Magnetism PDF

# The Magnetic Field It is thought that the word 'magnet' comes from the name of the ancient country of Magnesia, which is now part of Turkey. In this region, large numbers of black stones were found which had the power to draw pieces of iron to them. The black stone became known as lodestone or le...

# The Magnetic Field It is thought that the word 'magnet' comes from the name of the ancient country of Magnesia, which is now part of Turkey. In this region, large numbers of black stones were found which had the power to draw pieces of iron to them. The black stone became known as lodestone or leading stone, because of the way it could be used to find directions, and it eventually came to be used in the compass. Today, it is known as the mineral magnetite, and it has been found in many countries. ## The Magnetic Field There is a region all around the magnet in which the pull of the magnetic force from the magnet acts on magnetic materials. This region is called the **magnetic field**. The field around a magnet can be shown by using a piece of card and iron filings. The card is laid over the magnet and the iron filings are sprinkled over the paper. Each iron filing has such a small mass that it can be moved by the magnetic force of the magnet if the paper is gently tapped. The iron filings line up as shown in Figure 18.4. The pattern made by the iron filings is called the **magnetic field pattern**. If you look closely at Figure 18.4, you can see that some of the iron filings close to the magnet form lines that arch over from one end of the magnet to the other. These lines are called **lines of force**. ## Lines of Force and Magnetic Strength We have seen that the distance between the lines of force varies along the length of the magnet. This observation can be used to set up another enquiry to see if there is a link between the distance between the lines of force and the magnetic strength. ## The Magnetic Fields When Two Magnets Meet We have seen how the magnetic force from a magnet forms a magnetic field which can be investigated to show a magnetic field pattern as seen in Figure 18.4. We can now take this knowledge forwards to see what happens to the magnetic fields and their field patterns when two magnets are brought together. ## Looking at How Magnetic Field Patterns Can Change Each magnet has its own magnetic field pattern, but when another magnet is brought close to it, the field patterns of both magnets change. This is due to the way the magnetic fields interact. If two similar poles are brought together, a certain pattern develops between the two magnets, but if two opposite poles are brought together, a different field pattern is produced. ## The Earth's Magnetic Field At the centre of the Earth is the Earth's core. It is made from iron and nickel and is divided into two parts: the **inner core**, made of solid metal, and the **outer core** made of liquid metal. As the Earth spins, the two parts of the core move at different speeds and this is thought to generate the magnetic field around the Earth, making it seem as though the Earth has a large bar magnet inside it. The Earth spins on its axis, which is an imaginary line that runs through the centre of the planet. The ends of the line are called the geographic north and south poles. Their positions on the surface of the Earth are fixed. **Magnetic north** - towards which the free north pole of a magnet points - is not at the same place as the geographic north pole (see Figure 18.9) and it changes position slightly every year. The magnetic north pole originally got its name because it is the place to which the north poles of bar magnets point. In reality it is the Earth's south magnetic pole because it attracts the north poles of magnets. Similarly, the magnetic south pole is really the Earth's north magnetic pole because it attracts the south poles of bar magnets. However, for most purposes the old, incorrect names for the magnetic poles are still used. ## Science In Context ### Early Discoveries About Magnetism Probably the first use of a magnet in direction-finding was in the practice of Feng Shui by the Ancient Chinese civilisation. They used a device called a luopan (see Figure 18.10), which contained a magnet to find a south-pointing direction. They then read off a scale around the magnet to decide on the final position of building foundations. The first evidence of the magnet being studied scientifically for navigation is found in the writings of Chinese scientist Shen Kuo (1031-1095). He performed experiments on magnetic needles, described how magnets pointed north and south, and how other directions (east and west) could be found using a scale around the magnet. The knowledge of using magnetite for direction-finding is believed to have slowly passed to other countries as they traded with one another. Petrus de Peregrinus (also known as Peter the Pilgrim) was a French engineer who lived in the 13th century. He experimented on the way magnets could attract and repel each other and how they could point north and south. He believed that the magnet pointed to the outer sphere of the heavens. Compasses at that time were made by floating a magnetic needle on water, but Peregrinus showed that attaching the needle to a pivot made the compass easier to use. William Gilbert (1544-1603) was an English scientist and doctor to Queen Elizabeth I. He made many experiments magnets and disproved beliefs such as 'garlic destroys magnetism' and 'rubbing a diamond on a piece of iron makes the iron into a magnet'. Gilbert suspended a magnetic needle so that it could move both horizontally and vertically, and discovered that the needle also dipped as it pointed north-south. He extended his investigation by using a model of the Earth made out of a sphere of lodestone (magnetite). He put a compass with a pivot at different places on the surface of his model Earth, and showed that the dip varied with the position of the compass on the sphere, just as it did with compasses at different places on the surface of the Earth. From this investigation, Gilbert described the Earth as behaving as if it contained a huge magnet. In time, from this early work on magnetism, a compass was developed, like the one shown in Figure 18.13, to help people in every country find their way around the world. ## The Link Between Magnetism and Electricity Hans Christian Ørsted (1777-1851) was a Danish physicist who studied electricity. In one of his experiments, he was passing an electric current along a wire from a battery when he noticed the movement of a compass needle which had been left near the wire. This chance observation led to many discoveries about how magnetism and electricity are linked together and it has many modern applications. When an electric current passes through a wire, it generates a magnetic field around the wire. A compass can be placed at different positions on a card around the wire (see Figure 18.14) and the lines of force can be plotted. When the current flows up through the card, the field shown in Figure 18.15a is produced. When the current flows down through the card, the field shown in Figure 18.15b is produced. Lines of force on diagrams of magnetic fields show not only the direction of the field as given by a plotting compass, but also the strength of the field in different places. The lines of force are close together where the field is strong and further apart where the field is weaker. If the wire is made into a coil and connected into a circuit, a magnetic field is produced around the coil as shown in Figure 18.16. ## The Electromagnet An electromagnet is made from a coil of wire surrounding a piece of iron. When a current flows through the coil, magnetism is induced in the iron and the coil and iron form a strong electromagnet. When the current is switched off, the electromagnet completely loses its magnetism straightaway. This device, which can instantly become a magnet and then instantly lose its magnetism, has many uses. For example, a large electromagnet is used in a scrapyard to move the steel bodies of cars (see Figure 18.17). ## How Can You Make and Test an Electromagnet? **Making an Electromagnet** You will need: - a 5 cm iron nail, 3 m of thin, plastic covered copper wire and a ruler. 1. Measure 25 cm from one end of the copper wire, then wind the next 50 cm of wire around the nail. 2. The turns of the wire should form a single layer over the nail - they should be close together and all wound in the same direction. **How to Test an Electromagnet** You will need: - your electromagnet, a switch, three cells, three wires, a clamp and stand. 1. Set up your electromagnet on the clamp and stand so that it is a few centimetres above the bench or table top. 2. Attach one end of a wire to one end of the wire of your electromagnet. 3. Attach the other end of the wire to a cell. 4. Attach the end of another wire to the other end of the wire of your electromagnet. 5. Attach the other end of this wire to one terminal of a switch. 6. Connect the cell and switch with the third wire to complete the circuit. 7. Hold some paper clips under one end of your electromagnet, switch on the circuit and record how many paper clips are attracted to your electromagnet. **Does the number of cells in the circuit affect the power of your electromagnet?** 1. Make a plan to answer the question, and if your teacher approves, try it. 2. What does your investigation show?