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phy124:lab_4 [2010/02/10 20:13]
mdawber
phy124:lab_4 [2010/02/19 12:33] (current)
mdawber
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====== PHY 124 Lab 4 - Magnetic Force and Induction ====== ====== PHY 124 Lab 4 - Magnetic Force and Induction ======
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+[[http://​www.ic.sunysb.edu/​Class/​phy122ps/​labs/​dokuwiki/​pdfs/​124lab4worksheet.pdf|Important! You need to print out the 4 page worksheet you find by clicking on this link and bring it with you to your lab session.]]
+
+If you need the .pdf version of these instructions you can get them [[http://​www.ic.sunysb.edu/​Class/​phy122ps/​labs/​dokuwiki/​pdfs/​phy124lab4.pdf|here]].

===== Goals ===== ===== Goals =====

The purpose of this laboratory is in Part I to observe the magnetic force on moving electrons due to the magnetic field from a bar magnet. The purpose of Part II is to observe the induction of a voltage in a coil by the change of the magnetic flux through the coil. In Part III the voltage induced in a coil by an alternating current (AC) from a second coil is observed. These two coils are arranged similar to a transformer. The purpose of this laboratory is in Part I to observe the magnetic force on moving electrons due to the magnetic field from a bar magnet. The purpose of Part II is to observe the induction of a voltage in a coil by the change of the magnetic flux through the coil. In Part III the voltage induced in a coil by an alternating current (AC) from a second coil is observed. These two coils are arranged similar to a transformer.
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+===== Video =====
+<​flashplayer width=640 height=480>​file=http://​www.ic.sunysb.edu/​Class/​phy122ps/​labs/​phy122vid/​lab4vidhq.flv</​flashplayer>​

===== Equipment ===== ===== Equipment =====
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===== Part II - Induction ===== ===== Part II - Induction =====

-==== Induction using a Bar Magnet ====+==== A - Induction using a Bar Magnet ====

Your goal is to verify Faraday’s Law and Lenz’ Law (see Ch19 sheet 4) of magnetic induction. ​ For this part you will only be making qualitative observations. Your goal is to verify Faraday’s Law and Lenz’ Law (see Ch19 sheet 4) of magnetic induction. ​ For this part you will only be making qualitative observations.
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**Note:** Here the magnetic field which contributes dominantly to the magnetic flux inside the coil is the magnetic field inside the magnet, indicated by the high density of field lines at either end of the bar. The average magnetic field is directed parallel to the axis of the bar and goes straight from the south to the north pole inside the bar.  This is the direction of the magnetic field you are concerned about for this part of the lab.  You are no longer interested in the field outside of the magnet like you were in Part I.  (see the sketch for Part I above) **Note:** Here the magnetic field which contributes dominantly to the magnetic flux inside the coil is the magnetic field inside the magnet, indicated by the high density of field lines at either end of the bar. The average magnetic field is directed parallel to the axis of the bar and goes straight from the south to the north pole inside the bar.  This is the direction of the magnetic field you are concerned about for this part of the lab.  You are no longer interested in the field outside of the magnet like you were in Part I.  (see the sketch for Part I above)
-\Insert the bar magnet north pole first from left to right, slowly into the coil and observe the needle deflection. Note whether the needle deflection stays at maximum or falls back to zero when the magnet motion ceases. Then withdraw the magnet slowly from the coil and observe the needle deflection. Note the direction of the needle deflection. Do it a couple of times.+
+Insert the bar magnet ​**north pole first from left to right**, slowly into the coil and observe the needle deflection. Note whether the needle deflection stays at maximum or falls back to zero when the magnet motion ceases. Then withdraw the magnet slowly from the coil and observe the needle deflection. Note the direction of the needle deflection. Do it a couple of times.

Repeat this procedure with a fast motion of the bar magnet. Repeat this procedure with a fast motion of the bar magnet.
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{{124l4fig4.jpg}} {{124l4fig4.jpg}}

-==== Induction using an electro-magnet ====+==== B - Induction using an electro-magnet ====

You make an “electro-magnet” by passing a DC current through the smaller coil of the induction set. If you use the right-hand-rule to find the magnetic field lines of this small coil, you will notice that they are identical to the magnetic field lines of the bar magnet. ​ The small coil with a current through it is a magnet with a north and a south pole like the bar magnet. This part of the lab will be exactly the same as Part IIA, except that you simply replace the bar magnet by the electro – magnet as shown below. You make an “electro-magnet” by passing a DC current through the smaller coil of the induction set. If you use the right-hand-rule to find the magnetic field lines of this small coil, you will notice that they are identical to the magnetic field lines of the bar magnet. ​ The small coil with a current through it is a magnet with a north and a south pole like the bar magnet. This part of the lab will be exactly the same as Part IIA, except that you simply replace the bar magnet by the electro – magnet as shown below.
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Connect the small coil to the DC power supply as shown. As you did for the large coil, inspect your small coil carefully to get the correct winding of the coil. On Execution Sheet 3 choose the incomplete sketch of the coil with the correct winding as in your set up and draw the wire connections (dashed lines) to the coil. Draw the directions of the current on the wires of the small coil, taking into account how they are connected to the DC power supply (which end of the small coil is connected to the positive terminal of the power supply? the negative terminal?). Use the right-hand-rule (see Ch18 sheet 25) to determine the north pole of the electro-magnet and label it in the sketch. Connect the small coil to the DC power supply as shown. As you did for the large coil, inspect your small coil carefully to get the correct winding of the coil. On Execution Sheet 3 choose the incomplete sketch of the coil with the correct winding as in your set up and draw the wire connections (dashed lines) to the coil. Draw the directions of the current on the wires of the small coil, taking into account how they are connected to the DC power supply (which end of the small coil is connected to the positive terminal of the power supply? the negative terminal?). Use the right-hand-rule (see Ch18 sheet 25) to determine the north pole of the electro-magnet and label it in the sketch.

-Place the small coil inside the large coil with the DC power off. Turn the knobs on the DC power supply to maximum (clockwise). Turn the power on and observe the needle. Note! The needle deflection is very small. If you can hardly see any needle deflection, insert the bar labeled “steel” into the small coil, which enhances the strength of the electro-magnet. After the power has reached a maximum value, check whether the needle stays at maximum deflection or goes back down to zero. Turn the power off and observe the needle. Do this a couple of times.+Place the small coil inside the large coil with the DC power off. Turn the knobs on the DC power supply to maximum (clockwise). Turn the power on and observe the needle. Note! The needle deflection is very small. If you can hardly see any needle deflection, insert the bar labeled “steel” into the small coil, which enhances the strength of the electro-magnet. After the power has reached a maximum value, check whether the needle stays at maximum deflection or goes back down to zero. Turn the power off and observe the needle. Do this a couple of times.

Record your observations on your worksheet. Record your observations on your worksheet.

-Now we will apply Lenz's law to see if the direction of motion agrees with what we expect from our understanding of magnetic induction. ​The form below will guide you through a set of questions from which you can deduce the direction the needle should have moved when you turned on and when you turned off the power supply.+Now we will apply Lenz's law to see if the direction of motion agrees with what we expect from our understanding of magnetic induction. ​As before the form below will guide you through a set of questions from which you can deduce the direction the needle should have moved when you turned on and when you turned off the power supply.

{{124l4fig6.jpg}}{{124l4fig4.jpg}} {{124l4fig6.jpg}}{{124l4fig4.jpg}}
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-===== Induction using alternating current (AC) power =====+===== Part III: Induction using alternating current (AC) power =====

Your goal here is very similar to PART II B. However you excite your electro-magnet (small coil) with an AC voltage which then induces an AC voltage in the large coil, essentially working like a transformer (see Ch19 sheet 24). Your goal here is very similar to PART II B. However you excite your electro-magnet (small coil) with an AC voltage which then induces an AC voltage in the large coil, essentially working like a transformer (see Ch19 sheet 24).
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When you have done this your set up should look like the picture below. When you have done this your set up should look like the picture below.

-{{124l4fig7.jpg}}+{{124l4fig7.jpg?600}}

Set the AC generator to a sine wave (push the button on the top right in) of 1000 Hz (push the MULT 100 button in and set the Frequency to 10). Use MAIN OUT HI and turn the AMPLITUDE to maximum (all the way clockwise) if you are using the coils with metal ends, or, if you are using the coils with the black plastic ends set the AMPLITUDE about halfway. Set the oscilloscope to: Set the AC generator to a sine wave (push the button on the top right in) of 1000 Hz (push the MULT 100 button in and set the Frequency to 10). Use MAIN OUT HI and turn the AMPLITUDE to maximum (all the way clockwise) if you are using the coils with metal ends, or, if you are using the coils with the black plastic ends set the AMPLITUDE about halfway. Set the oscilloscope to:
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BE CAREFUL! ​ When observing the signals from Channel 1 and Channel 2 remember that the signal for each channel has its own VOLT/DIV setting. ​ Take this into account when labeling your sketch and when computing the amplitude of each wave. BE CAREFUL! ​ When observing the signals from Channel 1 and Channel 2 remember that the signal for each channel has its own VOLT/DIV setting. ​ Take this into account when labeling your sketch and when computing the amplitude of each wave.

-From the ratios of the voltages you can calculate the ratio of the number of turns in the two coils. ($\frac{V_{2}}{V_{1}}=\frac{N_{2}}{N_{1}}$ See Ch 19 Sheet 24) +From the ratios of the voltages you can calculate the ratio of the number of turns in the two coils. ($\Large ​\frac{V_{2}}{V_{1}}=\frac{N_{2}}{N_{1}}$ See Ch 19 Sheet 24)

Are the frequencies of the two waves the same? Discuss with your TA whether they are or aren't and whether you observation is consistent with your expectation. Are the frequencies of the two waves the same? Discuss with your TA whether they are or aren't and whether you observation is consistent with your expectation.
phy124/lab_4.1265850787.txt · Last modified: 2010/02/10 20:13 by mdawber