Previously From Page 220 Forces appear whenever two objects interact. From Page 225 Unbalanced forces cause the motion of a body to change. Unit 6 Magnets How do magnets work? Magnets are interesting things they can attract or repel each other without touching. It is very difficult to explain how magnets work, but you can learn a lot about what magnets can do without knowing a lot of advanced scientific ideas. We use magnets every day without really thinking about it. They are great for holding letters and notes to metal fridge doors but did you know there is a magnet inside the fridge that helps keep the door closed tightly? In this Unit, you will learn: how we represent magnetic fields; how electromagnets work. about magnetic and non-magnetic materials; how magnets attract and repel each other; Key words attracts compass core electromagnet magnet magnetic field magnetic field lines magnetic material magnetic pole north pole permanent magnet repels solenoid south pole 246 MART CIECE
Magnets Which materials are magnetic? A bar magnet is an example of a permanent magnet. It will remain magnetic for a long time. A bar magnet can attract pieces of iron and steel. Any material that is attracted by a magnet is called a magnetic material. The table below gives some examples. How could you show that a magnet s attraction can work through a non-magnetic material for example, through paper? Magnetic materials iron steel (most types) nickel ferrite (used for fridge magnets) on-magnetic materials copper stainless steel aluminium paper What are the forces between two magnets? A bar magnet has two ends; one is its north pole, the other is its south pole. If two magnets are placed so that their ends are close together as shown, they may attract or repel. A south pole attracts a north pole; a north pole attracts a south pole. A north pole repels another north pole; a south pole repels another south pole. We summarise this as Like poles repel, unlike poles attract. Like poles means two of the same type. Unlike poles means two of the opposite type. The magnetic force between two poles is stronger if they are closer together. Will two south poles attract or repel each other? Draw a diagram to show this. PHYIC 247
Unit 6 What are the poles of a magnet? If a bar magnet is hung so that it is free to rotate, it will turn until one end points towards the north this end is called the magnet s north pole. The other end points south, of course, and is called the south pole. This is how a compass works. The magnet turns because it is in the Earth s magnetic field. There is a lot of iron and nickel inside the Earth s core and they make it behave like a giant magnet, with one magnetic pole close to each geographical pole. A magnet s north pole is attracted towards the north. What kind of magnetic pole (north or south) must there be in the north? north omething scientists don t know: does a north pole always come with a south pole? Every magnet we have discovered, no matter how large or how small, appears to have both poles together. ome scientists think that they may be able to discover particles that are just north poles or south poles. But we haven t found any of these magnetic monopoles yet. If there was a magnetic monopole that could move freely near the Earth, what do you think would happen to it? 248 MART CIECE
Magnets What is a magnetic field? A magnet can attract magnetic materials near to it, without touching them. We say that there is a magnetic field around the magnet. We can show up a magnet s field using iron filings. We draw magnetic field lines to represent the pattern. The lines come out of the north pole and go into the south pole. pole. A compass will point in this direction. Where the field lines are closer together, the field is stronger. The arrows on the lines show the direction of the field, from a north pole to a south The magnetic field of a bar magnet is strongest at its poles true or false? Explain how you can tell. What is the field surrounding two magnets? The magnetic field surrounding two magnets depends on whether they are attracting or repelling. Draw the magnetic field pattern for two south poles, repelling each other. PHYIC 249
Unit 6 How are magnets and magnetic materials used? Magnets and magnetic materials are used in: compasses (for showing direction); fridge magnets; the stripe on a credit card that contains stored information; magnetic memories such as computer hard disk drives; loudspeakers, headphones and some microphones; electric motors and generators. The disc in a computer hard drive is coated on both sides with a magnetic material. This is the drive s memory. A pattern of tiny magnetised areas is used to store digital information. List five places where you might find magnets in your home. What is an electromagnet? An electromagnet is another way of making a magnetic field. A coil of wire is connected to a battery or a power supply. A current flows in the coil. A magnetic field appears all round the coil. One end of the coil is a north pole, the other end is a south pole. The magnetic field of the electromagnet is the same shape as the field of a bar magnet. If the current is reversed, so that it passes through the coil in the opposite direction, the field will be reversed the north pole will be at the other end. The coil of wire is sometimes called a solenoid. It is usually made of copper wire it doesn t have to be a magnetic material. When the current is switched off, the coil stops being magnetic. The field disappears. + battery coil current 250 MART CIECE
Magnets Electric current and magnetism are closely related. The connection between electricity and magnetism was first discovered by Hans Christian Oersted in 1820. He noticed that, when he switched on an electric circuit, the needle of his compass moved. Oersted made his observation during a public lecture. The audience watched as he repeated it several times to make sure that what he saw was really true. Plan an investigation to test whether three different electrical items are also magnetic when they are switched on and when they are switched off. How can an electromagnet be made stronger? An electromagnet can be made stronger in three ways: 1. Use a coil with more turns of wire. 2. Put a bigger current into the coil. 3. Add an iron core to the coil. Each of the electromagnets shown has the same current. Put them in order, from strongest to weakest. PHYIC 251
Unit 6 How are electromagnets used? Electromagnets have many uses because they can be turned on and off easily, and because the direction of the magnetic field can be reversed. Electric bell an electromagnet moves a striker backwards and forwards very uickly, producing the ringing sound. Electric motor the electric current passing through the coil causes a rotor or axle to rotate and generate turning forces or moments (see page 229). Loudspeakers and headphones the electric current passing through the coil causes a magnetic piston to vibrate and produce sounds (see page 270). Brain scanner we can map the electric currents inside our brains using a person-sized electromagnet, to show whether our brains are healthy. Particle accelerators we can bend beams of electrically charged particles such as electrons and protons, and speed them up or slow them down, using huge electromagnets. A scrapyard crane uses an electromagnet to lift steel. The steel falls off when the current is switched off. You need to remember that: Like poles repel; unlike poles attract. ome materials, such as iron, nickel and most types of steel, are magnetic. A magnet creates a magnetic field around itself. Magnetic field lines come out of the magnet s north pole and go into its south pole. We can show the direction of magnetic field lines using a compass. The Earth is a large magnet because its core contains iron and nickel. An electromagnet is a coil with an electric current flowing through it. The greater the current and the more turns on the coil, the stronger the electromagnet. An iron core makes an electromagnet stronger. ext time Unit 7 In electricity, like charges repel; opposite charges attract (page 254). Unit 8 An electric current in a circuit causes electromagnets to work or motors to turn on (page 261). 252 MART CIECE