Magnetism and Electromagnetic Induction

11 Grade 10 General Science questions on Magnetism and Electromagnetic Induction, with worked marking schemes. 11 questions are available on this topic in full.

1. Question 1

   4 marks

   A learner places a bar magnet so that it lies in the east-west direction and then hammers it repeatedly.

   (a) Explain, using the domain theory, how this demagnetises the magnet. (3 marks)

   (b) State one disadvantage of using hammering to demagnetise a magnet. (1 mark)

   (4 marks)

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2. Question 2

   1 mark

   An e.m.f. is induced in a conductor whenever

   A. it is made of a magnetic material such as iron

   B. it is connected to a galvanometer in a circuit

   C. the magnetic field linking it changes

   D. a steady current flows through it near a magnet

   (1 mark)

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3. Question 3

   3 marks

   Two bar magnets are placed on a bench with their north poles facing each other a short distance apart. Draw the magnetic field pattern between and around the two facing poles. Show the direction of the field lines and mark the neutral point with the letter X. (3 marks)

   (3 marks)

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4. Question 4

   1 mark

   Which of the following methods is most suitable for demagnetising a steel bar magnet in the laboratory without damaging it?

   A. Leaving it lying north-south beside a strong bar magnet overnight

   B. Pulling it slowly out of a coil carrying alternating current

   C. Wrapping it tightly in aluminium foil and storing it in a dry box

   D. Cooling it in iced water while it lies along the north-south line

   (1 mark)

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5. Question 5

   4 marks

   (a) Describe how a steel bar can be magnetised by the electrical method. (3 marks)

   (b) State how the polarity of the magnetised bar can be confirmed. (1 mark)

   (4 marks)

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6. Question 6

   6 marks

   Learners in a Grade 10 class are making a simple electric bell from locally available materials such as a large iron nail, insulated copper wire, a hacksaw blade, a bottle top and a dry cell.

   (a) Explain why the core of the electromagnet should be made of soft iron rather than steel. (2 marks)

   (b) Explain how the bell rings continuously while the bell switch is pressed. (4 marks)

   (6 marks)

   Marking scheme

   (a) (2 marks) - Soft iron is magnetised easily when current flows and loses its magnetism immediately when the current stops (1 mark) - Steel would keep its magnetism, so the armature would stay attracted and the bell would not keep ringing (1 mark) (b) (4 marks) - When the switch is pressed, current flows through the coil and magnetises the soft iron core, forming an electromagnet (1 mark) - The electromagnet attracts the armature (e.g. the springy hacksaw blade carrying the hammer), so the hammer strikes the gong (bottle top) (1 mark) - The movement of the armature breaks the circuit at the contact point, so the current stops and the core is demagnetised (1 mark) - The springy blade returns the armature, the contact is remade and the cycle repeats rapidly, giving continuous ringing (1 mark)

7. Question 7

   1 mark

   When iron filings are sprinkled on a sheet of paper placed over a bar magnet and the paper is tapped gently, the filings are most crowded near the ends of the magnet. This shows that

   A. field lines start and end at the centre of the magnet

   B. the magnetic field is strongest at the poles

   C. the poles of the magnet carry the most electric charge

   D. iron filings are repelled by the middle of the magnet

   (1 mark)

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8. Question 8

   1 mark

   Falling water at a hydroelectric power station such as Gitaru turns a turbine connected to a generator. The main energy change taking place in the generator is from

   A. kinetic energy to electrical energy

   B. potential energy to chemical energy

   C. electrical energy to magnetic energy only

   D. electrical energy to kinetic energy

   (1 mark)

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9. Question 9

   6 marks

   A coil of insulated copper wire is connected to a sensitive galvanometer, and a bar magnet is pushed into the coil. Explain three factors that determine the magnitude of the e.m.f. induced in the coil. (6 marks)

   (6 marks)

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10. Question 10

    4 marks

    Other than generating electricity in power stations, explain two applications of electromagnetic induction in day-to-day life. (4 marks)

    (4 marks)

    Marking scheme

    Any two of the following; 1 mark for the application and 1 mark for the explanation (4 marks): - Transformers (e.g. on electricity poles and in phone chargers): a changing current in the primary coil produces a changing magnetic field that induces an e.m.f. in the secondary coil, stepping voltage up or down - Bicycle dynamo: the turning wheel rotates a magnet near a coil, inducing an e.m.f. that lights the bicycle lamp - Induction cookers: an alternating current produces a changing magnetic field that induces currents in the base of the pot, heating it - Metal detectors at security checkpoints: a changing magnetic field induces currents in metal objects, which are detected by the device - Moving-coil microphone: sound makes a coil vibrate in a magnetic field, inducing an e.m.f. that matches the sound - Wireless (induction) charging of phones: a changing field from the charging pad induces an e.m.f. in a coil in the phone - Card readers / magnetic strip readers: the moving magnetic strip induces small e.m.f.s in a reading coil

11. Question 11

    4 marks

    Describe how a plotting compass can be used to map the magnetic field around a bar magnet on a sheet of paper. (4 marks)

    (4 marks)

    Marking scheme

    Any four of the following, 1 mark each (4 marks): - Place the bar magnet on a sheet of white paper (away from other magnets and iron objects) and trace its outline - Place the plotting compass near the north pole of the magnet - Mark a dot at the position of the north end (tip) of the compass needle - Move the compass so that the south end (tail) of the needle is on the dot just marked, and mark a new dot at the tip - Repeat until the compass reaches the south pole or the edge of the paper - Join the dots with a smooth curve and add an arrow showing the direction in which the compass needle points (north to south) - Repeat from other starting points near the north pole to obtain several field lines

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