Potential dividersEdexcel A-Level Physics: Subtopic test

1. 1
   A potential divider is made from a 12 V battery of negligible internal resistance, a 4.0 kΩ resistor R₁ and a 2.0 kΩ resistor R₂ connected in series. R₁ is joined to the positive terminal and R₂ to the negative terminal. The output p.d. is taken across R₂.

   (a)

   What is the output p.d. across R₂?

   [1 mark]

   • A8.0 V
   • B6.0 V
   • C2.0 V
   • D4.0 V

   (b)

   What is the current in the circuit?

   [1 mark]

   • A3.0 mA
   • B2.0 mA
   • C6.0 mA
   • D2.0 A

   (c)

   A load of resistance 2.0 kΩ is now connected across R₂. Calculate the new output p.d.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A garden light switches on automatically at dusk. A 5.0 V supply of negligible internal resistance is connected across a 10 kΩ fixed resistor in series with a light-dependent resistor (LDR). The output p.d. is taken across the fixed resistor, and the lamp is switched on when this output falls below 2.0 V. The LDR has a resistance of 2.0 kΩ in daylight and 40 kΩ in the dark.

   (a)

   What is the output p.d. across the fixed resistor in daylight?

   [1 mark]

   • A0.83 V
   • B2.5 V
   • C4.2 V
   • D5.0 V

   (b)

   What is the output p.d. across the fixed resistor in the dark?

   [1 mark]

   • A1.0 V
   • B4.0 V
   • C2.5 V
   • D0.20 V

   (c)

   Explain why the output p.d. falls when it gets dark.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A uniform resistance wire PQ of length 1.00 m and resistance 20 Ω is connected across a 6.0 V cell of negligible internal resistance. A sliding contact J is placed on the wire at a distance of 0.35 m from Q, and the output p.d. is taken between Q and J.

   (a)

   Calculate the output p.d. between Q and J when nothing else is connected to the circuit.

   [3 marks]

   (b)

   A 10 Ω resistor is now connected between Q and J, with the contact still at 0.35 m. Calculate the p.d. across the resistor.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer designs a controller that switches on a fan in a greenhouse. A 9.0 V supply of negligible internal resistance is connected across a negative temperature coefficient (NTC) thermistor in series with a fixed resistor R. The output p.d. is taken across R and the fan switches on when this output rises above 4.5 V. The thermistor has a resistance of 5.0 kΩ at 20 °C and 1.0 kΩ at 35 °C.

   (a)

   Explain how this circuit switches the fan on as the greenhouse warms, including why the output p.d. changes.

   [6 marks]

   (b)

   The engineer wants the fan to switch on at 35 °C. Evaluate whether R = 1.0 kΩ or R = 5.0 kΩ is the better choice.

   [6 marks]

   Total for question 4: 12 marks

End of questions