ElectromagnetsIB MYP Physics: Subtopic test

1. 1
   A scrapyard in Johannesburg uses a crane with an electromagnet to lift old steel car bodies. The electromagnet is a coil of insulated copper wire wound round a soft iron core and connected to a d.c. supply.

   (a)

   What is the main advantage of using an electromagnet instead of a permanent magnet on the crane?

   [1 mark]

   • AIt can be switched off to release the load
   • BIt does not need a power supply
   • CIt attracts aluminium as well as steel
   • DIt has only one pole

   (b)

   Which change makes the electromagnet stronger?

   [1 mark]

   • AUsing fewer turns of wire
   • BReplacing the iron core with a wooden core
   • CIncreasing the current in the coil
   • DConnecting the supply the other way round

   (c)

   Explain why a soft iron core is used in the crane's electromagnet.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a car, the driver's ignition switch controls the starter motor through a relay. The starter motor needs a very large current, while the circuit with the ignition switch carries only a small current.

   (a)

   A small current passes through the coil of the relay. What happens next?

   [1 mark]

   • AThe coil becomes a permanent magnet
   • BThe coil becomes an electromagnet and attracts an iron armature, which closes the switch in the starter motor circuit
   • CThe iron armature melts and joins the two circuits
   • DThe current in the starter motor circuit is reduced to zero

   (b)

   What is the shape of the magnetic field around a long straight wire carrying the starter motor current?

   [1 mark]

   • AStraight lines parallel to the wire
   • BStraight lines from one end of the wire to the other
   • CThere is no magnetic field around a straight wire
   • DCircles around the wire

   (c)

   Explain why a relay is used to switch the starter motor instead of the driver's ignition switch.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician in Cairo makes a solenoid, which is a long coil of insulated copper wire, and connects it to a d.c. power supply through a switch. She uses a small plotting compass to investigate the magnetic field.

   (a)

   Describe the magnetic field around the solenoid when a current flows, and state how the right-hand grip rule shows which end is the north pole.

   [3 marks]

   (b)

   Describe three changes that increase the strength of the magnetic field of the solenoid, and state one advantage of an electromagnet over a permanent magnet.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A science class in Tokyo builds electromagnets from insulated copper wire, an iron nail and a battery, and tests how many steel paper clips each can lift. A recycling company in the same city uses large electromagnet cranes to move scrap steel, and a councillor asks whether it should replace them with mechanical grabs.

   (a)

   The class hypothesises that the number of turns of wire on the nail affects the number of paper clips it can lift. State a testable hypothesis with a scientific reason, identify the independent, dependent and one control variable, and outline a safe method.

   [6 marks]

   (b)

   Discuss and evaluate whether the recycling company should replace its electromagnet cranes with mechanical grabs.

   [6 marks]

   Total for question 4: 12 marks

End of questions