EC-7c  Induction - Test Coil

OBJECTIVES:

1. To observe the emf induced in a small detecting coil by the varying magnetic field produced by the varying current in a separate large coil.
2. To follow how how this emf depends on the shape, and the frequency of the inducing current. The computer will produce a voltage varying with time in the form of a triangular waveform with amplitude $V_{max} = 3 V$ and a frequency $f = 10 Hz$.

   Figure 5: The triangular waveform

   

INTRODUCTION:

The applied voltage varies linearly from $V_{min} = -3 V$ to $V_{max} = 3 V$ in $50 ms$. The slope of the voltage curve is therefore $\Delta V / \Delta t = 120 V/s$.
The resistance of the large coil is $R \simeq 7 \Omega$ so current flowing in the large coil will vary linearly from $I_{min} = -3/7=-0.43 A$ to $I_{max} =3/7 = +0.43 A$ in $50 ms$, Correspondingly $\Delta I / \Delta t = 17.2 A/s$. The strength of the magnetic field produced by the large coil is proportional the the voltage applied on it, and therefore the rate of change $\Delta B/ \Delta t$ is proportional to the slope of the curve that shows the voltage versus time.

EQUIPMENT:

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PC and interface.

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A computer controlled power amplifier.

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A stand holding a small detecting coil.

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A large coil with an ohmic resistance $R = 7 \Omega$. The radius of the coil is $r=0.1$ m.

PROCEDURE I: (10 min)

1.

Click on the Launch EC-7C icon below (web version) to initiate PASCO software window. The monitor should now show a scope window, and a window for the control of the signal generator; CLK on the frequency, backspace and enter a frequency of 10 Hz. CLK on the triangular waveform. DCLK anywhere on the face of the scope and enter 1 V for the trigger level.

2.

CLK on the amplitude, backspace and enter 3 V.

3.

Turn on the signal generator by CLKing on the ON button then DCLK on RECORD, you now see the voltage induced in the small coil together with the one applied on the large coil

4.

Adjust the size of the waves you see in the scope window using the size controls on the side of the scope window, then measure the height $h_1$ of the square wave using the cursor (5). Record this height.

QUESTIONS

1.

Explain why the induced voltage is a square wave.

2.

Observe the direction of the winding in the large coil. What is the direction of the magnetic field during the rising part of the triangular wave? Explain your thoughts.

3.

Observe the direction of the winding in the small coil. What is the direction of the induced emf during the rising part of the triangular wave? Explain your thoughts.

PROCEDURE II:

Move the small detecting coil 5 centimeters away from the large coil, measure the amplitude of the induced voltage. Repeat at distances of 10 and 20 centimeters. Plot a graph.

QUESTION

How does the strength of the magnetic field depend on distance? $\sim 1/r$ ?$\sim 1/r^2$? $\sim 1/r^3$?

PROCEDURE III:

1.

Replace the detecting coil to its original position at the center of the large coil. Decrease the amplitude of the triangular wave to 2 V.

2.

Measure the height of the square wave. record this value.

QUESTIONS

Q1

Did the slope of the triangular wave change? By how much?

Q2

Is the change in amplitude of the square wave what you would expect? Why?

PROCEDURE IV:

1.

Repeat procedure III with a frequency of 15 Hz.

2.

Compare the result with those of procedure I and III

PROCEDURE V:

1.

Loosen slightly the clamp holding the small coil; rotate the coil $90^o$ so that it is in the horizontal plane. Observe the resulting induced voltage.

2.

Rotate the coil another $90^o$ so that it is in the vertical plane again. Observe the resulting induced voltage. Compare the result with procedure III.