Potential dividersAQA A-Level Physics: Subtopic test

1. 1
   A potential divider consists of a 6.0 kΩ resistor in series with a 3.0 kΩ resistor connected across a 9.0 V supply of negligible internal resistance. The output is taken across the 3.0 kΩ resistor.

   (a)

   What is the output pd of the potential divider?

   [1 mark]

   • A6.0 V
   • B3.0 V
   • C4.5 V
   • D9.0 V

   (b)

   A load of resistance 3.0 kΩ is now connected across the output. What is the new output pd?

   [1 mark]

   • A3.0 V
   • B4.5 V
   • C7.2 V
   • D1.8 V

   (c)

   Explain why connecting the load across the output reduces the output pd.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A light-dependent resistor (LDR) is connected in series with a fixed 2.2 kΩ resistor across a 5.0 V supply of negligible internal resistance, forming a potential divider that controls a street lamp. The output is taken across the fixed resistor. The resistance of the LDR is 500 Ω in bright light and 20 kΩ in darkness.

   (a)

   What happens to the resistance of the LDR and to the output pd as the light intensity decreases?

   [1 mark]

   • ALDR resistance increases and output pd decreases
   • BLDR resistance increases and output pd increases
   • CLDR resistance decreases and output pd decreases
   • DLDR resistance decreases and output pd increases

   (b)

   What is the output pd in bright light?

   [1 mark]

   • A0.93 V
   • B0.50 V
   • C4.1 V
   • D5.0 V

   (c)

   Calculate the output pd in darkness.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A designer needs to supply a motor with a variable pd from 0 V to 12 V from a 12 V supply of negligible internal resistance. A variable resistor with a track of total resistance 100 Ω and a sliding contact is available.

   (a)

   Describe how the variable resistor should be connected so that it acts as a potential divider supplying a pd that can be varied from 0 V to 12 V.

   [3 marks]

   (b)

   The sliding contact is set so that the output pd is 4.5 V when no motor is connected. A motor of resistance 50 Ω is then connected across the output. Calculate the pd across the motor and explain why it is different from 4.5 V.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A greenhouse controller uses a negative temperature coefficient (NTC) thermistor in series with a variable resistor across a 6.0 V supply of negligible internal resistance. The output is taken across the variable resistor and passed to a switching circuit of very high input resistance, which turns the heater on when the output falls below 2.0 V. The thermistor has resistance 4.0 kΩ at 10 °C and 1.0 kΩ at 25 °C.

   (a)

   Explain how this circuit makes the heater switch on when it is cold, and calculate the resistance to which the variable resistor must be set so that the heater switches on at 10 °C.

   [6 marks]

   (b)

   A technician suggests swapping the thermistor and the variable resistor, so that the output is taken across the thermistor, with the variable resistor still set at 2.0 kΩ. Evaluate whether this change is suitable.

   [6 marks]

   Total for question 4: 12 marks

End of questions