Page 416 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
11 A student builds an electronic circuit to control a motor using two identical LDRs, P and Q (Figure 25.30). In a room with the same amount of light on each LDR, the resistance of each LDR is 500 Ω. When the light from a torch shines on either LDR its resistance becomes 200 Ω.
+9 V
P
B–
Q
+
A
300 Ω 300 Ω
M
Figure 25.30 For End-of-chapter Question 11.
Vout
0V –9 V
12 a
Explain what is meant by a virtual earth and use your explanation to show that the gain of the inverting amplifier in Figure 25.31 is − R1 .
a State the value of the output voltage Vout of the op-amp when:
i the potential at point A is greater than the potential at point B [1]
ii the potential at point B is greater than the potential at point A. [1]
b Calculate the potential at point A when a torch shines on P. [2]
c i Explain what happens in the circuit when a torch shines on P but not on Q. [1] ii Explain what happens in the circuit when a torch shines on Q but not on P. [1]
d Explain why it is difficult to know what will happen if no torch shines on P or Q. [1]
R2
R1
[5]
R2
– +
+12 V
–12 V Vout
Vin
Figure 25.31 For End-of-chapter Question 12.
b The value of R1 is 20kΩ and the value of R2 is 4kΩ. The value of Vin is 0.50V.
i Calculate the value of Vout.
ii A resistor of 4 kΩ is placed in parallel with R2. Calculate the new value of Vout. [2]
[2]
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