1. 1
   A homeowner fits an automatic garden lamp circuit. The circuit contains a light-dependent resistor (LDR) in series with a fixed resistor, a lamp and a 12 V supply, so that the lamp switches on as it gets dark.

   (a)

   As it gets dark, the light intensity falling on the LDR decreases. What happens to the resistance of the LDR?

   [1 mark]

   • AIt stays the same
   • BIt increases
   • CIt decreases to zero
   • DIt becomes infinite

   (b)

   Which statement correctly explains why metals such as copper conduct electricity well?

   [1 mark]

   • AMetals contain no charged particles
   • BMetals have very few free electrons compared with insulators
   • CMetals contain free (delocalised) electrons that can move easily through the structure
   • DMetals conduct because their ions are able to move freely through the structure

   (c)

   Calculate the current flowing through the lamp when the total resistance of the circuit is 60 $\Omega$, and state what physical quantity current measures in terms of charge flow.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A car's ignition system uses a relay: a small current from the ignition key switch flows through a coil wound around a soft iron core, and when energised the resulting electromagnet pulls a metal contact closed, switching on the large current needed to operate the starter motor.

   (a)

   The relay coil has a fixed current flowing through it. If the number of turns on the coil is increased, what happens to the strength of the magnetic field it produces?

   [1 mark]

   • AIt increases
   • BIt decreases
   • CIt stays the same
   • DIt becomes zero

   (b)

   When the ignition key switch is opened, current in the relay coil stops flowing. What happens to the magnetism of the iron core inside the coil?

   [1 mark]

   • AIt stays exactly as strong as before
   • BIt reverses direction but keeps the same strength
   • CIt becomes a permanent magnet
   • DIt loses almost all of its magnetism

   (c)

   State two changes that could be made to the relay coil to increase the strength of the magnetic field it produces when it is switched on.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A physics teacher rubs a polythene rod with a dry cloth and holds it near small pieces of paper, which jump up and stick to the rod. The teacher then touches the rod momentarily to a metal doorknob, causing a small spark and a crackling sound.

   (a)

   Explain, in terms of electrons, why the rod becomes charged when rubbed with the cloth, and why the small pieces of paper are attracted to it.

   [3 marks]

   (b)

   Explain why a spark and crackling sound occur when the charged rod touches the metal doorknob, and explain why this would not happen if the rod were instead held near a plastic ruler.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A hospital's MRI (magnetic resonance imaging) scanner contains a very strong electromagnet made from a coil of wire carrying a large current, wound around an iron-cored cylinder that surrounds the patient. Before a scan, the current in the coil is gradually increased from zero to its full operating value.

   (a)

   Explain, using ideas about magnetic fields, current and iron cores, how the electromagnet in the scanner produces its magnetic field and how the strength of the field changes as the current is increased from zero to full operating value. Also explain why an iron core, rather than no core, is used.

   [6 marks]

   (b)

   The scanner's magnet must be switched off completely between patients for safety checks. Explain what happens to the iron core's magnetism when the current is switched off, and describe two changes engineers could make to the coil to increase the strength of the magnetic field for future scanners, without changing the current.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A hobbyist builds a potential divider circuit using two resistors, of resistance 30 $\Omega$ and 60 $\Omega$, connected in series with a 9 V battery, to supply a smaller potential difference to an LED.

   (a)

   In the series circuit containing the two resistors, how does the current compare at different points around the circuit?

   [1 mark]

   • AIt is the same at every point in the circuit
   • BIt is largest near the battery and smallest near the LED
   • CIt is shared between the two resistors
   • DIt depends on the resistance of the component at that point

   (b)

   How is the total resistance of the 30 $\Omega$ and 60 $\Omega$ resistors, connected in series, related to their individual resistances?

   [1 mark]

   • AIt is less than either individual resistance
   • BIt is the sum of the two individual resistances
   • CIt is the difference between the two individual resistances
   • DIt is the average of the two individual resistances

   (c)

   Calculate the total resistance of the circuit and the current that flows through it.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   An electric doorbell contains a coil of wire wound around an iron core. When the doorbell button is pressed, current flows through the coil, and the resulting electromagnet attracts a metal armature, which strikes a bell and breaks the circuit, causing the process to repeat rapidly.

   (a)

   Which change to the doorbell's coil would decrease the strength of the electromagnet's magnetic field?

   [1 mark]

   • AIncreasing the current flowing through the coil
   • BAdding more turns to the coil
   • CDecreasing the current flowing through the coil
   • DAdding a soft iron core inside the coil

   (b)

   Ignoring the coil for a moment, what is the pattern of the magnetic field around a single straight current-carrying wire?

   [1 mark]

   • AField lines running parallel to the wire in one direction
   • BField lines radiating outward from the wire like spokes on a wheel
   • CNo magnetic field is produced around a straight wire
   • DConcentric circles around the wire, perpendicular to it

   (c)

   Explain, in terms of the iron core, why the bell's armature is no longer attracted the instant the circuit breaks.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   In a school laboratory, a 6 V supply is connected to two resistors, of resistance 12 $\Omega$ and 24 $\Omega$, connected in parallel with each other.

   (a)

   State two rules that apply to potential difference and current in a parallel circuit, and use one of these rules to state the potential difference across the 24 $\Omega$ resistor.

   [3 marks]

   (b)

   Calculate the current through each resistor, and hence the total current drawn from the supply.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A physics class investigates the magnetic field around a single bar magnet by placing a small plotting compass at different positions on a sheet of paper laid flat over the magnet, and marking the direction the compass needle points at each position to build up a field pattern.

   (a)

   Explain what a magnetic field is, how the plotting compass reveals its shape and direction, and describe how the field pattern would appear around the bar magnet, including where the field is strongest.

   [6 marks]

   (b)

   A second identical bar magnet is placed next to the first with its north pole facing the first magnet's north pole. Explain what would happen between the two magnets, and describe how the plotting compass could be used to identify a neutral point between them, referring to the origin of the compass needle's own magnetism.

   [6 marks]

   Total for question 8: 12 marks

End of questions