Forced Oscillation Lab Report

Vietnamese-German University

Electrical Engineering and Information Technology

Experimental Physics

Report

Forced Oscillation

Nguyen Phuc Hai (EEIT-2013-1594)

Nguyen Anh Huy (EEIT-2013-1603)

Instructor: Dr. Ho Trung Dung

Due Date: 2 Febuary 2015

1. Introduction

When an oscillating system is set up to execute its free motion, damping is the main reason that causes the reduction of the system’s successive maximum amplitudes. Furthermore, when an external periodic torque is applied on the system, we can observe and analyze that the amplitude is now a function depending not only on damping but also on the frequency and the amplitude of the external torque. The characteristic frequency of the free oscillation and the resonance curves of the forced oscillation are determined by performing the experiment.

1. Necessary Equipment

1 Torsion pendulum after Pohl

1 Variable transformer, 25 VAC/ 20 VDC, 12 A

1 Bridge rectifier, 30 V AC/1 A DC

1 Stopwatch, digital, 1/100 sec.

2 Digital multimeter

2 Connecting cord, l = 250 mm, yellow

2 Connecting cord, l = 750 mm, red

3 Connecting cord, l = 750 mm, blue

1. Preparation and Setting Up

The experiment which is set up and its electrical connection are shown in Fig. 1 and Fig. 2. For the first experiment (measuring the frequency of free oscillation), the DC current is provided to the eddy current break and can be adjusted by using the knob on the power supply which is indicated by the ammeter in series. In the second experiment (forced oscillation), we need an external torque created by the DC motor connected to the rectifier, which turns the AC supply to the DC current for the motor.

To measure the differential voltage influencing the torque, the motor is connected to a voltmeter in parallel.

[pic 1]

Figure 1: Experimental Set-up

[pic 2]

Figure 2: Electrical Connection of the Experiment

1. Experiment Procedure

• Free oscillation amplitude measurement

To determine the characteristic period and frequency of the torsion pendulum without damping (I=0):

-deflect the pendulum completely to one side.

        -measure the period and the amplitude for several oscillations.

For different damping value, we adjust the output current from the supply by adjusting the knob to these values:

-I ~ 0.25 A

-I ~ 0.45 A

-I ~ 0.6 A

After adjusting the current, we continue to determine the period and frequency by:

-deflect the pendulum completely to one side.

        -measure the period and the amplitude for several oscillations.

• Forced oscillation resonance amplitude measurement

We now use the DC motor to create an external periodic force and then measure the period and resonant amplitude with and without damping.

Now the damping current is adjust to these values:

        -I = 0 A (no damping)

        -I ~ 0.25 A

        -I ~ 0.45 A

To determine the resonant amplitude of the torsion pendulum:

-deflect the pendulum completely to one side.

-find the resonant amplitude by changing the frequency of the external force and also the differential voltage between two terminals of the DC motor.

1. Data Analysis

1. Amplitude of free moving oscillation with and without damping

Table 1