28 Which statement about magnetism is correct? A Aluminium is a ferrous metal. B A steel magnet can be demagnetised by heating it. C The core of an electromagnet is usually made of steel. D The magnetic field lines around a bar magnet are evenly spaced. 29 An electromagnet is used to separate magnetic metals from non-magnetic metals. Why is steel not suitable as the core of the electromagnet? A It forms a permanent magnet. B It has a high density. C It has a high thermal capacity. D It is a good conductor of electricity. 30 An old and expensive steel watch becomes magnetised. The owner wants to use the watch again . He must demagnetise the watch. What is the best method to do this? A Heat it until it glows red hot. B Pass direct current though it. C Place it in a plastic bag and put the bag in hot water for several hours. D Place it in a solenoid that carries alternating current and then slowly remove it.

28. B29. AThe best method to demagnetise the watch: D

28. This question tests knowledge of magnetism and the properties of materials in relation to magnetism. A. Aluminium is not a ferrous metal; ferrous metals contain iron, and aluminium does not. B. Heating a steel magnet can indeed demagnetise it. Heat disrupts the alignment of the magnetic domains within the steel, causing it to lose its magnetic properties. C. The core of an electromagnet is typically made of iron or soft steel, not just steel. These materials are chosen for their high permeability, which allows them to easily magnetize and demagnetize. D. The magnetic field lines around a bar magnet are not evenly spaced; they are denser at the poles and less dense at the center.29. This question is about the suitability of different materials for the core of an electromagnet. A. Steel is not suitable because it can form a permanent magnet. This would interfere with the electromagnet's ability to be turned on and off. B. The density of steel is not directly relevant to its suitability as a core for an electromagnet. C. The thermal capacity of steel is also not directly relevant to its role in an electromagnet. D. Being a good conductor of electricity is not a disadvantage for the core of an electromagnet; however, the key issue with steel is its tendency to retain magnetism.For the steel watch that has become magnetised: A. Heating it until it glows red hot could demagnetise it, but this is likely to damage the watch. B. Passing direct current through it would not demagnetise it; it might even magnetise it further. C. Placing it in a plastic bag in hot water for several hours is unlikely to have any effect on its magnetism. D. Placing it in a solenoid carrying alternating current and then slowly removing it is a common method for demagnetising objects. The alternating current creates a changing magnetic field, which helps to randomize the orientation of the magnetic domains in the steel, effectively demagnetising it.