Part 1 Part IV Which way do the electrons move? Out With Steel Bar Which way does the current flow? In Quantity Large Coil Voltage Large Coil Turns Small Coil Voltage Small Coil Turns Efficiency, f When you point the dot end to the left, which way is the beam deflected? Down Unit Turns Turns When you point the dot end to the left, which way is the magnetic field? Right Value 10.4 1500 1.2 175 0.989010989 When you point the dot end to the right, which way is the beam deflected? Up Uncertainty 0.2 0.2 0.1660263378 When you point the dot end to the right, which way is the magnetic field? :Left Without Steel Bar When you point the dot end up, which way is the beam deflected? Left Quantity Large Coil Voltage Large Coil Turns Small Coil Voltage Small Coil Turns Efficiency, f When you point the dot end up, which way is the magnetic field? : Down Unit Turns Turns When you point the dot end down, which way is the beam deflected? : Right Value 0.4 1500 0.8 175 0.6593406593 When you point the dot end down, which way is the magnetic field? : Up Uncertainty 0.2 0.2 0. 1653656343 Outside the magnet, the magnetic field flows from... North to Sout Based on the previous answer, which end of the magnet is north? Dotted Was your coil measured as perfect efficiency (f=1) with the steel bar, to within uncertainty Yes How did your measurement of f with the steel bar compare to without it? : "With" was higher Part 2 Which end of the magnet did you insert first? Dotted From above or below? Above What direction is the change in flux? Down Based on Lenz' law, what direction flux should the induced current generate? Up What direction induced current (as viewed from above) generates this flux? Counterclock What direction did your galvanometer (initially) deflect? Left What direction is your coil wound, from red to black (as viewed from above)? Clockwise What direction did you observe the induced current to flow (as viewed from above)? Counterclock What direction is the magnetic flux this induced current would produce? :UP Did your results match your expectation? : Yes When you inserted the magnet quickly, did the galvanometer deflect more or less? More When you use more turns, does the galvanometer deflect more or less? Less When you leave the magnet inside, what direction does the galvanometer deflect? It Doesn't What is the name of the law which describes these last three observations? : Lenz' Law Part 3 Which direction was the current flowing in the inserted coil (as viewed from above)? Clockwise What direction does the field from that current point (inside the inserted coil)? Down What direction is the change in flux (in the coil at rest) when you insert the small coil? Based on Lenz' law, what direction flux should the induced current generate? What direction induced current (as viewed from above) generates this flux? What direction did your galvanometer (initially) deflect? Left What direction is your at-rest coil wound, from red to black (as viewed from above)? Clockwise What direction did you observe the induced current to flow (as viewed from above)? | Counterclock What direction is the magnetic flux this induced current would produce? Did your results match your expectation?Introduction In this lab, we will validate Lenz' law and (qualitatively) Faraday's law, demonstrating that a change in magnetic flux generates a current opposing the change in flux. We will also be measuring the extent to which a transformer deviates from an ideal transformer. \"W\" \"9' the\"! Back to Top - 1 Oscilloscope - 1 Bar Magnet - 2 Solenoid Coils (8: Steel Bar) - 1 Galvanometer (roughly speaking, this is an analogue ammeter) - 1 DC Power Supply . 1 Function Generator 0 6 Leads with banana connectors . Record data in this Google Sheets data table Back to Top IIIIIIIIIIIIIIIIEMMMEIIIIIIIIIIIIIIII Ensure you are very familiar with both uses/versions of the right hand rule: for moving charges in a magnetic eld, and for EMFs induced by a time-varying magnetic eld. 1 The last part of this lab deals with something you don't touch on in class: non-ideal transformers. 2 For an ideal transformer, we assume that all the flux that the rst loop generates passes through the second loop. Our guiding equations, by Faraday's law (neglecting signs), are: 6'131 : N _ 1 V1 1 6t ( ) 6'I52 V2 7 NZ(St (2) Let's now assume that only a net fraction f of the flux passing through is going through the other coil, yielding $2 = fl. 1 (assuming V1 is the generating voltage and V2 is the induced voltage). This yields the result: Vz V1 F2 f (3) In this lab, we are therefore going to be measuring f using the equation: _VN1 \"W (4) Transformers typically make f as close to one as possible using some sort of magnetizable material, which "focuses" the magnetic eld inside the coil. We will measure 1' both with and without a steel bar inside the coil, and observe the difference. Back to Top Part I: Free Charges and a Bar Magnet Set the Oscilloscope to XY mode (push in that button) and VERT MODE to CH2. This shouid cause the screen to have a single stationary dot on screen. 1 Adjust the position knobs until the dot is in the middle of the screen. This dot results from the oscilloscope emitting a beam of electrons from the back of the machine to the front. We are going to be studying the effects of a magnet on this beam. Based on that information, fill out the first few questions. Now, we are going to introduce the magnet. Hold up your bar magnet flat across the screen with the dotted end pointing to the left. 2 Observe that the beam is deflected, and record the direction in which it is deflected. This deflection arises from a magnetic force in the direction of the deflection (i.e., if it were deflected left, the magnetic force would be to the left). Using the right-hand rule to work your way backwards to the magnetic field from the directions of velocity and magnetic force, deduce the direction of the magnetic field. 2 Then, repeat the above procedure with the dotted end on the right, top, and bottom, and record the direction of deflection and direction of magnetic field for each. Based on the above information and your knowledge of how magnets work, deduce whether the dotted or undotted end is magnetic North. 3 Part 1'1": Bar Magnet and a Coil Now, set the oscilloscope aside (we won't be needing it again until part IV). Take the large magnetic coil, and set it upright (such that the base sits on the table). Remove the smaller coil (and steel rod) from inside it, if necessary. Use banana cables to wire it to the galvanometer (red to red and black to black). When you do so, use the "1500" port on the coil (which indicates that port is connected to 1500 turns away from the black "zero" port). Insert the magnet into the coil. You should observe that the galvanometer deflects in some direction. The galvanometer measures current, so this means that the change in flux from the magnet produced a (small) current via induction. Now, let's do it a bit more carefully and keep track of all the directions involved. When you insert the magnet, keep track of what end of the magnet you are inserting, and the direction from which you are inserting it (presumably, from above). Based on these two pieces of information, what direction is the change in magnetic flux - up or down? Keep in mind when thinking about that question that the magnetic field inside the magnet flows in the opposite direction as the field outside the magnet, and here, most of the change in magnetic fiux comes from the field inside the magnet. Now, insert the magnet, and observe (and record) the direction of deflection of the galvanometer (initially, before it bounces back). Note that positive (right) on the galvanometer is flowing from red to black through it, and negative is flowed from black to red through it. The solenoid will therefore be opposite this - red to black inside the galvanometer is (continuing the loop) black to red inside the solenoid, and vice versa. Based on your galvanometer deflection and wiring, deduce what direction the current in your coil flowed. This will require knowing what direction (clockwise or counterclockwise) the coil turns when going, say, from red to black. To determine this, look closely at the wires emerging from the terminals for the banana cables. From all of the above information, deduce the direction of the magnetic ux produced by the induced current, and whether or not this agrees with Lenz' law. Finally, as a separate observation, try inserting the magnet more quickiy. How does the magnitude of galvanometer deflection compare to the magnitude when you insert it more slowly? What if you vary the number of turns (represented by the numbers on the base of the coil)? Also, when you just leave it inside, what does the galvanometer do? What law describes these behaviors? Part III: Electromagnet and a Coil Now, set aside the bar magnet. Take the second coil, and wire it up to the DC power supply (red to red, and black to black, 3V or less). If you insert this coil into the other one, like the previous part, you should observe a deflection. For optimal results, you should have the steei bar inside the small coil as you insert it. Note that the DC power supply makes a current from red to black inside the solenoid. Based on observation of the wires inside the solenoid (as in the last part), deduce whether the current flows clockwise or counterclockwise (as viewed from above, when the small solenoid is held with the same orientation you will hold it when you insert it into the large coil). Again, insert it, and check the direction of the (initial) deflection of the galvanometer. Does the direction of deflection match what you expect based on your knowledge of Lenz' law and your results from the previous part? Please remember to turn off the DC power supply and disconnect it from the small coil. Part IV: Transformers Now, unplug all previous instruments, and just look at the oscilloscope, function generator, and solenoid coils. You will now be wiring these things together (always red to red and black to black, of course). First, insert the smaller coil within the larger solenoid coil, and the steel bar inside the coil. Next, wire the function generator to channel 2 of the oscilloscope. Then, in parallel with Cha 2 of the oscilloscope (so with two more wires), wire the function generator to the large coil, as well. It is best to use the 1500 turns port here. Finally, wire the small coil to channel 1 of the oscilloscope. Now, unpress the "XY" button, and set the oscilloscope VERT MODE to DUAL. If you turn on the function generator (and have it set to a sine wave), you should now observe two sine waves on your screen, one for each coil. 4 Vary the TIME/DIV and VOLTS/DIV knobs, as well as the POS knobs, until both waves show are completely on screen and show 2-5 full periods. Note that the TIME/DIV controis both waves, but the VOLTS/DIV knobs are separate for each. Sketch the screen of the oscilloscope. Be sure to label the waves as to which is the small or large coil (tweak the POS knobs and see which one moves), and record this on your sketch. For this sketch, you may have two different VOLTS/DIV settings, which complicates your y-axis labels. A good way to draw this is to put the CH1 voltages on the left-hand side of the plot, and the CH2 voltages on the right-hand side. Measure the peak-to-peak amplitude of each wave (using the corresponding VOLTS/DIV settings). Record the amplitude of the wave for the larger coil as Vl and the amplitude of the wave for the smaller coil as V2 (with uncertainties). Note how many turns you are using in your larger coil as N1, and take N2 = 175. Take 1 turn as the uncertainty in the number of turns. Finally, take out the steel bar, and repeat this measurement. Back to Top Make sure you answer all the questions presented as a part of the procedure above (presented on the data table). Using N1, V1, V3, and N2, calculate f, the (unknown) "efficiency" of our transformer, both with and without the steel bar. Back to TOP Movement of conductor Magnetic field Motion Current induced Field in conductor Induced current Figure 13.18 Fleming's right-hand rule