4. ELECTRICITY AND MAGNETS

Transcription

1 4. ELECTRICITY AND MAGNETS 4.1 INTRODUCING ELECTRICITY AND MAGNETS Today almost everyone uses electricity. Electricity gives us light when we switch on a torch (flashlight), and sound when we switch on a radio or a cassette/cd player. The electricity usually comes from batteries. Batteries are also used in pocket calculators, to start cars and trucks, and for many other purposes. Modern homes are supplied with electricity through wires. This is called mains electricity. Mains electricity is much more powerful than electricity from a battery. It gives us the power for bright lights and television sets. It also runs household machines like irons, refrigerators and electric cookers, and office machines like air conditioners and computers. Mains electricity is safe if we are careful and use it properly. However, it is so powerful that it can be dangerous too! If we touch a live wire, we will get a painful electric shock. An electric shock can even kill, so we must be very careful when we use mains electricity. In this chapter, we will study electricity using batteries only. You will learn about conductors that allow electricity to pass through them, and insulators that stop electricity. You will also learn about light bulbs and switches, how a torch works and how to draw circuit diagrams. Working with batteries is completely safe, but you must NEVER try any experiments with mains electricity. That would be very dangerous! The second part of the chapter is about magnets. Later in your science course, you will learn how electricity can be used to make magnets. Some common kinds of magnets are shown on the right below. They are usually made of iron or steel and they are quite strange. They pull certain small objects, like needles or paper clips, towards themselves and cling on to them. If you have never played with a magnet, try to find one and see how strangely it behaves! Magnets are used in many electrical machines including radios and telephones. If there is an old radio that has been thrown out, you can find a magnet at the back of the loudspeaker. The loudspeaker is the part the sound comes out of. If you take the back off the radio, you can easily find it, because a needle or the metal part of a screwdriver will cling to it. You can turn the needle or the screwdriver into a magnet if you wish. Stroke the needle or the screwdriver against the magnet in the radio many times, always in the same direction. In a few minutes you will have a magnet of your own to do experiments with. 1. List at least seven different things that use batteries. 2. What is a magnet? What part of a radio uses a magnet? List at least nine different things that use mains electricity. (Hint: Look at the picture on the left above). What is the big difference between the electricity from a battery and mains electricity?

2 4. 2 THE ELECTRIC CIRCUIT Can you make a torch bulb light? Some bulbs need two batteries to light them properly, so you can use two batteries as well as the bulb. You are also allowed two pieces of wire. Try it yourself if possible. The illustration below shows a good answer. Before you continue, study it closely, and check that it works. SAFETY NOTE: NEVER do experiments with mains electricity. Use batteries only. MAINS ELECTRICITY CAN KILL The bulb. Notice that one wire must touch the metal point at the bottom of the bulb. The other wire must touch the metal side below the glass bulb. You will learn more about bulbs in Section 4.5. The batteries. Notice that the two batteries must point the same way. The top of one battery must touch the bottom of the other. One wire must touch the top of the two batteries, and the other wire must touch the bottom. You will learn more about batteries in Section 4.6. The wires. One wire carries the electricity from the top of one battery to the bulb. The other wire carries the electricity back from the bulb to the bottom of the other battery. That second wire is a surprise to some people! The electricity will only flow, and light the bulb, if it can get back to the battery again. We say that electricity only flows when there is a complete, unbroken circuit. The circuit is the path that the electricity flows along. A circuit always starts from one end of a battery and returns to the opposite end. The electricity flowing around a circuit is called an electric current. If there is any break in the circuit, the electric current will not flow and the bulb will not light. Try it and see! 1. Where must the two wires touch the bulb? Does it matter which wire touches which place? (If you are not sure, try it!) 2. (i) How must the batteries be placed, and (ii) where must the wires touch the batteries? 3. What is an electric circuit? What kind of circuit is needed before an electric current can flow? 4. The illustrations opposite show how some students joined up their batteries and wires to their bulbs. (i) Which of the bulbs will light? (ii) Why will each of the other bulbs not light? 39

3 4.3 CONDUCTORS AND INSULATORS Conductors allow electricity to flow through them but insulators do not. But how do we know if something is a conductor or an insulator? The illustration below shows a very simple way to find out. Testing conductors and insulators. First make a simple circuit as shown above. The circuit is not complete, so of course the bulb does not light! To check that the circuit is working properly, touch the ends of the two wires together. That completes the circuit and the bulb should light. To find out if something is a conductor, touch it with the ends of the two wires at the same time. If the object conducts electricity it will complete the circuit. The electric current will flow through it and the bulb will light. If the object is an insulator, the electric current cannot flow through it and the bulb will not light. Test as many objects as you can. Classify each object as a conductor or an insulator. The table opposite lists some common objects that we might test. It also shows the materials that each object is made of, and the results of the test. Study the results carefully. What do you notice about the things that are classified as conductors? Nearly all of them are made of metals such as copper, iron and aluminium. The only exception is the pencil lead. Pencil leads are not made of lead! They are made of a soft material called graphite, which is not a metal. After testing all known materials, scientists can generalise and say that all metals and graphite are conductors. Most other materials are insulators. Object What is it made from? Conductor or insulator? wire copper conductor covering on wire plastic insulator coin mixed metals conductor pen or comb plastic insulator pencil (outside) wood insulator pencil lead graphite conductor nail or needle iron or steel conductor window glass insulator saucepan aluminium conductor box or carton cardboard insulator 1. Which of the following will make the bulb light if we put them in the circuit above? a bush knife, a book, a food tin, a rock, iron from a roof, a cooking pot, a bucket, a gold ring. 2. What is an insulator? What are insulators made of? 3. Why is the covering on an electric wire made of plastic? 40

4 4.4 MAKING GOOD CONNECTIONS The conductors that carry the electric current in most circuits are wires or strips of metal. A connection is where two conductors are joined together. If anything electrical goes wrong, the most common reason is a bad connection! In a bad connection, the two conductors are not well joined, so the electricity cannot flow easily from one to the other. This section is about making sure all your connections are good ones. Connecting wires. Most wires are covered with an insulator such as plastic. The insulator makes sure that electricity flows only where we want it to go. It also protects us from electric shocks. When we want to connect a wire to another conductor, we must first strip a short piece of the plastic from the end. Use small scissors, or a razor blade, to cut around the plastic about 1 cm from the end. Be careful not to cut the wire inside, and not to cut yourself! When you have cut carefully all round, pull off the plastic cover and expose the bare wire inside. If there are many small wires, twist them gently together so they do not spread out. Keep the end of the wire clean. Dirty wires make bad connections. The bare end of the wire is often joined to a terminal. Some different kinds of terminals are shown in the box. There is usually a screw that has to be loosened with a screwdriver first. In some terminals, the wire is pushed inside a hole in the metal. If the bare wire is too long, cut it shorter. When the screw is tightened, it clamps the wire firmly onto the metal to make a good connection. Make sure the screw clamps onto the wire, not the plastic cover! Sometimes, the wire has to be wrapped round the screw. You should wrap the wire clockwise then twist it back around itself to form a small loop around the screw. When the screw is tightened, the head of the screw clamps down on the wire loop and makes a good connection. Connecting batteries. Batteries usually fit into a special battery clip. Often a metal spring pushes against the bottom of the battery. This makes a good connection. The spring also presses the top of the battery against a metal strip, or onto the bottom of the next battery, so it makes a good connection there too. The picture on the left shows a battery clip in a portable radio. The sketch on the right shows how students at one school made their own battery clip for doing experiments with electricity. They used only scrap materials. Battery clip in radio (with 2 out of 6 cells fitted) Battery clip made by students 40

5 4.5 LIGHT BULBS How does the bulb in a torch give light? Try to find a torch bulb and examine it closely. If possible, use a hand lens. The diagram shows the structure of a bulb. Remember only the wires and the other metal parts in the bulb are conductors. Glass is an insulator. When the bulb is connected into a circuit, the electric current flows up one of the wires, through the filament, and out through the other wire. The filament is a very thin wire made of a metal called tungsten. It is usually made into a tiny coil. The electric current flowing through the filament makes it so hot that it glows. If you touch a glowing bulb, you can feel the heat, even through the glass. The tungsten filament is very thin and fragile. The glass bulb protects it from getting broken. The bulb is filled with a gas called argon that prevents the filament from burning away. (You will learn more about burning, and about argon too, in Chapter 6). The ordinary light bulbs used in homes and other modern buildings work in exactly the same way. They are full of argon gas too. The only important difference is that they are bigger than a torch bulb and use a much more powerful electric current. They also get much hotter than a torch bulb, so you must not try to touch one when it is lit. An experiment with a torch bulb 1. Try very carefully to remove the glass from a torch bulb without breaking the filament. (Wrap a soft cloth around the bulb and then squeeze it very gently in the hinges of a door. Be careful you do not squeeze your fingers too! Take care with the broken glass and dispose of it safely). 2. Do not touch the filament, but observe it closely when you connect the bulb to two batteries. 3. Now that the filament is not protected by the argon in the bulb, it burns away in a puff of white smoke! After a bulb has been used for some time, the filament may become worn out and break. When that happens, the circuit is broken, the electric current cannot flow, and the bulb does not light. It has to be thrown away. If you look closely at a bulb that has stopped working, you can see the break in the filament. You may need to use a hand lens. Bulb holders. To make sure of good connections, special holders are made for different kinds of bulbs. The diagram shows one kind of holder used for torch bulbs in school laboratories. If you can find one, examine it closely. The bulb screws down until the contact at the bottom of the bulb meets a small metal strip. This metal strip leads to a screw terminal at one side of the holder. The metal sleeve that the bulb screws into is connected to the screw terminal at the other side of the holder. Wires can be connected to the screw terminals. 1. Draw and label a diagram of a torch bulb from memory. 2. What is the filament made of, and what does it do? 3. In a bulb holder the only metal parts are the sleeve that the bulb screws into, a small strip at the bottom and the two screw terminals. Why do you think all the rest is made of plastic? 41

6 4.6 CELLS AND BATTERIES A single battery on its own is often called a cell. The most common cells are the kind we use in torches, radios and cassette or CD players. They are called Leclanché cells after the Frenchman who invented them. There are many different brands and sizes but they are all the same inside. What are cells made of? The diagram shows a Leclanché cell that has been cut in half from top to bottom. It has three important parts. There is a positive terminal (which is marked with the plus sign, +), a negative terminal (which is marked with the minus sign, -) and an electrolyte (which is the mixture of chemicals inside the cell). Electricity leaves the cell from the + terminal, flows around a circuit to do its work, and then returns to the - terminal. In a Leclanché cell, the positive terminal is a rod made of graphite. It sticks out at the top of the cell. The negative terminal is the container. It is made of a metal called zinc. This terminal is normally exposed at the base of the cell. The electrolyte is a paste made of two chemicals called ammonium chloride and manganese dioxide. Battery clips have + and - signs to show which way round to insert each cell. positive terminal plastic seal zinc case graphite rod electrolyte negative terminal Voltage and power. Any Leclanché cell has 1.5 volts written on it somewhere. This tells us about the power that pushes the electric current through the circuit. Big fat cells, and small slim ones, all produce 1.5 volts, so they all have the same power. But of course a big cell gives power for much longer than a small one. That is also the main difference between an ordinary cell, and a cell marked heavy duty or high power. The heavy duty or high power cell can work at full power for a longer time. A job like turning the reel on a cassette player needs more power than we can get from 1.5 volts. When we connect cells together (top to bottom of course) the voltages add up. Two cells together give 3 volts. Cassette/CD players usually need 6 volts (a battery of four cells) or even 9 volts (six cells). This gives them enough power to spin the cassette or disc at a steady speed to produce a good sound! Other cells and batteries. Here are some notes about a few different kinds of cells and batteries: High voltage batteries give 4.5 or 9 or 12 volts. Each of these batteries has several 1.5 volts Leclanché cells inside it. In the picture, a red 9V battery has been opened so you can see some of the 6 small cells it contains. Alkaline cells use a different electrolyte. They give 1.5 to 1.6 volts and can work at full power longer than other cells. Lithium cells use different electrolytes too and use lithium for the negative terminal. They give 1.5 volts (some kinds give more) and can work at full power longer than other cells. Button cells are the very tiny, flat cells used in watches, calculators and cameras. They give about 1.25 to 1.55 volts. Car or truck batteries give 12 volts. They have lead terminals and a dangerous liquid electrolyte called sulphuric acid. These large, heavy batteries contain six, 2 volts cells. They are good for heavy work and are used to start cars and trucks. They can be recharged when they run down. 1. In an ordinary (Leclanché) cell, (i) what are the terminals made of, (ii) where are the + and - terminals, (iii) what is the voltage? 2. How can we make the voltages of two cells add together? 3. How many ordinary cells are there in a 4.5 volt battery? How many ordinary cells fill a battery clip for a 12 volt radio? 42 Cells and batteries In science, words sometimes have special meanings. The proper scientific name for a single 'battery' is a cell. A single cell usually gives between 1 and 2 volts. The exact voltage depends on what the terminals and the electrolyte are made of. The word battery is used for a group of cells working together. The 'battery' in a truck really is a battery - there are six separate cells inside it.

7 4. 7 SWITCHES AND CIRCUIT DIAGRAMS Switches. One of the reasons that electricity is so useful, is that it is very easy to switch on and off. Look at the circuit below. Can you see how the switch works? In the circuit above, we simply push down the metal strip to complete the circuit and switch on the bulb. When we let the strip go, it springs back up again. The circuit is broken and the bulb goes off. All switches work in basically the same way. If you have the chance, carefully examine a switch to check how it works. (WARNING: Do not take the cover off a switch that is connected to mains electricity - mains electricity can kill). How a switch works All switches work by completing and breaking an electric circuit. The base of the switch is always an insulator. No electricity can pass through the base. A conductor always leads electricity in to a switch. There is another conductor to lead the electricity out. When we switch on, a strip of metal joins the in conductor to the out conductor. The metal strip completes the circuit and the electric current flows. When we switch off, the strip of metal is pulled back again. The circuit is broken and the electric current stops. Domestic light switches Circuit diagrams. A circuit diagram is a simple way of showing a circuit. The circuit diagram for our switch circuit is shown below. As you can see, it looks quite similar to the actual circuit. Lines are used to represent the wires, and special symbols are used for cells, bulbs and switches. Two other symbols are also included in the table below. You will learn about resistors later in your science course. Variable resistors are used to control the volume in radios and cassette players. Look at the circuit diagrams below, then answer the questions to see if you understand them properly. 1. First a question about switches! Explain in a few words why the base of a switch must be an insulator. 3. List all the things you would need to make each of the two circuits (A and B) shown above. 2. Draw circuit diagrams for the two circuits illustrated at the top of Modules 4.2 and With standard cells in circuits A and B, (i) what voltage is used, and (ii) what is connected to the + side of the battery? 43

8 4. 8 TORCHES In different places a torch may be called a torchlight or a flashlight. You can use whatever name you like. Most torches have a plastic case and use two cells, a bulb, a switch and a metal strip as a conductor. The diagram on the left shows what you might see if you could look inside a torch. On the right is the circuit diagram for the same torch. cell cell Let us start by looking at the two cells. The metal spring connects the negative terminal of one cell to a metal strip leading to the switch. The spring also pushes the two cells together, so they make good contact with each another. The positive terminal of the top cell, is pressed firmly against the bottom of the bulb. Now look at the bulb. The top cell contacts the terminal at the bottom of the bulb. The metal terminal on the side of the bulb, screws into a circular metal plate. In some torches the bulb does not screw in, instead it is just pushed against the circular metal plate. To complete the circuit, the circular metal plate touching the side of bulb must be connected to the negative terminal of the battery. The switch on the side of the torch is joined to a strip of metal inside the plastic case. One end of this metal strip is connected to the spring that presses on the negative terminal of the battery. The other end almost touches the circular metal plate holding the bulb. When the switch is pushed forwards, the metal strip contacts the circular metal plate. This completes the circuit. Now an electric current can flow. The path of the current is traced below. With your finger, follow this path on the diagram of the torch and then on the circuit diagram. + terminal bulb circular metal metal strip metal - terminal of battery filament bulb holder (switch) spring of battery If you can, try to take a torch apart carefully. Every torch is a little bit different, but you should be able to see how it works. Try to follow the path of the electric current, all the way from the positive terminal of one cell and back to the negative terminal of the other. Notice especially how the switch is used to make the circuit complete, and to break it again. You may notice that the bulb sits in the middle of a tiny, curved, silvery dish. This is called a reflector. It reflects the light from the bulb into a strong beam. You will learn more about light and reflection in Chapter Think about the connections in a torch. What connects to each of the following? (i) the - terminal of the battery, (ii) the + terminal of the battery, (iii) the metal on the side of the bulb, (iv) the two ends of the metal strip in the switch. 2. Write down the path of the electric current through the bulb. Start with the terminal at the bottom of the bulb. 3. What is the purpose of the curved reflector? 45

9 4.9 MAGNETS AND MAGNETIC MATERIALS Magnets are usually made of iron or steel. They attract certain other objects towards themselves and try to cling onto them. The illustration shows a bar magnet and a horse-shoe magnet clinging onto some pins. Making a magnet Any piece of steel can be made into a magnet by stroking it with a strong magnet. The diagram shows a steel rod being magnetised using a bar magnet. Instead of a steel rod, you could magnetise any other object made of steel such as a needle, a knife, a pair of scissors, a screwdriver or a teaspoon. Always use the same part of the magnet to stroke with, and always stroke in the same direction. Between strokes, keep the magnet well away from the object being magnetised. If you do not have a strong bar magnet, take the loud speaker out of an old radio and use the magnet Magnetic materials. Magnetic materials at the back of it. are materials that a magnet attracts and tries to cling onto. You already know that magnets attract objects such as pins or nails, which are made of steel. So steel is a magnetic material. Try testing as many objects as you can with a strong magnet. Find out which objects are attracted. The table below shows the results obtained by the pupils in one class. Object Made from Magnetic? knife/scissors steel magnetic pencil wood/graphite not magnetic pen or comb plastic not magnetic needle/pin steel magnetic window glass not magnetic coin metal mixture not magnetic iron from roof iron magnetic book paper not magnetic cooking pot Iron magnetic saucepan aluminium not magnetic copper wire copper not magnetic case of cell zinc not magnetic Study the table carefully. As you can see, only a few things are attracted by a magnet. Notice that metals like aluminium, zinc and copper are not magnetic. All the magnetic objects in the table are made of iron or steel. Two uncommon metals, called nickel and cobalt, are also magnetic. So the only magnetic materials are iron, nickel and cobalt. Steel is also magnetic, but steel is just a solid solution of other materials in iron. 1. What are magnets usually made of? 2. Magnets attract magnetic materials. What does the word attract mean? 3. In an exam, a pupil said that conductors are magnetic and insulators are not. Explain what his mistake is. 46

10 4.10 MAGNETIC POLES Magnets have another very strange and unexpected property. If a magnet is free to swing, one end always tries to point north! Two different ways to see this are illustrated below. We can hang up a bar magnet, using a strip of paper and a thin thread. Very thin nylon fishing line is good because it has no twist. We must also make sure that there is no iron or steel nearby. You will find that one end of the magnet always swings around until it points north. If we remove the magnet from the paper sling, and put it back pointing in the opposite direction, it slowly swings round until the same end points north again. The end that points north is called the north pole of the magnet. The opposite end is called the south pole. Many bar magnets have the north pole permanently marked in some way - sometimes the letter N is used. To find the north and south poles of a small magnet such as a needle, a good method is to tape the needle to a flat cork and float the cork in a bowl of water. The cork will swing round so that the north pole of the magnet points north and the south pole points south. If you turn it around, it will soon swing back. The simple experiment on the right shows that most of the strength of a bar magnet is at the two poles. There is very little magnetism in the middle. Attraction and repulsion. Here is a simple activity you can try with two magnets. First find and mark the north and south poles of each magnet. Place one magnet on a smooth surface so it is free to move. Bring the north pole of the other magnet towards the north pole of the free magnet. You may be surprised to see that the two north poles push away from each other. We say that they repel each other. The same thing happens if we try to bring What part of a bar magnet is strongest? To find out what part of a bar magnet is strongest you will need a collection of small objects made of iron or steel. You could use pins, or paper clips, all the same size. Find out how many pins or paper clips you can hang in a string from one end of the magnet. Now find out how many you can hang from the other end, and how many from the middle. You will find that the two poles at the ends of the magnet are both equally strong. But the middle of the magnet is much weaker. two south poles together. But if we bring a north and south pole together, they pull towards each other. We say they attract each other. We can generalise and state that as a principle or law: Like magnetic poles repel each other, but unlike poles attract each other. repulsion repulsion attraction attraction 1. When we hang up a magnet to find which way it points: (i) Why must there be no iron or steel nearby? (ii) Why must the thread have no twist in it? 2. What is the meaning of the word repel? 3. You have a metal bar. (i) How would you find out if your bar is a magnet or not? (ii) If it is a magnet, how would you find which end is the N pole? (iii) Now you have two metal bars. You know that one is a magnet and the north pole is marked. What is the quickest way find out about the other bar? 47

11 4.11 THE MAGNETIC COMPASS As you have already learnt, when a magnet is free to swing, one end always points towards the north. This property of magnets is used in the magnetic compass. A simple magnetic compass has a magnetised pointer that swings on a pivot at the centre of a circular card. The card is marked with the directions North, East, South and West. These are called the cardinal points of the compass. The compass may also be marked with degrees from 0 o to 360 o. The degrees corresponding to each direction are shown in the table below. To use the compass, the North point on the card must always be lined up with the pointer. Magnetic variation A magnetic compass does not point exactly North. The difference between true north and the magnetic north on a compass is only a few degrees. This angle is called is called the magnetic variation. Magnetic variation varies slightly from place to place. It also changes slowly from year to year. In a ship's compass, magnetic needles are fixed to the underside of the compass card. The card rests on a pivot at the centre, and floats in a liquid in a sealed container with a glass cover. The liquid keeps the compass card level and steady, even when the ship is thrown about in a rough sea. There is a line on the glass cover called the lubber line. The lubber line points in the same direction as the ship. The reading of the compass card against the lubber line shows the direction in which the ship is travelling. Direction North East South West Degrees 0 o 90 o 180 o 270 o A pocket compass (The compass must be rotated so the N on the compass card is under the compass needle that points north) Magnetic compasses A ship s compass are used by sailors, explorers and map makers. The compass tells them in what direction they are travelling. It can also tell them the direction, or bearing, of important landmarks such headlands or high mountains. Compasses are still used in this way, but now anyone with the right kind of radio can find exactly where they are with satellite navigation systems. Natural magnets Magnets were used in China more than 2000 years ago and in North Africa soon after that. The magnets they used were lodestones, which occur naturally. We now know that lodestones are made of a substance called magnetic iron oxide, or magnetite. The Chinese and the North Africans knew that, when a lodestone was free to swing, one end always pointed to the north. They used lodestones to make simple magnetic compasses to help them steer their ships far out of sight of land. 1. The person in the sketch above is taking a bearing of a church on a hill. (i) What does taking a bearing mean? (ii) Why will the bearing have to be corrected for variation? 2. Give each of these directions in degrees: (i) Northeast, (ii) South, (iii) South-west, (iv) North-west. 3. Look at the ship's compass illustrated above. In what direction was the ship travelling? 4. The case of a magnetic compass is made of metals like copper, zinc or aluminium. Why are iron and steel not used? 48

12 4. 12 MAGNETIC FIELDS The force of a magnet can be felt some distance away from the magnet. When we bring a magnet close to an object made of iron or steel, we can feel the magnet attracting the object from a distance. Similarly, when two magnets attract or repel each other, the forces can be felt long before they touch. The area around a magnet, in which we can feel the force of the magnet, is called a magnetic field. The magnetic field of a bar magnet is shown in the diagram. The lines in the diagram are called lines of force. We can trace lines of force, using a small compass called a plotting compass. The north pole of the compass points, along the line of force, towards the south pole of the magnet. Remember, unlike poles attract. Lines of force never cross one another. lines of force lines of force plotting compasses Plotting magnetic fields with iron filings. An easy way to show (plot) a magnetic field is with iron filings. Iron filings are the tiny grains of iron obtained when a piece of iron is scraped with a file. These grains are attracted by a magnet. The black sand from volcanic islands often contains iron and can be used instead of iron filings. To plot a magnetic field, first cover the magnet with a sheet of white paper so the iron filings will not stick to the magnet. Then gently sprinkle a few iron filings on top of the paper. The iron filings are attracted by the magnetic field and settle along the lines of force. You may need to tap the paper gently to help the filings to settle. The diagrams show the patterns of the iron filings for two pairs of bar magnets. One pair are attracting each other; the other pair are repelling. Notice how the lines of force show the unlike poles pulling together (attraction) and the like poles pushing apart (repulsion). If you make a good magnetic field with iron filings and want to keep it, you can try "fixing" it with ladies' hair spray! The magnetic field of the earth. The earth itself has a strong magnetic field, as if there was a huge bar magnet through the centre of the earth. The south pole of this huge magnet is somewhere near the geographical north pole. That is why the north pole of a compass needle points north - it is attracted to the huge magnetic south pole there! magnetic field magnetic north true north magnetic field Of course there is not really a huge magnet through the centre of the earth. The earth's magnetic field is caused by streams of electrical particles from the sun as they shoot past the earth. Later in your science course you will learn how magnets can be made using electricity - and how electricity can be made using magnets too! 1. What is a magnetic field and how can you 'plot' it? 2. What actually causes the magnetic field of the earth? true south magnetic south 3. A horse-shoe magnet is a bar magnet that is bent into a curve. Predict what its magnetic field would be like and make a sketch of your idea. (Check with iron filings if you can). 49