1. 1
   A security company installs an electric door-release lock on a fire exit. The lock contains a coil of insulated wire wound around a soft iron core, forming an electromagnet. When the fire alarm system sends a current through the coil, the electromagnet attracts a metal bolt, releasing the door so people can escape.

   (a)

   What is meant by the core of this electromagnet being made from a magnetically soft material rather than a magnetically hard material?

   [1 mark]

   • AIt loses its magnetism almost immediately once the current is switched off
   • BIt becomes permanently magnetised and stays magnetic once the current stops
   • CIt is not attracted to magnetic fields at all
   • DIt cannot conduct electric current

   (b)

   Which change to the electromagnet's coil would increase the strength of the magnetic field it produces, for the same current?

   [1 mark]

   • AReplacing the iron core with a wooden core
   • BIncreasing the number of turns of wire on the core
   • CReducing the current in the coil
   • DUsing bare, uninsulated wire instead of insulated wire

   (c)

   State two ways, other than the number of turns, in which the strength of the magnetic field produced by this electromagnet's coil could be increased.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A workshop uses a step-down transformer to convert the mains supply to a safer voltage for power tools. The transformer's primary coil has 2000 turns and is connected to the 230 V mains supply. The secondary coil is designed to output 115 V.

   (a)

   Which equation relates the primary and secondary voltages of a transformer to the number of turns on each coil?

   [1 mark]

   • AVp x Vs = Np x Ns
   • BVp + Vs = Np + Ns
   • CVp / Vs = Np / Ns
   • DVp / Np = Vs x Ns

   (b)

   For an ideal transformer operating at 100% efficiency, what is true about the electrical power in the primary and secondary coils?

   [1 mark]

   • AOutput power is always greater than input power
   • BInput power is always greater than output power
   • CPower in the primary coil is unrelated to power in the secondary coil
   • DInput power always equals output power

   (c)

   Calculate the number of turns needed on the secondary coil to produce an output of 115 V.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An engineer is testing a straight, current-carrying wire held perpendicular to a uniform magnetic field produced by two flat permanent magnets. When a current flows through the wire, it experiences a force that pushes it out of the region between the magnets.

   (a)

   Explain how the left-hand rule can be used to predict the direction of the force on the current-carrying wire, and name the three quantities its fingers and thumb represent.

   [3 marks]

   (b)

   Describe how the force on the current-carrying wire would change if (i) the current in the wire were reversed, and (ii) the strength of the magnetic field were increased while the current stayed the same.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An amusement park's drop-tower ride uses a metal fin fixed to the bottom of each falling car. As the car falls, the fin passes between rows of fixed permanent magnets mounted along the tower, and this interaction slows the car down before it reaches the ground. Separately, small electromagnets mounted at the top of the tower are used to hold each car in place before it is released to fall.

   (a)

   Explain, in terms of electromagnetic induction, why the metal fin experiences forces that oppose its motion as it falls between the rows of magnets, and describe what would happen to the size of these forces if the car fell faster.

   [6 marks]

   (b)

   Explain, in terms of magnetism, how the electromagnets at the top of the tower are able to hold a car in place, and explain why using an electromagnet is more practical for this job than using a permanent magnet of the same size.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A security company designs a walk-through metal detector for an airport. Inside the detector's frame is a solenoid - a coil of insulated wire wound into a cylindrical shape - connected to an alternating current supply.

   (a)

   Which statement correctly describes the magnetic field pattern produced inside a solenoid carrying a current?

   [1 mark]

   • AA field that only exists at the two ends of the coil
   • BA strong, uniform field running along the length of the inside of the coil
   • CA field identical to that of a single straight wire
   • DNo magnetic field is produced inside a solenoid

   (b)

   How could the strength of the magnetic field produced by the solenoid be increased without changing the current?

   [1 mark]

   • AAdding a soft-iron core inside the solenoid
   • BSpacing the turns of wire further apart
   • CUsing fewer turns of wire
   • DUsing thicker insulation on the wire

   (c)

   State two factors, other than adding a core, that affect the strength of the magnetic field produced by a solenoid.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   An induction hob heats cooking pans using a coil hidden beneath a ceramic glass surface. When switched on, an alternating current in the coil creates a rapidly changing magnetic field. Placing a pan with a magnetic (steel) base on the hob causes the pan itself to heat up, even though the hob's surface stays relatively cool.

   (a)

   Why does a voltage get induced in the base of the steel pan when it is placed on the switched-on induction hob?

   [1 mark]

   • ABecause the pan is directly connected to the mains electricity supply
   • BBecause the ceramic surface conducts electricity into the pan
   • CBecause the pan's base is a permanent magnet
   • DBecause the changing magnetic field of the coil passes through the metal base of the pan

   (b)

   A pan with a plastic (non-conducting) base is placed on the same induction hob. What would be observed?

   [1 mark]

   • AThe pan would heat up more than a steel pan
   • BThe hob's ceramic surface would stop working
   • CThe pan would not heat up, since no current can be induced in a non-conducting material
   • DThe plastic base would become magnetised

   (c)

   State two factors that would affect the size of the voltage induced in the base of a pan placed on the hob.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A laptop's mains adapter contains a step-down transformer. The primary coil is connected to the 230 V mains supply and draws a current of 0.2 A. The transformer is 100% efficient and steps the voltage down to 20 V for charging the laptop battery.

   (a)

   Calculate the current supplied to the laptop by the secondary coil of the transformer.

   [3 marks]

   (b)

   Calculate the number of turns on the secondary coil if the primary coil has 4600 turns, and use your answer to explain whether this transformer is a step-up or step-down transformer.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A science centre's hands-on exhibit contains two devices built around the same design of rectangular coil placed between the poles of a permanent magnet. In the first device, visitors pass a direct current through the coil to make it spin as a simple electric motor. In the second, identical device, visitors turn a handle to spin the coil by hand, which generates a voltage as an electromagnetic induction generator.

   (a)

   Explain how passing a direct current through the coil in the first device causes it to spin, and explain what would happen to the direction of its spin if the direction of the current were reversed.

   [6 marks]

   (b)

   Explain how turning the handle to spin the coil in the second device generates a voltage, and describe two ways in which the visitor could increase the size of the voltage generated.

   [6 marks]

   Total for question 8: 12 marks

End of questions