Chapter 20 - Magnetism

1.   

State the properties of a magnet.

  • A magnet can attract another magnetic material such as iron, steel, cobalt and nickel.
  • A magnet has two poles, the North pole and South pole.
  • Like poles repel and unlike poles attract.
  • A freely suspended magnet always points in a North-South direction.

 2.   

With the aid of a diagram, describe how a magnetic material such as a steel bar can be magnetised using the electrical method.

magnetisation of steel. magnetization of steel using direct current (d.c.)
  • A steel bar to be magnetised is placed in a solenoid (wire coil) as shown.
  • The solenoid is connected to a direct current (d.c.) supply and switched on. The current flowing through the solenoid produces a magnetic field that will magnetise the steel bar.
  • The resistance of the rheostat can be reduced to increase the current flowing in the circuit hence increasing the strength of the magnet.

3.   

With the aid of a diagram, describe how a magnet can be demagnetised using the electrical method.

Demagnetisation using alternating current (a.c.)
  • A magnet to be demagnetised is placed in a solenoid.

  • An alternating current is allowed to flow through the solenoid.
  • The magnet is withdrawn in an east-west direction, far away from the solenoid while the current is still flowing.

 4.   

With the aid of a diagram, describe how the magnetic field lines can be plotted with a compass.

plotting magnetic field lines
  • Place a bar magnet on a sheet of paper. Mark the outline of the magnet.
  • Put a small compass near to the North-pole of the magnet.
  • Using a pencil, mark dots 1 and 2, the position of the South and North pole of the compass respectively.
  • Move the compass so that the S-pole of the compass is on dot 2 and mark dot 3 against the N-pole of the compass.
  • Repeat the above method to plot other lines of force on either side of the magnet to obtain a magnetic field pattern around the magnet.

5.   

State the differences between the magnetic properties of steel and iron.

  • Iron can be magnetised and demagnetised easily while steel is harder to magnetised and demagnetised.

6.   

You are given three steel bars. Only two of the bars are magnets. Explain how you would identify the magnets without using any other equipment.

  • Using two of the three bars, check for repulsion between the ends of the two bars. The bars whose ends repel are magnets.

7.   

You are given three apparently identical metal bars, one steel, one soft iron, and one copper. How would you identify them without damaging them in any way?

  • Place all 3 bars in a solenoid with direct current flowing through it to magnetise it.
  • Bring an iron paper clip close to one end of each bar.
  • The bar that does not attract the paper clip at all is the copper.
  • Switch off the current.
  • The bar that still attracts the paper clip is the steel (as it retained its magnetism).