Night and Day

We wish to control the brightness of an indoor light bulb using a photoresistor located outside, on the roof of a house, as shown in the figure above. As the sun comes up (and the amount of light falling on the photoresistor increases), the resistance of the photoresistor decreases. Since the photoresistor is outside, it does not see the light from the light bulb (only from the outside world).

We will model each light bulb as a resistor with an unknown resistance. Consider the following ways we could connect these components to each other (shown below and on the next page). For each, indicate what happens as the sun comes up (the resistance of the photoresistor decreases):

• the brightness of the bulb increases
• the brightness of the bulb decreases
• the brightness of the bulb stays the same
• there is not enough information given to answer the question

Justify your answer for each.

1) Circuit 1

2) Circuit 2

3) Circuit 3

4) Circuit 4

5) Circuit 5

 
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