Induction Introduction Equipment Background Procedure Analysis Questions References and Tools 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\" \"'3' \"'35\"! Back to Top . 1 Oscilloscope . 1 Bar Magnet . 2 Solenoid Coils (& Steel Bar) . 1 Galvanometer (roughly speaking, this is an analogue ammeter) - 1 DC Power Supply 0 1 Function Generator 0 6 Leads with banana connectors . Record data in this Google Sheets data table Rat-II in 'I'nn Background Ensure you are very familiar with both uses/versions of the right hand rule: for moving charges in a magnetic field, and for EMFs induced by a time-varying magnetic field. 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 first loop generates passes through the second loop. Our guiding equations, by Faraday's law (neglecting signs), are: V1 = M1- (1) St V2 = N2 St (2) Let's now assume that only a net fraction f of the flux passing through is going through the other coil, yielding P2 = f91. 1 (assuming V1 is the generating voltage and V2 is the induced voltage). This yields the result: V2 V1 N1 (3) N2 In this lab, we are therefore going to be measuring f using the equation: V2 N1 f = VI N2 (4) Transformers typically make f as close to one as possible using some sort of magnetizable material, which "focuses" the magnetic field inside the coil. We will measure f both with and without a steel bar inside the coil, and observe the difference. Back to TopPart 1: Free Charges and a Bar Magnet Set the Oscilloscope to XY mode (push in that button) and VERT MODE to CH2. This should 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 clot 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 ieft. 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 wouid 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 11': 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 gaivanometer 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 flux 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 coii 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 cabies. From all of the above information, deduce the direction of the magnetic flux produced by the induced current, and whether or not this agrees with Lenz' law. Finally, as a separate observation, try inserting the magnet more quickly. 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? .____ ._..._...__. __ _-... _.. _.._ _ _ r"'' __'_r., _.._. _.--_..\"--- ._ ....._ _.._ _..._.. __... 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 controls both waves, but the VOLTS/DIV knobs are separate for each. Sketch the screen of the oscilloscope. Be sure to labei 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 V1 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: Va, and N2, calculate f, the (unknown) "efficiency" of our transformer, both with and without the steel bar. Back to Top A D E G H K M N Part Part IV Which way do the electrons move? With Steel Bar Which way does the current flow? Quantity : Large Coil Voltage Large Coil Turns Small Coil Voltage Small Coil Turns Efficiency, When you point the dot end to the left, which way is the beam deflected? Unit Turns Turns When you point the dot end to the left, which way is the magnetic field? Value 175 When you point the dot end to the right, which way is the beam deflected? Uncertainty When you point the dot end to the right, which way is the magnetic field? Without Steel Bar When you point the dot end up, which way is the beam deflected? Quantity Large Coil Voltage Large Coil Turns Small Coil Voltage Small Coil Turns Efficiency, When you point the dot end up, which way is the magnetic field? Unit Turns Turns When you point the dot end down, which way is the beam deflected? Value 175 When you point the dot end down, which way is the magnetic field? Uncertainty Outside the magnet, the magnetic field flows from.. Based on the previous answer, which end of the magnet is north? Was your coil measured as perfect efficiency (f=1) with the steel bar, to within uncertainty How did your measurement of f with the steel bar compare to without it? Part 2 Which end of the magnet did you insert first? From above or below? What direction is the change in flux? 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? What direction is your coil wound, from red to black (as viewed from above)? What direction did you observe the induced current to flow (as viewed from above)? What direction is the magnetic flux this induced current would produce? Did your results match your expectation? When you inserted the magnet quickly, did the galvanometer deflect more or less? When you use more turns, does the galvanometer deflect more or less? When you leave the magnet inside, what direction does the galvanometer deflect? What is the name of the law which describes these last three observations? Part 3 Which direction was the current flowing in the inserted coil (as viewed from above)? What direction does the field from that current point (inside the inserted coil)? 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? What direction is your at-rest coil wound, from red to black (as viewed from above)? What direction did you observe the induced current to flow (as viewed from above)? What direction is the magnetic flux this induced current would produce? Did your results match your expectation?Which of the following magnetic forces are we testing in this lab? |:| Magnetic force on a free charge |:| Magnetic forces on wire(s) |:| Magnetic force on a permanent magnet Question 2 (3 points) Which of the following will generate a (time-dependent) magnetic flux that will be used to test induction in this lab? |:l Free charge |:| Permanent magnet [7'] Wire coil Question 3 (3 points) Saved If we have a beam of electrons moving towards you, and a (permanent) magnet placed near this beam with the north pole facing to the left, we expect the charges to be deflected... (Eum :1 Left :1 Down 3 Not at all 'ERgm Question 4 (3 points) When we have a magnet sitting stationary in a coil, with the north pole facing up, we expect the induced current to be flowing... :: Clockwise :1: Counterclockwise :: There is no induced current Question 5 (3 points) Suppose we place coil A inside coil B. Coil A has a decreasing current flowing in the clockwise direction. Coil B will have an induced current flowing... if :3 Clockwise if Counterclockwise a: There is no induced current