Series and parallel circuits and conservation lawsEdexcel A-Level Physics: Subtopic test

1. 1
   A string of decorative lights consists of 20 identical lamps connected in series to a 230 V supply.

   (a)

   What is the potential difference across each lamp?

   [1 mark]

   • A230 V
   • B5.8 V
   • C11.5 V
   • D4600 V

   (b)

   One lamp filament breaks, so that lamp becomes an open circuit. What happens to the other lamps?

   [1 mark]

   • AThey all go out, because the current is zero everywhere in the circuit
   • BThey stay on at the same brightness
   • CThey get brighter, because the p.d. is shared between fewer lamps
   • DThey get dimmer but stay on

   (c)

   Explain, using conservation of energy, why the potential differences across the 20 lamps add up to the p.d. of the supply.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two resistors, of resistance 6.0 Ω and 12 Ω, are connected in parallel across a 12 V battery that has negligible internal resistance.

   (a)

   What is the total current drawn from the battery?

   [1 mark]

   • A0.67 A
   • B1.0 A
   • C2.0 A
   • D3.0 A

   (b)

   What is the combined resistance of the two resistors?

   [1 mark]

   • A18 Ω
   • B4.0 Ω
   • C9.0 Ω
   • D0.25 Ω

   (c)

   Explain why the combined resistance is less than the resistance of the 6.0 Ω resistor alone.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A circuit contains a 9.0 V battery of negligible internal resistance. A 15 Ω resistor is connected in series with a parallel combination of a 30 Ω resistor and a 60 Ω resistor.

   (a)

   Calculate the total resistance of the circuit.

   [3 marks]

   (b)

   Calculate the current in the 60 Ω resistor.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician is building resistor networks for a test rig, using resistors from the stock cupboard.

   (a)

   Derive the equations for the combined resistance of two resistors R₁ and R₂ in series and in parallel, using the conservation of charge and the conservation of energy.

   [6 marks]

   (b)

   The technician needs a combined resistance of 8.0 Ω to connect across a 6.0 V supply. The only resistors available are 24 Ω resistors, each of which can safely carry a maximum current of 0.30 A. Determine how many resistors are needed and how they should be connected, and evaluate whether the arrangement is safe.

   [6 marks]

   Total for question 4: 12 marks

End of questions