Q#181: [Current of Electricity > Resistance] (Past Exam Paper – June 2013 Paper 12 Q38)
In circuit below, reading VT on voltmeter changes from high to low as temperature of the thermistor changes. Reading VL on voltmeter changes from high to low as level of light on the light-dependent resistor (LDR) changes.
Readings VT and VL are both high.
What are the conditions of temperature and light level?
Solution 181:
Answer: C.
Ohm’s law: V = IR. Therefore, for the voltmeter to read a high value, the resistance of the component to which it is connected must be high.
For both reading VT and VL to be high, the resistance of the thermistor should be low (so that the resistance of the fixed resistor, to which the voltmeter is connected, is much higher than the resistance of the thermistor) and the resistance of LDR should be high (since the voltmeter is connected across it).
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. ΔR = k ΔT.
Depending on k, resistance may increase or decrease with temperature (For A-level, k is normally taken to be only negative.
That is, resistance decreases with an increase in temperature).
A photoresistor or light-dependent resistor (LDR) or photocell is a light-controlled variable resistor.
The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity.
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