Questions on Characteristic Curves - Filament Lamp
Q4. The characteristic shown below is that of a filament lamp.
(a) Explain why, as the voltage is increased either positively or negatively from zero, the characteristic has the form shown in the diagram
An increase in voltage (whether positive or negative in polarity) gives increase in current 
An increase in temperature produces an increase in resistance.
Current in the filament produces heat energy in the filament wire. The heat produced is proportional to the square of the current (P = I2R) this heat energy raises the temperature of the filament wire, and as the filament gets hotter its resistance increases.
Ohm's law is obeyed (or a straight line graph is obtained) initially because the heat energy developed in the filament at low currents is not sufficient to change its temperature - and therefore resistance - dramatically.
The gradient of the characteristic line then decreases at higher voltage magnitudes because the hotter filament wire has greater resistance therefore a given increase in voltage will produce a snaller increase in current than at lower voltages.
Negative voltage and current produces same effect as positive voltage and current
(5 marks MAX)
(b) At a certain point on the characteristic, the power developed in the lamp is 20 W and the current is 90 mA. Calculate the resistance of the filament at this point on the characteristic.
P = I2R
20 = (90 x 10-3)2 R
R = 2.5 x 103 
or 2.5 k
(2 marks)
(Total 7 marks)
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