Potential dividersEdexcel International A Level Physics: Subtopic test

1. 1
   A 9.0 V supply of negligible internal resistance is connected across a 2.0 kΩ resistor in series with a 4.0 kΩ resistor. A voltmeter of very high resistance measures the output p.d. across the 4.0 kΩ resistor.

   (a)

   What is the output p.d.?

   [1 mark]

   • A3.0 V
   • B4.5 V
   • C9.0 V
   • D6.0 V

   (b)

   The 2.0 kΩ resistor is replaced by a resistor R so that the output p.d. becomes 7.5 V. What is R?

   [1 mark]

   • A0.50 kΩ
   • B0.80 kΩ
   • C1.5 kΩ
   • D4.8 kΩ

   (c)

   Calculate the current in the resistors and the power dissipated in the 4.0 kΩ resistor.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A light-meter circuit has a 6.0 V supply of negligible internal resistance, a light-dependent resistor (LDR) and a 10 kΩ fixed resistor, all in series. The output p.d. is measured across the fixed resistor with a voltmeter of very high resistance. In bright light the LDR has a resistance of 2.0 kΩ; in dim light its resistance is 40 kΩ.

   (a)

   What is the output p.d. in bright light?

   [1 mark]

   • A1.0 V
   • B3.0 V
   • C5.0 V
   • D6.0 V

   (b)

   What happens to the output p.d. as the light intensity falls?

   [1 mark]

   • AIt decreases
   • BIt increases
   • CIt stays the same
   • DIt rises to the supply p.d.

   (c)

   Calculate the p.d. across the LDR in dim light.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A temperature-sensing circuit has an NTC thermistor in series with a 4.0 kΩ fixed resistor across a 5.0 V supply of negligible internal resistance. The output p.d. is taken across the fixed resistor. At 20 °C the resistance of the thermistor is 12 kΩ and at 60 °C it is 1.0 kΩ.

   (a)

   Calculate the output p.d. of the circuit at 20 °C.

   [3 marks]

   (b)

   A fan is to switch on when the output p.d. reaches 3.0 V. Calculate the resistance of the thermistor at this point and explain whether this happens above or below 20 °C.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician designs a potential divider to supply a sensor with 3.0 V from a 12 V supply of negligible internal resistance. The output is taken across a lower resistor of resistance 1.5 kΩ, which is in series with a fixed upper resistor. The output voltmeter has a very high resistance.

   (a)

   Calculate the resistance of the upper resistor and the power dissipated in each resistor, and show that these are consistent with the power supplied by the 12 V supply.

   [6 marks]

   (b)

   The technician replaces the lower resistor with a variable resistor of range 0 to 10 kΩ, keeping the 4.5 kΩ upper resistor. Calculate the range of output p.d. and evaluate whether the circuit could also supply a 9.0 V device.

   [6 marks]

   Total for question 4: 12 marks

End of questions