Lab 7: Magnetism Introduction Magnets need no introduction (i.e. introduction to be added in future revision).

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CSUEB Physics 1780 Lab 7: Magnetism Page 1 Lab 7: Magnetism Introduction Magnets need no introduction (i.e. introduction to be added in future revision).

1 CSUEB Physics 1780 Lab 7: Magnetism Page 1 Lab 7: Magnetism Introduction Magnets need no introduction (i.e. introduction to be added in future revision). Experiments The purpose of these experiments is to provide an introduction to the study of magnetism. The experiments this time will not be very quantitative, rather they will be much more phenomenological. A. Magnets Recall that a magnet is a dipole, it has a North pole end and a South pole end. If you break a magnet you can t get a North Pole all by itself, instead you get two smaller dipole magnets. Like poles repel, and opposite poles attract. 1. Magnetic Materials: Only a few metals are magnetic (e.g. iron, steel, any others?). Experiment with your bar magnet on various materials. Question 1. (a) Which metals are magnetic? (Is a nickel coin magnetic?) (b) Which metals are not? (c) Why can you pick up several paper clips in a line, even though the paper clips cannot do it themselves? 2. Compass: Tie a string on the center of your magnet. The magnetic field of the earth points in the direction of north, and the dipole magnet tries to line up with it. Recall that the definition of the North pole of a magnet is North seeking, i.e. it should be the pole which is pointing north. Question 2. (a) Does the north end of your magnet point north? [Note, sometimes the polarity gets reversed) (b) What color is the north end of your magnet? What color is the south end? (c) Is the north geographic pole of the earth a North or South magnetic pole? Explain. 3. BIG Horseshoe Magnet: You should have a really powerful magnet at your lab station. BE CAREFUL! Don t get your cell phone near it. If you are wearing any magnetic jewelry, also be cautious, as it can really grab and you might pinch a finger. Using your bar magnet, determine the North and South poles of your big magnet. Label them (masking tape?). Question 3. Check the side of your magnet. Does it tell you the strength (either in units of Gauss or Tesla?). Note 10,000 Gauss= 1 Tesla.

CSUEB Physics 1780 Lab 7: Magnetism Page 2 B. Electromagnets Magnetism can be created from a flow of electric current! 1. Oersted s Experiment: This experiment we aren t able to do today, because we need both BIG currents and very sensitive little compass magnets.

Electromagnet (with an Iron Core): A coil of wire (carrying current) mimics a magnet.

2 CSUEB Physics 1780 Lab 7: Magnetism Page 2 B. Electromagnets Magnetism can be created from a flow of electric current! 1. Oersted s Experiment: This experiment we aren t able to do today, because we need both BIG currents and very sensitive little compass magnets. Its included here for the future. Basically he showed that a current flowing in a wire deflected a magnet. This was the first evidence of a connection between electricity and magnetism. 2. Electromagnet (with an Iron Core): A coil of wire (carrying current) mimics a magnet. The strength of the electromagnet is proportional to the current in the wire, the number of turns, and the cross section area of the loops. If you put a core in the middle (of soft iron) it can increase the magnetic field by a big factor. We have two different types of ironcore electromagnets that you can use. Try to keep the current under 4 or 5 Amps, else the wire may get hot. Question 4. (a) Does your electromagnet pick up paper clips like a magnet? (b) Using your bar magnet, determine the North end of your electromagnet. (c) Reverse the direction of the current flowing in the electromagnet. Does it change which end of the electromagnet is North? (d) Draw a sketch in your notebook, and try to determine your fingers (of your right hand) curl in the direction that the current is flowing, would your thumb point in the north or south pole end of the electromagnet? 3. Solenoid Coil (air core electromagnet): You should have a long (6 inch) cylindrical coil, which is called a solenoid. We suggest inserting a momentary contact switch, so that the current will flow only as long as you hold down the button. Again, try to keep the current under 5 Amps, or less if the wires get hot. Put your bar magnet near one end. You should be able to either get it to suck the magnet in, or push it out.

3 CSUEB Physics 1780 Lab 7: Magnetism Page 3 Question 5. (a) How much current are you using? (b) Sketch the type of coil you are using (approximate dimensions) (c) Describe what happened in your experiment. C. Electromotive Force Michael Faraday became famous for inventing the electric motor and electric generator. 1. Ampere s Experiment: Magnetic Force on a Current: The force F (in Newtons) on a wire of length L (in meters) carrying current I (in Amps) in magnetic field B (in Teslas) is: F=ILB (1) The direction of the force is interesting, it is perpendicular to both the magnetic field and direction of current. If the current is flowing parallel to the magnetic field there will be no force! This time, we need you to insert a 2 ohm resistor (perhaps 1 ohm) in series so that the current will not exceed what the power supply can deliver (i.e. it won t blow a fuse). Go from the power supply to the resistor box, then a wire from there through the magnet to the momentary switch, and then back to the power supply. All the elements are in series. Have the instructor check you BEFORE you turn it on! You should see the wire move when you tap the switch. Try also reversing the direction of the current and see what changes. Question 6. (a) Make a sketch of your circuit, label the resistance used, and the amount of current used (b) Make a sketch of the direction of magnetic field, direction current is flowing, and the direction of the force on the wire. (c) Comment also on what happens when you reverse the direction of the current. (d) Knowing the current, the magnetic field strength of your magnet, and the length of wire (only the length inside the field counts), estimate the force on the wire.

perpendicular to both the magnetic field and direction of velocity. If the velocity is parallel to the magnetic field there is no force! So, a wire has charge already in it.

4 CSUEB Physics 1780 Lab 7: Magnetism Page 4 2. Generating Electricity: Magnetic fields can affect charges, but only if they are moving! The force F (in Newtons) on a charge q (in Coulombs) moving with velocity v (in m/s) in a magnetic field B (in Teslas) is given F=qvB (2) Again the direction of the force is interesting, it is perpendicular to both the magnetic field and direction of velocity. If the velocity is parallel to the magnetic field there is no force! So, a wire has charge already in it. By physically moving the wire through a magnetic field we give all of these charges velocity. If we move the right way, the force will be along the wire, i.e. a current will be created. The voltage V generated (in this situation its called an electromotive force, measured in volts) is given to be: V=LvB (3) Where L is the length of the wire (inside the magnetic field). Simply connect a wire to an old-fashion Ammeter, i.e. one with a needle rather than the newfangled digital ones (a voltmeter would be better). Move the wire through the magnetic field to generate electricity. Question 7. (a) Summarize your result. Does the current depend upon speed as expected? (b) Does the current depend upon the direction you move the wire? explain (c) Suppose instead you made a double (or triple) loop and moved it through the magnet (see second figure). Comment.

CSUEB Physics 1780 Lab 7: Magnetism Page 5 3. Faraday s Law of Induction: Suppose instead of moving the wire, we move the magnet? By Galileo s principle, motion is relative, so it should still work.

5 CSUEB Physics 1780 Lab 7: Magnetism Page 5 3. Faraday s Law of Induction: Suppose instead of moving the wire, we move the magnet? By Galileo s principle, motion is relative, so it should still work. However, equations (2) and (3) apply to the velocity of the charge (or wire), which is zero. Faraday came up with a generalized equation that showed the change in magnetic flux (i.e. change in magnetic field, which can be due to moving magnet) would generate a voltage. The voltage V generated in a loop of wire is equal to the number of turns N in the coil times cross section area A of the coil, times the rate of change of the magnetic field B in time: B V NA (4) t So, simply moving the magnetic in and out of the coil should generate current. Try it! Question 8. (a) Summarize your result (sketch, etc). (b) Does the current depend upon speed at which you move the magnet? (c) Does the current depend upon the direction you move the magnet? (d) How about if you hold the magnet still but move the coil? (e) How about if you spin the magnet near the coil?

CSUEB Physics 1780 Lab 7: Magnetism Page 6 D. AC and Transformers A transformer transmitts electricity from one coil to another (aka mutual induction ).

Build the circuit: Put an AC signal through a (primary) coil by using a Function Generator (aka signal generator ). This will create a changing magnetic field.

6 CSUEB Physics 1780 Lab 7: Magnetism Page 6 D. AC and Transformers A transformer transmitts electricity from one coil to another (aka mutual induction ). The first coil (primary) is fed an AC signal, hence the magnetic field is constantly reversing. The second coil (secondary) hence has a voltage generated in it by Faraday s law of induction. 1. Build the circuit: Put an AC signal through a (primary) coil by using a Function Generator (aka signal generator ). This will create a changing magnetic field. It usually works better if you use square wave and around 200 Hertz. Be sure to turn the amplitude to maximum. Connect your secondary coil directly to a speaker. Two different ways of doing it are shown above. The first uses the big Helmholtz Coils as the primary and your solenoid coil as a secondary. Or, you can use your solenoid coil as the primary, and your electromagnet coil as the secondary. Experiment with moving the secondary coil into different orientations (e.g. rotate the coil so the axis is parallel to the first, then perpendicular). Also change the position of the secondary (far away from primary, closer, inside, etc). Make sketches of which configuration gives the maximum and minimum signals. Question 9. (a) Summarize your results (sketch, etc). (b) Which orientation gives the maximum signal? (c) Which configurations give the minimum signal? (d) Which frequencies are best? (high, low?) (e) How about if you reverse the coils, make the primary the secondary the secondary the primary. Does it still work? 2. Vibrating Magnet: One last idea to try, is to hold a magnet in your hand while it is inside (near) the primary. Can you feel the magnet physically vibrating? [This would work much better if we had more powerful small magnets]. This is essentially how a speaker works. If you glue the magnet to a paper cone, you will hear a sound!

EB Education Revision Guide. How to work with Magnetism and Electromagnetism

EB Education Revision Guide. How to work with Magnetism and Electromagnetism EB Education Revision Guide How to work with Magnetism and Electromagnetism Magnets Magnetic fields Magnets have two poles, north and south. They produce a magnetic field, this is a region where other