Magnetic fields in the correct orientation for sending positive ions through this apparatus. (5pts) 6) What are the basic equations for each component of the Mass Spectrometer? (5pts) 7) Singly ionized (one electron removed) atoms are accelerated and then passed through a velocity selector consisting of perpendicular electric and magnetic fields. The electric field is E = 180 V/m and the magnetic field is B = T. The ions next enter a uniform magnetic field of magnitude B = T that is oriented perpendicular to their velocity. How fast are the ions moving when they emerge from the velocity selector? If the radius of the path of the ions in the second magnetic field is 18.2 cm, what is their mass? (5pts)

3 8) A rectangular 12 cm by 18 cm circuit carrying an A current is oriented with its plane parallel to a uniform T magnetic field. Find the magnitude and direction of the magnetic force on each segment (ab, bc, etc.) of this circuit Find the magnitude of the net force on the entire circuit. You can use the diagram. (8pts) 9) The amount of meat in prehistoric diets can be determined by measuring the ratio of the isotopes nitrogen-15 to nitrogen-14 in bone from human remains. Carnivores concentrate N-15, so this ratio tells archaeologists how much meat was consumed by ancient people. Suppose you use a velocity selector to obtain singly ionized (missing one electron) atoms of speed 9.70 km/s and want to bend them within a uniform magnetic field in a semicircle of diameter 25.0 cm for the C-12. The measured masses of these isotopes are 2.32 x kg (N-14), 1.99 x kg (C-12), and 2.49 x kg (N-15). Find the separation of the N-14 and N-15 isotopes. You need to show your work. Show how you got the equations that you used. If you use the derived equation and get the right answer you will receive 4 pts. (12pts) Chapter 20 Equations - W=qV=1/2mv 2, F E =qe, F B =qvb=ilb, F C =mv 2 /r, B=µ o I/2πr, B=µ o I/2r [µ o = 4π x10-7 T m/a, charge on an electron = -1.6 x C, m e = 9.11 x kg, m p = 1.67 x kg]

4 10) In the diagram the rod is falling downward due to gravity in a magnetic field as shown. (5pts) According to an induced current will begin to flow around the circuit loop because the magnetic flux through the circuit loop is. This induced magnetic field will the external B-field and points in the direction to the external B-field. The falling rod will feel an and will soon be equal to the force of gravity pulling the rod down. The rod will reach velocity. a) Lenz s Law, increasing, oppose, opposite, upward force, terminal b) Faraday s Law, increasing, oppose, opposite, upward force, terminal c) Lenz s Law, decreasing, oppose, opposite, downward force, zero d) Faraday s Law, increasing, oppose, opposite, downward force, zero 11) Why is the iron core used in the transformer? If I wanted to make this transformer a step-down transformer which would be greater N 1 or N 2? Why? Why do you need an AC power source? Be brief. (5pts) a) b) c) 12) After watching the Magnetic Storm video, what were the three pieces of eveidence that showed scientists that the Magnetic Field of the Earth is decreasing in amplitude? (5pts) a) b) c) 13) How did scientists discover that Earth s Magnetic Field changed over time and sometimes abruptly? Be brief. (5pts)

5 14) Tether Power. Space debris is a serious problem in near earth orbit. What will cause the satellite to decay in orbit? Show your work. (5pts) 15) A magnet is falling through the center of a semiconductor. Draw this demo. Show the magnetic fields, the forces, the currents, magnetic flux and Lenz s Law in each region; a) as the magnet is approaching the semiconductor, b) in the middle of the semiconductor and c) after it passes through the semiconductor. (10pts) 16) A single loop of wire with an area of m 2 is in a uniform magnetic field that has an initial value of 3.67 T, is perpendicular to the plane of the loop, and is decreasing at a constant rate of T/ s. What emf is induced in this loop? If the loop has a resistance of ohms, find the current induced in the loop. (8pts)

6 17) A metal loop is held above the N pole of a bar magnet, as shown in the figure, when the magnet is suddenly dropped from rest. (5pts) Just after the magnet is dropped, the induced current in the loop, as viewed from above it, is directed clockwise, counterclockwise or zero? Show your work. Prove your answer. 18) The conducting rod ab shown in the figure makes frictionless contact with metal rails ca and db. The apparatus is in a uniform magnetic field of T, perpendicular to the plane of the figure. (12pts) a) Find the magnitude of the emf induced in the rod when it is moving toward the right with a speed 9.20 m/s. b) In what direction does the current flow in the rod? Prove it! c) If the resistance of the circuit abcd is a constant 8.00 ohms, find the direction of the force required to keep the rod moving to the right with a constant speed of 9.20 m/s. d) Find the magnitude of the force mentioned in part (C). Show your work. Chapter 21 Equations - Magnetic Flux Φ=BAcosθ, Faraday s Law E = -NBAcosθ/Δt = vbl, The Transformer Equation V 2 /V 1 =N 2 /N 1