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Magnets and the Magnetic field - Prelab 1. In Part 1, what is the name of the equipment that will provide the electron beam? Will this piece of equipment also supply the magnetic field? If not, what piece of equipment will generate the magnetic field? 2. Predict what will happen when the beam of electrons passes through the magnetic field. Will it deflect, and if so how? 3. In Part 2, what is the purpose of the the iron filings on the paper above the bar magnet? 4. In Part 4, what is the purpose of the compasses placed on the Ampere’s Apparatus? 5. In Part 5, what kind of magnet replaces the bar magnet in the diagram? 1 Magnets and the Magnetic field Part 1: The force on a moving charge in a magnetic field; The Helmholtz Coil Equipment: Helmholtz Coil Horseshoe magnet Dark room Dim the lights and proceed to the Helmholtz Coil setups . Follow instructions regarding warm up time etc. and obtain the blue electron beam. This beam is a current, composed of electrons (negative charge carriers). In most uses of this apparatus, the coils are electrified to provide a magnetic field. For this exploration, the horseshoe magnet will provide the magnetic field, as it is easier to manipulate. 1. Predict what would happen if the horseshoe magnet were positioned so the electron beam would pass through the magnetic field of the magnet. Would the beam be attracted, to one of the poles, repelled from one of the poles, would there be some other deflection or would the beam not be deflected at all? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ____________ 2. Try it! Use the horseshoe magnet to investigate how the electron beam is deflected. What is the relative orientation of magnet and beam when the deflection is the greatest?___________________________________________________ ___________________________________________________________ ___________________________________________________________ _______________ 3. What is the relative orientation of magnet and beam when the deflection is minimal (or even absent)? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ _________________________________ 2 4. Explain how why the direction of deflection of the electron beam seems to contradict the right-hand rule for force (see text for reference). Part 2: The magnetic field of a magnet Equipment: Bar magnet Other magnets of various shapes Compass Mini compasses Iron filings Plexiglass sheet Plain white paper Textbook Caution: Do not let the iron filings come into direct contact with the magnet or the plexiglass. The filings are very difficult to remove. 1. Trace a bar magnet on a piece of paper, labeling north and south poles. Put that piece of paper aside for the moment. Place the bar magnet under the plexiglass sheet, noting where the north and south poles are. Put a fresh sheet of paper above the plexiglass sheet. The idea is to keep the iron filing on the paper, not on the plexiglass or the magnet. Carefully sprinkle iron filings over the paper in the area above the magnet. Tap the edge of the paper very gently until you see a pattern emerge. What does this pattern represent?__________________________________________________ _ 2. On the sheet of paper on which you traced the magnet, carefully draw the pattern you see in the filings. Be sure to indicate on your drawing which end of the magnet is N and which is S. How does it compare to the figure of iron filings around a bar magnet in your text? Where is the highest concentration of filings? As the distance away from the poles increases, how does the pattern change? Explain carefully! ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ______ 3 3. Carefully remove the paper from the magnet and return the iron filings to their container. Replace the paper over the magnet. Place a compass on the paper and note the effect on the compass needle as you move the compass to various locations on the paper around the location of the magnet. On your previously drawn sketch of the pattern of iron filings, draw arrows showing the direction that the north (painted) end of a compass points at a number of locations around the magnet. Be sure to look at locations at both ends and around both sides of the magnet, and at differing distances from the magnet. Attach your drawing to this lab. 4. According to your text, what does the direction of the compass needle indicate? Would you describe the compass directions as generally parallel, perpendicular, or tangent to the field lines you have drawn? Do your observations with the compass suggest that the magnetic field is stronger at some locations than others? If so, where is it the strongest? Part 3: Are static (non-moving) charges magnets? Do magnets experience a force in an electric field? Equipment: Black plastic strip wool Electroscope Compass 1. Charge the strip by rubbing it vigorously with the wool. Check to make sure the rod is charged by looking for an effect on the electroscope. What effect on the electroscope do you see when the rod is charged? ___________________________________________________________ ___________________________________________________________ ______________ 2. What kind of field causes this interaction?_________________________ 3. Move the compass close to the charged strip. Does the field from the strip affect the compass? Are magnets affected by a static electric field?______________________________________________________ ___________________________________________________________ ___________________________________________________________ _ 4 4. For comparison, bring one end of a bar magnet close to the compass and note the effect. Turn the magnet around so that the other pole is close to the compass. Record your observations._____________________________________ ___________________________________________________________ ___________________________________________________________ 5. What kind of field causes this effect on the compass? 6. Now bring the bar magnet close to the electroscope (discharge electroscope first). Does the magnetic field from the bar magnet affect the stationary charges residing on the electroscope? ___________________________________________________________ ___________________________________________________ 7. According to your observations, are static charges magnets? Explain your answer. ______________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___ Part 4: Oersted’s Experiment - Are moving charges magnets? Equipment: Ampere’s Apparatus 3 D-batteries, in holders (or power supply) 5 mini-compasses regular compass connecting wires switch Connect the Ampere’s Apparatus in series with a switch and the battery pack/power supply as instructed. Note the direction the current will travel when the switch is closed. Place the mini compasses on the “shelf” of the apparatus and the larger compass under the horizontal rod. Make sure all the north (red) arrows point in the same direction. Report any errant compasses to the authorities. 1. If current acts like a magnet, what would you expect to happen to the compasses when the switch is closed? ___________________________________________________________ ___________________________________________________________ ______ 5 2. Try it! Close the switch. What happens? Based on the results, are moving charges magnets? How do you know? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________ 3. According to the mini magnets, what is the shape of the magnetic field due to a current carrying wire? How does that compare with the field shown in your text?_______________________________________________________ ___________________________________________________________ ___________________________________________________________ ______________ 4. What do you think will happen if the direction of current is switched?___________________________________________________ ___________________________________________________________ ______________________ 5. Try it! Switch the leads and close the switch. What happens? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ _______________________________ 6. Using your text as a reference, explain how this experiment with the Ampere Apparatus confirms the validity of the right -hand rule for fields: ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________ 7. Is a current-carrying wire is a magnet? Explain: 6 Part 5: Force and Torque on a current loop We have seen that moving charge acts as a magnet. A current carrying wire deflects a compass. The magnetic field of a horseshoe magnet will exert a force on an electron beam. But what if the moving current is in the shape of a loop and can move about some pivot point? Equipment 3 D-batteries, in holders (or power supply ring stand and clamp wooden dowel connecting wires switch horseshoe magnet (replaces bar magnet in the diagram) piece of non-magnetic wire to make into a square-shaped loop short copper wire “hangers” 1. Set up the apparatus as shown. If you put the bottom, horizontal portion of the “loop” between the ends of the horseshoe magnet while the switch is open, do you think the loop will swing? Explain why or why not. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________ 2. Do you think it will swing once the switch is closed? Why or why not? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ____ 3. If you think the loop will swing, use the right-hand rule for force to predict which way it will go. Prediction: ____________________ 4. Do the experiment by closing the switch. Did the results match your predictions? If not, what was different?___________________________________________________ ___________________________________________________________ ___________________________________________________________ _______________________________ 7 5. If you switch the directions of the leads, what do you think will happen? Why? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________ 6. Try it. Did your results match your predictions? If not, why not?_______________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ __________________________________________________________ 7. Notice that the loop is pivoted about the dowel, so the force due to the magnetic field actually caused a rotation. What do we call the tendency of a force to cause rotation? _______________ 8. Now investigate what happens when you put one of the vertical sides of the loop in between the horseshoe magnet and close the switch. Do you think there is a magnetic force exerted on the vertical portion of the wire by the magnetic field? If so, does this force result in a torque? Why or why not? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ _________________________________ Part 6: Force and Torque on a current loop - Exercises The diagrams below represent pivoted loops in the presence of magnetic fields. The pivot is the rod in the center of the loop. The directions of currents and fields are indicated. Use the right-hand rule to determine the direction of the force on each side of the wire and indicate the direction of those forces on each diagram. Then describe what the overall effect on each current loop (will it rotate and if so in what direction) in the space on the right. 8 1. 2. 3. 4. 9 5. 6. 7. Based on your diagrams, under what conditions will a magnetic field cause rotation torque in a current carrying loop? What must be true about the direction of magnetic field, magnetic force and the axis of rotation relative to one another?_________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ _______________ 8. What do you think would happen to the magnitude of the torque if there were multiple loops of wire bundled together, all carrying the same current in the same direction? 10