Unit 3 Practice Problems ---Magnetism BEarth = 5 x 10" T 1. *A student holds a bar magnet in each hand. If both hands are brought close together, will the force be attractive or repulsive if the magnets are held so that (a) the two N-poles are brought close together and (b) an N-pole and an S-pole are brought together? (Repulsive, Attractive) 2. The figure to the right shows five disk magnets floating above each other. The N-pole of the top most disk faces up. Which poles are on the top-side of the other magnets? ( S, N, S, N) 3. The figure to the right shows a magnet attracting a nail to it which, in turn, has attracted many paper clips that are attached to the nail. IN a. Which end of the nail is the N-pole? (the head) b. Which ends of the three paper clips (from top to bottom) are S the N-poles? (the tops) C. Which of the items are permanent magnets? (only the bar magnet) d. Which of the items are temporary magnets? (the nail and the paperclips) What I call the 1" and 2" Right Hand Rules (the rules that enable you to determine the direction of the magnetic field around a current carrying straight wire and a current carrying coiled loop of wire) 4. *A long, straight wire has a (+) current traveling East through it. What is the direction of the magnetic field around the wire? (down in front of the wire, up in back of the wire) 5. *A cross-section of a long, thin wire with a (+) current in it is to the right. The current is shown as coming out of the page (toward your face). What is the direction of the magnetic field around the wire? (counter-clockwise) O 6. *What if the cross-section of wire had the (+) current moving into the page (away from your face)? (clockwise) 7. *A student makes a magnet by winding wire around a large nail as shown in the figure to the right. The magnet is connected to the battery as shown. Which end of the nail, the pointed end or the head, will be the N-pole? (the pointed end) Battery8. The diagram to the left shows a current (+) carrying wire wrapped around a piece of iron to create an electromagnet. The right hand in the diagram is exhibiting the use of the 2 Right-Hand Rule. What is the direction the (+) current in the wire is moving? (down in back, up in front---same as #7) 9. What is the direction of the magnetic field around the coiled loop of wire in the diagram to the right? (to the left inside of the coil, exiting the left side of the coil, looping back around and entering the coil in the right-side; the left side of the coil acts as an N-pole and the right side of the coil acts as a S-pole of a bar magnet) The Magnitude of the Instantaneous Force of the Magnetic Field on a charge or a current carrying wire (F = BIL sin 0 and F = Bqv sin 0) 10. *A straight wire 0.10 m long carrying a current of 2.0 A is at right angles to a magnetic field. The force on the wire is 0.04 N. What is the strength of the magnetic field? (0.2 T) 11. A wire 0.50 m long carrying a current of 8.0 A is at right angles to a 0.40 T magnetic field. How strong a force acts on the wire? (1.6 N) 12. A wire 0.75 m long carrying a current of 6.0 A is at right angles to a uniform magnetic field. The magnitude of the force acting on the wire is 0.60 N. What is the strength of the magnetic field? (0.13 T)13. A copper wire 0.40 m long carries a current of 6.0 A and weighs 0.35 N. A certain magnetic field is strong enough to balance the force of gravity on the wire so the wire can float in mid-air above the ground. What is the strength of the magnetic field? (0.146 T) 14. *An electron passes through a magnetic field at right angles to the field at a velocity of 4.0 x 10" m's. The strength of the magnetic field is 0.50 T. What is the magnitude of the force acting on the electron? (3.2 E-13 N) 15. A force of 5.78 x 10"N acts on an unknown particle traveling at a 90" angle through a magnetic field. If the velocity of the particle is 5.65 x 10* m/'s and the field is 3.20 x 10"- T a. what charge does the particle carry? (3.2 E -19 C) b. How many elementary charges does this represent? (2 e) 16. *A proton is moving with a speed of 2.3 x 105 m/'s when it encounters a magnetic field of 0.80 T. The direction of the magnetic field makes an angle of 50.0" with respect to the velocity of the proton. What is the magnitude of the magnetic force on the proton? (2.26 E -14 N) 17. A charged particle is fired from a gun with a velocity of 5.2 x 104 m/s at an angle of 35 with respect to a 0.0045-T magnetic field. If the magnetic field exerts a force of 0.0026 N on the particle, determine the magnitude of the charge. (1.94 E -5 C) Concepts 1. How do magnets affect televisions and speakers? 2. Describe how a temporary magnet differs from a permanent magnet. 3. Draw a bar magnet and show the magnetic field lines (including direction) around it. 4. Draw the magnetic field lines between two like magnetic poles and then between two unlike magnetic poles. 5. If you broke a magnet into two pieces, would one piece just be an S-pole and the other piece just be an N-pole? Explain. 6. Why will dropping or heating a magnet weaken it? 7. Can you set a resting electron into motion with a a. magnetic field? Explain. b. An electric field? Explain. 8. What are three ways to increase the strength of the magnetic field around a coiled electromagnet? 9. A conventional (+) current flows in a wire (see diagram below). Draw the magnetic field around it. O10. In the following diagram of a cross-section of a wire, the conventional current is coming straight out of the page at you. Sketch the magnetic field around the wire. 11. If the diagram to the right of a cross-section of wire, the conventional current is moving into the page (away from you). Sketch the magnetic field around the wire. 12. Sketch the magnetic field around the diagram to the right. + 13. When a bar magnet is placed near a TV screen, the picture becomes distorted. Why