TJC 2023 IP4 Magnetism Notes

TJC IP4 PHYSICS IP4 Physics Topic 15 – Magnetism Page 1 of 10 Student’s Copy Topic 15: Magnetism Content • Laws of magnetism • Magnetic properties of matter • Magnetic field Learning Outcomes Candidates should be able to: (a) state the properties of magnets (b) describe induced magnetism (c) describe electrical methods of magnetisation and demagnetisation (d) draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets (e) describe the plotting of magnetic field lines with a compass (f) distinguish between the properties and uses of temporary magnets (e.g. iron) and permanent magnets (e.g. steel)

TJC IP4 PHYSICS IP4 Physics Topic 15 – Magnetism Page 2 of 10 Student’s Copy 1 Introduction We can observe that the magnets interact even when they are they are not in direct contact. In electrostatics, we used the idea of an electric field to account for the interaction between charges that were separated from one another. In the same way, we can use the idea of magnetic field to account for the magnetic interaction. Every magnet has two poles. This is where most of its magnetic strength is most powerful. These poles are called north and south poles. The poles are called this as when a magnet is hung or suspended the magnet lines up in a north - south direction. Like poles repel each other while unlike poles attract. The attraction or repelling of two magnets depend on how close they are to each other and how strong the magnetic field of each magnet is. 1.1 Properties of Magnets LO (a) All magnets have the following properties: • Magnetic poles: Sprinkle iron filings onto a bar magnet and most of them attracted to the two ends of the bar magnet. These two ends are known as poles of the magnet. They are the locations where the magnetic effects are the strongest. • North and South poles: When a bar magnet is suspended freely, it will come to rest in the North-South direction. The end of the bar magnet that points to the northern end of the Earth is called the north-seeking pole, North pole or N pole. The other end is known as the south-seeking pole, South pole or S pole. • Law of magnetism: Like poles repel, unlike poles attract. [parallel to what has been taught in Static Electricity: Law of electrostatics] • Attracts magnetic materials : Cobalt,Iron, Nickel & Steel (Mnemonic: CoINS) 1.2 The Earth’s Magnetic Field Because of Earth's magnetic field, a compass placed anywhere on Earth turns so that the "north pole" of the magnet inside the compass points roughly north, toward Earth's north magnetic pole in northern Canada. This is the traditional definition of the "north pole" of a magnet, although other equivalent definitions are also possible. One confusion that arises from this definition is that if Earth itself is considered as a magnet, the south pole of that magnet would be the one nearer the north magnetic pole, and vice-versa. (Opposite poles attract and the north pole of the compass magnet is attracted to the north magnetic pole.) The north magnetic pole is so named not because of the polarity of the field there but because of its geographical location. N

TJC IP4 PHYSICS IP4 Physics Topic 15 – Magnetism Page 3 of 10 Student’s Copy 1.3 Magnetic and Non-magnetic Materials Magnetic materials (with ‘tiny’ magnets or magnetic domains) Non-magnetic materials Attraction by magnet yes no Magnetism can be magnetised, can be used to make magnets cannot be magnetised, cannot be used to make magnets Examples Cobalt, Iron, Nickel & many alloys based on these metals e.g. Steel (alloy of iron and carbon) and alnico (alloy of almumium, nickel and cobalt) wood, copper, paper, aluminium, brass and glass Every magnet has a maximum strength. This happens when ALL the magnetic domains are pointing in the same direction. The magnet is said to be magnetically saturated and cannot be any stronger. Magnetised Bar Unmagnetised Bar Magnetic domains are aligned Magnetic domains point in random directions Magnetic domains at the ends splay out due to the mutual repulsion between like poles Resulting magnetic effect of all magnetic domains are cancelled out Free poles at the end No free pole No magnetic effect Question 1: When a bar magnet is broken into half, does each smaller piece still possess an N pole and an S pole? Does each smaller piece behave like a magnet? 1.3.1 Magnetic Material In an unmagnetised state, millions of atoms arrange themselves in groups (magnetic domains) each with its atomic magnets pointing in different directions. Their magnetic effects cancel out. A group of atomic magnets pointing in the same direction is called a magnetic domain. In a magnetised magnetic material, the magnetic domains are arranged so that they point in a certain direction. Ferromagnetic materials consist of clusters of aligned atoms called magnetic domains, which behave like tiny magnets. When the material is unmagnetised, these domains are randomly oriented so that their magnetic fields cancel each other out. When the material is subject to a strong magnetic field, the magnetic domains line up and the material is said to be magnetised. Ferromagnetic materials include such metals as steel, iron, nickel and cobalt. Examples of non-magnetic metals include copper, aluminium and lead. domains point all ways domains point all ways all domains point the same way all domains point the same way

TJC IP4 PHYSICS IP4 Physics Topic 15 – Magnetism Page 4 of 10 Student’s Copy How is a magnet identified? Testing for evidence of magnetism: Only repulsion between a specimen and a magnet allows us to conclude that the specimen is a magnet. Specimen One end of specimen brought near to N pole Other end of specimen brought near to N pole bar magnet repulsion attraction bar magnet attraction repulsion soft iron rod (magnetic material) attraction attraction wooden rod stationary stationary 2 Magnetic Induction (Induced Magnetism) LO (b) When a paper clip is attracted to a permanent magnet, we say the paper clip has become an induced magnet. Magnetic induction is the process whereby an object of a magnetic material becomes a magnet when it is near (without contact) or in contact with a magnet. Worked Example 1: How does a magnet attract iron paper clips? When 1 iron paper clip is brought near a permanent magnet, it is attracted to the magnet. The iron paper clip has become an induced magnet (temporary magnet). This induced magnet (iron paper clip) is able to attract other iron paper clips. The S pole of the permanent magnet induces an N pole in the top end of the iron paper clip nearer to the magnet, while the further end of the iron paper clip becomes an induced S pole. Since opposite poles attract, the S pole of the permanent magnet attracts the induced N pole of the iron paper clip; and like poles repel, the S pole of permanent magnet repels the induced S pole of the iron paper clip. Since the magnetic strength/ force decreases with increase in distance, there is a net attractive force acting on the iron paper clip towards the magnet. Thus, the iron paper clip is attracted upwards to the permanent magnet. Induction always precedes attraction permanent magnet iron paper clips

TJC IP4 PHYSICS IP4 Physics Topic 15 – Magnetism Page 5 of 10 Student’s Copy 3 Magnetisation LO (c) 3.1 Stroking method of Magnetisation Single Touch Divided touch • Stroke continuously in the same direction. • Polarity of each end of the induced magnet is opposite to the pole which leaves the end. 3.2 Electrical Method of Magnetisation Magnetisation using direct current: 3 ways for increasing magnetic field strength: • increase current (increase the emf of battery/ decrease the resistance of rheostat); • increase the no. of turns (per unit length) of coil/solenoid • Use a soft-iron core (which concentrates magnetic field lines) • A direct current passes thr