RF OSCILLATORS

Experiment 3physics

Experiment 4applied physics

FORCED OSCILLATIONS4.1OBJECTIVES

1. Study forced oscillations for I=0A and plot resonance curve for each value of current.2. Determine the natural frequency o and Torsion constant or Torsion coefficient D.4.2 SAFETY NOTES:

1. The current through the eddy current brake should not exceed 2 A for a long time.

2. Avoid overheating of the coils by measuring too long with large current I >1A.

3. Do not touch buttons and knobs of motor and power supply unnecessarily.

4. Make connections carefully and call teacher to check the connections before turning on the power supply.

4.3 EQUIPMENTS/ REQUIREMENTS:

1. 1 Torsion Pendulum

2. 1 Power Supply for Torsion Pendulum

3. 1 Stop watch

4. 1 Ammeter (max. 3ADC)

5. 1 Connecting lead red 50 cm

6. 1 Connecting lead blue 50 cm

4.4 THEORY:

Forced Oscillations:

Consider that a damped oscillator (torsion pendulum) is driven by an external torque . Then the oscillation of the system can be defined by the following equation: (5)

where is the driving frequency and 0 is the driving torque.

The general solution of the equation is:

with (6)

where, 0 is the natural frequency andis the phase shift between the oscillator and the external torque.

(7)

In this case the resonance frequency is

(8)

4.5 REFERENCES:

1. Physics by Halliday Resnick and Krane.2. Fundamental of Physics by Halliday Resnick and Walker.3. Physics for Scientists and Engineers by Serway and Jewett.4.6TECHNICAL DATA:

1. Characteristic frequency: approx. 0.5 Hz

2. Weight: approx. 6 kg

3. Moment of inertia: appox.3kgm24. Supply voltage: 0-24 V DC

5. Maximum eddy current: 2 A

4.7DESCRIPTION:1. Scale ring2. Pendulum body

(2a) pointer for deflection,

(2b) pointer for phase relation,

(2c) coiled spring

3. Exciter

(3a) push rod

(3b) drive wheel for eccentric

4. Electromagnet for eddy-current brake

5. Exciter motor

(5a) coarse control for excitation voltage,

(5b) fine control for excitation voltage

4.8EXPERIMENTAL SETUP:

4.9PROCEDURE:1. Make connections as shown in figure 3.5. Set the pointer to the zero on the scale by turning the drive wheel.

2. Set the current I=0. Set the excitation voltage U=5V of the exciter with the help of knob 5a (see in figure 3.3). This will change the frequency of exciter.

3. Measure timet for 5 rotation of the drive wheel and calculate the frequency of exciter .

= n/t

4. Measure the amplitude of the oscillation after it has reached a steady state.

5. Now turn off the motor and allow the pendulum to become at rest. Increase the excitation voltage in very small step. It will increase the frequency of exciter.

6. Turn on the motor. Measure timet for 5 rotation of the drive wheel and calculate the frequency of exciter and read off the amplitude for stead state.

7. Take at least ten readings by repeating above procedure.

8. Plot graph for amplitude A (Scd) Vs timet sec. Determine resonance frequency and discuss your graph.4.10OBSERVATIONS:

Timet secFrequency HzAmplitude A scdTimet secFrequency HzAmplitude A scd

4.11GRAPH:

1. Plot graph for frequency versus max amplitude and determine resonance frequency.

4.12CALCULATIONS:

1. From graph resonance frequency is __________Hz.

4.13RESULT AND DISCUSSION:

Lab Instructor Arshia Aijaz Page| 3 1Lab Instructor Arshia Aijaz Page| 4 3

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