Observing the switching on and off of a filament lamp: results

Discussion of results

Because of the high sampling rate a large file of data was accumulated. The data was copied as text and imported into Excel. Using this table of data, a suitable portion was selected for graphing.

The following graph shows the plot for the complete data set.

The following graph shows the period immediately after the application of a p.d. across the lamp.

Note that in all the above graphs, the p.d. plotted is that across the resistor, not across the lamp. Since the supply voltage remained constant, a decreasing p.d. across the resistor, implies that the p.d. across the lamp must be increasing.

The instant that the p.d. was applied can be clearly seen. What is also very clear is that the lamp takes some 150 ms to reach maximum brightness.

A 5.1 ohm resistor was placed in series with the 6 volt lamp. The voltage input of DrDAQ was connected across the resistor.

The supply voltage was set to 9 volts. This was checked with a digital multimeter and found to be 9.42 volts. The value remained constant during both switching on and off.

This arrangement allowed the current flowing to be calculated, by application of Ohm's law to the data collected across the resistor.

As the supply p.d. remained at 9.42 volts, then from a knowledge of the p.d. across the resistor, the p.d. across the lamp could be found. This, together with the current, allowed calculation of the resistance of the filament during switch on.

The correlation between lamp brightness and its resistance is shown in the following graph:

Answers to questions

Answer 1

Bearing in mind that mains AC has a frequency of 50 Hz (in the UK), and this slow response to changing p.d. is likely to reduce any effect of flicker.

Answer 2

Until the lamp filament reaches a high enough temperature it will not emit any light. Consequently, although its temperature, and hence resistance, increases there is no change in levels of illumination.

Answer 3

The lamp will be emitting infrared radiation before any light is emitted. Detectors capable of responding to both infrared and light would therefore extend the range.

Further study

The very large spike at the moment of switch on, is probably due to the low resistance of the lamp when it is at room temperature. During this time the p.d. across the resistor will be a maximum, and will fall away with increasing temperature and resistance of the lamp filament.

Another possibility was that it was a spike produced by the power supply, and that this occured too rapidly to be detected by the digital multimeter. The experiment was repeated using ADC-40 connected across the power supply. [DrDAQ was used simultaneously in the manner described above.] Whilst the spike remained across the resistor, no such effect was detected across the power supply. This appears to rule out any surge during switch on of the power supply.

The original display also shows what happens during lamp switch off. It can be seen that some 180 ms elapses before the emission of light ceases.

This might be the time that it takes the filament to cool down.

Another explanantion might be the electromagnetic induction. At the moment of switch off, the magnetic field generated within the coiled filament, will begin to colapse. According to Lenz's law this will induce an emf in the opposition to the dropping emf. What is observed is certainly opposing the switch off. As can be seen, the emf does not drop instantaneously to zero.

Further, very large induced emf's can be deliberately generated, eg. to operate neon indicators in switches.

Comparing switch-on with switch-off

Note: during switch-on the p.d. across the lamp increases as the p.d. across the resistor drops away. Knowing the total p.d. we can find the p.d. across the lamp and the current flowing.

As far as switch-off is concerned the p.d. is supposed to drop to zero [a back-emf possibly preventing this happening instantaneously]. Hence the actual applied p.d. is unknown during this time. Therefore cannot find either p.d. across the lamp or its resistance.

The two period cannot be compared:

1. during switch-on the resistance of the lamp is increasing with temperature - so the brightness increases with decreasing current!
2. during switch off both the brightness and the current are falling.

The difference between these two times is seen from the curves:

1. falling p.d. across the resistor [and hence falling current] and rising brightness during switch on.
2. compared to falling p.d. and brightness during switch off.

This is perhaps more clearly seen in the screen shot below:

Credits, comments and further info

This experiment was written by Dr. R. Robinson, Maesydderwen Comprehensive School, Ystradgynlais, Powys.