Experiment 6: Induction

Transcription

1 Experiment 6: Induction Part 1. Faraday s Law. You will send a current which changes at a known rate through a solenoid. From this and the solenoid s dimensions you can determine the rate the flux through it changes. Putting this into Faraday s law tells you how many volts should be induced in some wire wound around the outside of the solenoid. You then use a computer with a voltage sensor to obtain a graph of the induced voltage which shows whether Faraday s law predicted correctly. 1. Measure the solenoid s outside diameter. (The nearest millimeter is good enough, but closer is better.) If you need help with the vernier caliper, ask. Notice that you can see the tube around which the coil is wound protruding through the end pieces. Use this to measure the coil s inside diameter. Find the average diameter then the average radius from that. Also measure the coil s length. 2. Wrap a piece of insulated wire around the solenoid ten times and connect it to a voltage sensor plugged into channel A of the computer interface. Connect the solenoid to Output 1 of the interface. 3. Connect the interface to the computer with a USB cable and turn both on. (The button at the upper left of the interface should turn blue.) Open PASCO Capstone on the computer. 4. On the computer screen, click signal generator at the bottom of the column on the left. Click 850 Output 1. Set the frequency at 50 Hz and the amplitude at 5 V. Set the wave form for Positive Ramp. Click On. 5. Click Hardware Setup at the upper left. Click the yellow circle by Channel A on the picture then on Voltage Sensor. Click the yellow circle by Output 1 on the picture then on Output Voltage- Current Sensor. Click Hardware Setup again to hide that window. 6. To have the computer show graphs of the current in the solenoid and the voltage induced in the 10 loops of wire, a. In the column on the right, double click Scope, which is second from the top. Click <Select measurement> by the vertical axis and select Output Current, Ch 01 (A). b. In the toolbar at the top, click on (Add new y axis to scope display.) By the vertical axis which appears on the right, click <Select measurement> and select Voltage, Ch A (V). c. Click where it says Continuous Mode near the lower left. Click Fast Monitor Mode on the

2 menu which appears. Click Monitor at bottom left. Two curves should appear. The voltage probably looks like a horizontal line because only a small fraction of a volt is induced. Stretch the scale on the right until it shows up by dragging the numbers along the axis away from the origin. Also adjust the other axes as needed for a clear view of what is going on. d. Near the left of the toolbar at the top, click (Activate and Control Scope Trigger) to stabilize the display. 7. The graph of the current should rise fairly steadily for a few milliseconds and then it should suddenly drop and become curvy. Record the highest current reached and the time to get there. The best way is to click on then drag the crosshairs to the point you want the coordinates of. Right click on the box containing the coordinates. Click Tool Properties, then Numerical Format then Horizontal Coordinate. Check the box by Override default number format and under Number Style select Significant Figures. Click OK. 8. The voltage graph should be fairly constant and then suddenly drop. (If the wires are connected the other way, it will be a fairly constant negative voltage which suddenly jumps up.) While taking this reading, have the ten loops bunched up around the middle of the solenoid; its field is weaker near the ends. The graph may vary a little. Record what looks like a good average voltage. 9. Calculations. For the time interval between t = 0 and t = t f, a. What is the final magnetic field in the solenoid, B f? b. What is the final flux through the solenoid, Φ f? c. The flux through the ten loop coil you made is the same as through the solenoid. Calculate the emf induced in the ten loop coil. 10. In your conclusion, compare the measured and calculated voltages. Parts 2 through 4 are a series of short demonstrations. The "write up" for them has been integrated into the answer sheet. You only need to attach a discussion for part 1. Part 2. Coil and Magnet. Sign out a neodymium magnet. Caution: Keep the magnet wrapped in tape and cardboard, as you found it. Without this padding, they have pinched people's fingers, and also gotten chipped. Connect a galvanometer (a sensitive ammeter) to a coil of wire. Notice the effect on the meter of moving the magnet relative to the coil or the coil relative to the magnet. In particular, notice what happens when you use different speeds. Answer the questions about this.

3 Part 3. Transformer. 1. DC: Instead of using a magnet, use another coil as an electromagnet to produce the magnetic field. Include an ammeter in the circuit with the electromagnet and keep the current in it around one amp (Don't go much higher). The two coils should be placed against each other with a piece of iron through their centers. Investigate these questions: Do transformers work on steady DC? Do transformers work on DC if you chop it (turn it off and on)? 2. AC: Replace the DC meters with an AC ammeter and voltmeter as shown. Switch the primary coil from the DC to the AC terminals of your power supply. Do transformers work on AC? Part 4. DC generator and DC motor. a. Generator. You are given a little machine, consisting of a coil between the poles of a horseshoe magnet. Connect it to the galvanometer, then give the motor a good spin with your finger. Unlike parts 1 through 3, the strength of the field at the coil's position is not changing. In view of that, how do you explain what you saw? b. Motor. A DC motor and a DC generator are basically the same device. In one case you put mechanical work in and get electricity out. In the other, you put electricity in and get mechanical work out. Connect the device to the DC terminals of the power supply. Meters with a needle are better because digital meters might not be steady enough to read. On the ammeter, use a scale suitable for an amp or so; on the voltmeter, a scale suitable for a few volts. Before switching on the power supply, set its knob at the second mark on the left. Keep an eye on the meters; be sure neither goes off scale. Turn on the power and adjust the knob so that the motor runs at a moderate speed. You'll probably have to push it to get it started. Record the meter readings, and answer the questions on the answer sheet.

4 PHY 122 Report on Experiment 6: Induction Name Part 1. Coil s outside diameter: Inside diameter: average: Coil s length: L = N = 570 turns r av = t i = 0 I i = 0 t f = I f = Measured V = Calculations: Part 2: A) A coil is connected to a galvanometer. A magnet is placed against the coil, stationary. Once the effects of putting it there pass, how far is the needle from zero? Starting against the coil, then moving the magnet away very slowly, along the coil s axis, how far does the needle go from zero? Starting against the coil, then moving the magnet away very quickly, along the coil s axis, how far does the needle go from zero? (In each blank put "FLUX", "FIELD STRENGTH", or "RATE THE FLUX CHANGES":) This

5 last result shows that the induced emf depends on the, not or because and depend only on how far away the magnet is, but depends on how fast it's moving. Part 3. A transformer made from two identical coils linked by an iron bar is connected to a DC power supply. A galvanometer is connected to the secondary coil to detect any induced current. With steady DC in the primary, the galvanometer shows The power supply is turned off and on, like the primary current in a car s ignition coil. I observe The transformer is now connected to an AC power supply and AC meters. This time, I observe Part 4. A coil between the poles of a permanent magnet is spun. A galvanometer connected to this coil (does/ does not) show an induced current. Faraday's law says there must be a changing flux for induction to happen, but B from this permanent magnet does not change. The explanation for what I saw is The same device is now connected to a power supply and used as a simple DC motor. While running, V = and I =. The power taken in by the motor is The energy it uses in one minute is