TP Physique2

I. GOALS :

- Learn to use the oscilloscope as a means of measurement in the alternating regime.

- Analyze the behavior of a variable and repetitive electrical signal (potential) in the alternating regime.

- Study the behavior of the phase shift in the different associations: resistance-resistance (R-R), resistance-coil (R-L) and resistance-coil-capacitor (R-L-C).

II. EQUIPMENT USED :

• Low frequency generator (GBF).
• Voltmeter or multimeter.
• Two resistances of 1kW et 100W, a capacitor C=1µF and an induction coil L=9mH.
• An oscilloscope.

III. USE OF THE OSCILLOSCOPE :

An oscilloscope is a device used to analyze with great precision the behavior of a variable and repetitive electrical signal (potential). The oscilloscope screen is nothing more or less than a graph of potential versus time on which we can visualize the behavior of the potential during an entire cycle. The oscilloscope automatically identifies the repeating cycle and displays it on the screen.

III-1. Potential measurement

The oscilloscope screen is made up of two perpendicular axes crossing in the middle and divided into small squares (parallel to the axes) of length 1cm. These squares are divided in turn into 5 small divisions. By connecting a signal to the channel input, a curve V(t) appears on the screen where the horizontal axis represents time and the vertical axis represents potential. The measurement of time or potential, corresponding to a point on the plot, has a value equal to the product of the value of the scale with that of the number of divisions. In practice, we measure the peak-to-peak voltage (V_cc) which represents twice the maximum value of the voltage (V_max).

Questions :

1.  .What do the voltage V_eff measured by the voltmeter and the maximum voltage V_max measured by the oscilloscope represent ?

In practice, to measure a voltage with the oscilloscope, you must follow the following steps:

1. Assemble the figure below.

2. Turn on the generator (GBF).

3. Change the frequency by turning the button frequency of the generator.

4. Change the voltage value with the button amplitude of the generator.

5. Turn on the oscilloscope and choose the mode CH1 (chanal1).

6. Check that the CH1 channel switch is in AC (alternatif current).

7. Adjust the 0V level by pressing the button GBD of chanal CH1.

8. Adjust the curve using the vertical and horizontal position buttons.

9. Adjust the voltage gauge to allow a good voltage reading.

III-2. Phase shift measurement

a-Temporal difference method

Here we only define the phase shift of two signals which have the same period. . Let two signals of type : 

Where  A₁ and A₂ et are the amplitudes also called peaks.

The frequency  is linked to the period  by the relation:

The phase shift  between two signals is defined by :

Where Δt represents the time difference between the two signals.

Remark : If we define signal 1 as that which passes through the origin O, we can say that :

If OM > 0  : Signal 2 is phase delayed compared to signal 1.

If OM < 0 : Signal 2 is in phase advance compared to signal 1.

 b-Lissajous method

In the oscilloscope, we can eliminate the time component of the signals et by pressing the button XY. We obtain an ellipse from which the phase shift between the two signals will be calculated by the formula :

There are special cases for Lissajous curves :

If Δφ = 0 or π  : The ellipse is reduced to a line, one of the diagonals of the rectangle.

IV. Manipulation :