- #1
kamhogo
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Homework Statement
These questions are related to an experiment where we had to record the current for varying values of potential difference (10 readings), then repeat the process with the applied potential difference was reversed
a) Why is the resistance of a light bulb not constant?
b) Does the resistance of diode change when polarity is reversed? Why?
Homework Equations
· ∆V=IR (linear resistors)
· R = 1/ (dI/dV) (non-linear elements)
· I = eIe
3. The attempt at
a solution
a) The light bulb is non-linear because its resistance varies with increasing/sustained current.
The resistance is not constant because as you increase the current (by increasing the voltage difference), the filament of the bulb heats up. This has for effect to increase the resistance. Higher temperature makes the ions in the filament vibrate more, making it more difficult for electrons to pass between them (—› reduced electrons flow —› reduced drift speed —› reduced electron current —› reduced conventional current). As the resistance decreases, the current flowing through the filament increases. However, the light bulb does behave like a linear element when the current flowing through it is not sustained (the filament does not have time to heat up enough to affect conductivity). This explains why the experimental current-vs-voltage graph of the light bulb appears to be a linear one with slope 1/R. Indeed, during the experiment, the switch allowing the current to pass through the light bulb was maintained in a closed position only long enough to obtain the voltage and current readings. Thus the current was not sustained
b) The resistance of the diode seems to change when the polarity is reversed. For negative values of applied potential difference, the current was zero. (Is this normal or an error from the experiment)
If someone could tell me if my thinking makes any sense and why the diode resistance changes when the polarity changes + why it increases so abruptly , that would be well-appreciated :)