﻿ Work task - Voltamper characteristic of bulb
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Classic bulb V-A characteristic

1. Measure the dependence of the electric current (2.2 V/0.18 mA) on the voltage applied to the bulb.

2. Write the values obtained in the voltamper characteristic of the bulb. Determine the form of the dependence using theory.

3. Calculate the static resistance values from the measured values and construct a graph of the DC resistance of the filament of the bulb at the applied voltage.

4. On the graph, find the differential resistance values (using tangent construction) for the "cold" bulb and then for the filament lamp.

5. From the static resistance change, estimate the fibre temperature in each case – it is also possible to plot the dependence of the fibre temperature on the current (or applied voltage or power).

Measurement procedure

1. We launch the remote experiment VeLMA – Classic bulb V-A characteristic

2. Use the slider (PC) or the + and buttons (tablet) to set different voltage values on the bulb.

3. At each moment we read the values of the electric voltage and the electric current that are written in the table.

4. We continuously measure the measured values in the table and, if there are large gaps between the measured dependencies, we measure the missing parts.

5. We watch a light bulb on the webcam and record the moment (voltage and current) when the thread is hot.

6. After the required number of measurement data (files and save all experimental values) remote role terminate.

7. We open the selected experimental data file in the spreadsheet (MS Excel, Oo Calc, Kingsoft Spreadsheets…).

8. We draw the dependence of the electric current on the connected electric voltage in the diagram. Check the shape of the recorded dependence with the assumed shape.

9. Use formula (1) to calculate the static resistance values for the different voltages of the measured interval. Plot the dependence of the static resistance on the applied voltage on the graph.

10. Determine the direction of the tangent using a regression line in the area of the voltamper characterisation corresponding to the non-incandescent bulb. The direction of the regression line corresponds to the so-called differential (dynamic) bulb resistance – see formula (2).

11. Similarly, determine the differential resistance of the light bulb (preferably 2.2 V / 0.18 mA).

12. Using the formula for the temperature dependence of the electrical resistance of a metallic conductor R = R0 · [1 + α · (t – t0)], determine the approximate working temperature of the filament of the lamp, assuming that the temperature of the fibre at the beginning of the experiment corresponds to the laboratory temperature t0 = 25 °C (thermal coefficient of tensile strength of the tungsten fibre: α = 4,83 ·10–3 K–1).

$t = t 0 + R – R 0 α · R 0$
Graph of temperature versus current (voltage, power)

Using the above formulae, we can derive some additional dependencies.

1. We plot the dependence of the approximate temperature of the fibre on the current passing through the bulb.

2. We plot the dependence of the approximate temperature of the fibre on the voltage applied to the bulb.

3. We plot the dependence of the approximate temperature of the fibre on the input to the bulb (the product of voltage and current).

4. The dependence obtained (in particular from point 15) is compared with our assumption based on the concept of heat radiating bodies (e.g. Stefan-Boltzmann law).