1. 1
   During a thunderstorm, static charge builds up between a cloud and the ground until a lightning strike occurs, rapidly discharging the cloud.

   (a)

   Which statement best explains why a lightning strike happens?

   [1 mark]

   • AAn imbalance of charge builds up with no conducting route between the cloud and the ground, and the charge suddenly discharges when the insulating air breaks down
   • BThe cloud and the ground are permanently connected by a conductor, allowing a small current to flow continuously
   • CElectrons are created inside the cloud and destroyed when they reach the ground
   • DThe ground becomes positively charged by friction with the cloud, which has no effect on the air between them

   (b)

   During the lightning strike, a charge of 15 C flows between the cloud and the ground in a time of 0.0005 s. Which equation should be used to calculate the current during the strike?

   [1 mark]

   • A$I = \dfrac{Q}{t}$
   • B$V = I \times R$
   • C$P = I \times V$
   • D$E = m \times c \times \Delta\theta$

   (c)

   Using the equation identified above, calculate the current that flows during the lightning strike, given that 15 C of charge flows in 0.0005 s.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A greenhouse uses an automatic circuit containing a thermistor to control a heater and a light-dependent resistor (LDR) to control a grow-light, so that both switch on under the correct conditions.

   (a)

   Which statement correctly describes how the resistance of the thermistor changes as the greenhouse temperature falls?

   [1 mark]

   • AThe thermistor has no resistance at any temperature
   • BThe resistance of the thermistor decreases as temperature falls
   • CThe resistance of the thermistor stays constant regardless of temperature
   • DThe resistance of the thermistor increases as temperature falls

   (b)

   As it gets dark in the evening, the light intensity falling on the LDR decreases. Which statement correctly describes what happens to the resistance of the LDR, and the effect this has on the current flowing through the grow-light circuit at constant potential difference?

   [1 mark]

   • AThe resistance of the LDR decreases, so the current in the circuit increases
   • BThe resistance of the LDR increases, so the current in the circuit decreases
   • CThe resistance of the LDR stays the same, so the current is unaffected
   • DThe resistance of the LDR increases, so the current in the circuit also increases

   (c)

   Explain why using an LDR in this circuit allows the grow-light to switch on automatically as it gets dark, and explain, using the idea of resistance, how the current through the LDR changes at this point.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An electrician wires two identical lamps in series with a 12 V battery, then rewires the same two lamps in parallel across the same battery.

   (a)

   Explain how the total resistance of the two lamps in series compares with their total resistance in parallel, and explain why the lamps glow more brightly when connected in parallel.

   [3 marks]

   (b)

   The electrician then adds a variable resistor in series with one of the parallel lamps and gradually increases its resistance. Explain what happens to the current through that lamp and to the current through the other (unchanged) lamp, and explain how this affects the total current drawn from the battery.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student carries out a required practical investigating the I–V characteristics of a filament lamp, a diode and a resistor at constant temperature, by connecting each component in turn to a variable power supply, an ammeter and a voltmeter, and recording current at a range of potential differences of both polarities.

   (a)

   Describe how the student's circuit should be set up and used to obtain reliable results, and explain what shape of I–V graph would be expected for the resistor and how it differs from that of the filament lamp.

   [6 marks]

   (b)

   Describe the I–V characteristic the student would expect to obtain for the diode in this investigation, and explain, in terms of resistance, why the diode behaves so differently depending on the direction of the applied potential difference.

   [6 marks]

   Total for question 4: 12 marks

End of questions