CIS int’l School of Tunis IGSCE-Physics-Chapter 16 Chapter 16: Magnetism What you will need to know in this chapter: • • • • The difference between hard and soft magnetic materials, and nonmagnetic materials. How to describe magnetic forces between magnets. How to draw magnetic fields. The difference between permanent magnets and electromagnets. 1- Magnets A magnet is a rock or a piece of metal that can pull certain kinds of metals towards itself. The force of magnets is called magnetism. There are three main types of magnets: • Permanent magnets once they're magnetized these objects do not easily lose their magnetism. • Temporary magnets in this type when the magnetic field is removed the object gradually loses its magnetism. • Electromagnets are a type of magnet in which the magnetic field is produced when an electric current is applied to it. Magnetism works over a distance this means that a magnet does not have to be touching an object to pull it. When two magnets are brought close together, there is a force between them. The north pole (N pole) of one will attract the south pole (S pole) of the other. Two north poles will repel each other, and two south poles will repel each other. This can be summarized as: • like poles repel • unlike poles attract. 2- Magnetic materials Magnetic materials are attracted by a magnet and can be magnetized. Though they can be magnetized, not all pieces of magnetic material are magnets. They first need to be magnetized. 1 CIS int’l School of Tunis IGSCE-Physics-Chapter 16 In contrast, non-magnetic materials are not attracted by a magnet and cannot be magnetized. Examples include plastic and rubber. A compass needle is a permanent magnet. Like many bar magnets, it is made of hard steel. Magnetic materials may be classified as hard (permanent) or soft (temporary). • • Hard magnets: retains magnetism well, but difficult to magnetize in the first place. Soft magnets: easy to magnetize, but loses its magnetism fast. 3- Induced magnetism Permanent magnets are made of hard magnetic materials. A permanent magnet can attract or repel another permanent magnet. It can also attract other unmagnetized magnetic materials. For example, a bar magnet can attract steel pins or paper clips, and a fridge magnet can stick to the steel door of the fridge. Steel pins are made of a magnetic material. When the north pole of a permanent magnet is brought close to a pin, the pin is attracted. The attraction tells us that the end of the pin nearest the magnetic pole must be a magnetic south pole. This is known as induced magnetism. The two objects will have opposite polarities. In this example the pin will have a south pole induced in the end nearest the north pole of a bar magnet. When the permanent magnet is removed, the pin will return to its unmagnetized state. 4- Magnetic fields A magnet affects any piece of magnetic material that is nearby. We say that there is a magnetic field around the magnet. You have probably done experiments with iron filings or small compasses to illustrate the magnetic field of a magnet. The magnetic field of a single bar magnet using magnetic field lines. 2 CIS int’l School of Tunis IGSCE-Physics-Chapter 16 The pattern tells us two things about the field: • Direction: the direction of a magnetic field line at any point is the direction of the force on the north pole of a magnet at that point. We use a convention that says that field lines come out of north poles and go into south poles. • Strength: lines that are close together indicate a strong field. We can also draw the field lines for two or more magnets. 5- Electromagnets A typical electromagnet is made from a coil of copper wire. A coil like this is sometimes called a solenoid. When a current flows through the wire, there is a magnetic field around the coil. The coil does not have to be made from a magnetic material. The point is that it is the electric current that produces the magnetic field. You can see that the magnetic field around a solenoid is similar to that around a bar magnet. One end of the coil is a north pole, and the other end is a south pole. he field lines emerge from the lefthand end, so this is the north pole. There is no way to change the strength of a permanent magnet, but there are ways to increase the strength of an electromagnet: • Increase the current flowing through it: the greater the current, the greater the strength of the field. • Increase the number of turns of wire on the coil i.e. packing more turns into the same space to concentrate the field. The magnetic field inside an electromagnet is uniform. This means that the field lines are parallel and the same distance apart. The field around the outside of a solenoid is similar to that around a bar magnet: • One end of the solenoid is the north pole and the other end is the south pole. Field lines emerge from the north pole and go into the south pole. • The field lines are closest together at the poles, showing that this is where the magnetic field is strongest. 3 CIS int’l School of Tunis IGSCE-Physics-Chapter 16 Electromagnets have the great advantage that they can be switched on and off. This is the basis of several applications. For example, the electromagnetic cranes that move large pieces of metal and piles of scrap around in a scrapyard. The current is switched on to start the magnet and pick up the scrap metal. When it has been moved to the correct position, the electromagnet is switched off and the metal is released. Summary 1. Magnets have a north pole and a south pole. 2. Magnetic field lines are drawn with an arrow from north to south, which is the direction of the magnetic force. 3. Like poles repel and unlike poles attract. 4. Magnetic elements include iron, cobalt, and nickel. 5. A permanent magnet can attract unmagnetized magnetic materials by inducing magnetism in them. 6. A hard (soft) magnetic material is difficult (easy) to magnetize and demagnetize. 7. A magnetic field is a region of space around a magnet or electric current in which a magnet will feel a force. 8. A magnetic field pattern around a (bar) magnet can be produced using a plotting compass. A magnetic field line is the line (direction) of force on the north pole of the magnet, which is why a magnet (for example, a compass needle) lines up with it. 9. An electromagnet (or solenoid) is a magnet created when a current is passed through wire, which is usually shaped into a coil. 10. The strength of an electromagnet increases with the number of turns in the coil, the strength of the current and if it has a soft iron core. 11. The closer together magnetic field lines are to each other, the stronger the magnetic field. 12. Magnetic forces are due to interactions between magnetic fields. Practice 1- What is a magnetic field? 2- Magnetic field strength decreases as … 3- Describe the key features of field lines in a magnetic diagram. 4 CIS int’l School of Tunis IGSCE-Physics-Chapter 16 4- Two nickel bars are placed close to the N-pole of a bar magnet. The nickel bars become magnetized. What is the pole induced at P, the pole induced at Q, and the type of magnetic force between P and Q? 5- A wire perpendicular to the page carries an electric current in a direction out of the page. There are four compasses near the wire. Which compass shows the direction of the magnetic field caused by the current? 6- Which of these magnetic field patterns is correct? 7- Karamveer investigated how the strength of an electromagnet varied with current. He passed a current through an electromagnet so that it attracted a small steel plate. For each current he passed through the coil, he suspended more masses from the bottom of the steel plate until the steel was pulled away from the electromagnet. Current (A) 0 2 4 6 8 10 Force (N) 0.00 0.60 1.20 1.90 2.40 3.00 a. Plot a graph of force against current. b. State how the strength of the electromagnet varies with the current passing through the coil. c. State two other ways that the strength of an electromagnet can be increased. d. In terms of forces, state why the steel plate falls from the electromagnet once the suspended masses exceed a certain value. 5
