Electricity and Magnetism

Transcription

1 Electricity and Magnetism Teachers Guide Introduction Electricity and magnetism are two closely related concepts that are often considered together as electromagnetism. These concepts play an enormous role in everyday life from powering electronics, to wireless communication, directing compass needles, storing data, and even holding molecules together. Individual atoms can be considered to be very small magnets, with a north or south pole. In most metals the atoms line up in an alternating directions so that, overall, the atoms magnetism cancel out; this is why metals like aluminium and tin are not magnetic. In iron, nickel, and cobalt, however, small groups of atoms tend to line up in the same direction creating small magnetised areas called domains. In a normal piece of iron, nickel, or cobalt the domains themselves are arranged randomly (even though although the atoms in each domains are aligned) and so overall the metal is not magnetic. However, if you bring a magnet close to a normal piece of iron, the domains within it will be forced to align themselves and so the piece will become magnetic this explains why objects like paperclips or nails do not usually attract each other, but can be made to do so by a magnet, like in the picture below.

2 When thinking of magnetism we often imagine the classic horseshoe- or bar-magnets, however, any moving charge will generate a magnetic field the movement of electricity through a wire generates a small magnetic field (an effect that is used to create electromagnets). The reverse process is also true, that is, the movement of a wire through a magnetic field will produce electric current this is how power stations generate electricity. In this set of experiments we will explore the properties of magnets and demonstrate their relationship with electricity. Ideas for investigations 1. Magnetism and magnetic fields Every magnet is surrounded by an invisible magnetic field it is this field that influences and attracts objects. In this demonstration we will use iron filings to visualise this field and observe the north and south poles of the magnet. Equipment and consumables: Bar magnet (Available from Jaycar electronics for $1.50 tinyurl.com/nzqerex, or Surplustronics.co.nz for $1.00 tinyurl.com/lzdgfw7) Iron filings (Available from Modern Teaching aids, NZ for $13.69) Wax/white candle/white crayon White card

3 Method: 1. Cover the piece of card with a thin layer of wax (white candle or crayons are suitable). Try to create as smooth a layer as possible. 2. Place a bar magnet underneath the centre of the card. 3. Sprinkle the iron filings over the top of the card. You should see the filings start to arrange themselves along invisible magnetic field lines. 4. Once you are happy with the arrangement you can set the filings in place by heating gently (you can do this with a candle, an oven on low heat, or even with a hairdryer if you are careful). The wax will melt and hold the iron filings in place. Alternatively, transparent plastic cases containing iron filings are available from some educational stores, and can be used as a mess-free method of demonstrating magnetic field lines. The example shown below can be found in Warehouse Stationery stores for $9.95 (found online at tinyurl.com/mqgbyht) Investigating the properties of magnetic fields: Try using different shaped magnets how does the shape of the magnet influence the shape of the magnetic field? You should find that most magnets have two poles, although fridge magnets often have several, usually in a pattern of alternating north and south poles it is this pattern that causes the skipping behaviour when two fridge magnets are slid past east other. Place several magnets under the card how do the poles of each magnet interact with each other? How does the arrangement of the magnets affect the interactions between the poles? Try placing two bar magnets end-to-end so that their north and south ends are in contact (they should attract). Try holding two north or two south ends with a small gap between them (they should repel you may need to hold them in place with tape).

4 2. Making a compass In the same way that magnets have a magnetic field, so too does the Earth. In fact, the poles of a magnet are labelled according to their attraction to the Earth s North and South Poles, that is a magnet s north pole is attracted to the Earth s North Pole, while the opposite is true for its south pole. In this experiment we will make our own compass by magnetising a needle and demonstrate that it aligns with the Earth s magnetic field in the same way as a standard compass. Equipment and consumables: Bar magnet (Available from Jaycar electronics for $1.50 tinyurl.com/nzqerex) Compass Large sewing needle A bottle cap Bowl Water Method: 1. Choose one end of a bar magnet (north or south) and use only this end to drag along the sewing needle. Drag the magnet in one direction only (from eye to point), making sure that on the reverse stroke you leave at least 5 cm space between the magnet and the needle to avoid cancelling the magnetisation (see the diagram below). 2. Repeat this process, dragging the magnet in one direction, for thirty seconds. 3. Rest the needle across the bottle cap (see diagram below) and float in a bowl of water. You should find that the needle will rotate to point in one direction,

5 regardless of its original orientation use your compass to confirm that that your needle aligns itself in a North/South direction. 4. If your needle does not align itself, repeat steps 1 and 2 for a longer period. The length of time depends on the strength of your bar magnet and the size of the needle (given enough time, it is possible to magnetise a large nail). 3. Making an electromagnet Any moving charge will generate a magnetic field. This means that the flow of electric current (the movement of negatively charged electrons) through a wire will create a magnetic field similar to that seen for the bar magnet in section 1, above this is the basis for electromagnets. The magnetic field generated by a single wire is quite small, but if we tightly coiling the wire we can concentrate this field and produce a much stronger magnet. In this experiment you will create an electromagnet and investigate methods of improving its strength. Equipment and consumables: Paperclips Scissors/knife Insulated copper wire, approximately 2 metres (0.4 mm enamel wire works well and is available from most electronics stores. $8.90/roll at Jaycar electronics tinyurl.com/odld2b6, or $8.50/roll Surplustronics.co.nz tinyurl.com/o4v8f53) Iron nail AA battery

6 Method: 1. Take the copper wire and coil it tightly around the iron nail. Leave about 5 cm of wire free at each end of the coil. 2. Remove 1 cm of insulation from each end of the wire. For wire with a plastic coating, this may be achieved using wire-strippers, scissors or a knife. For laminated wire you can use sandpaper, or a cigarette lighter to burn off the coating (wipe away the soot afterwards). 3. To activate the magnet, attach one end of the copper coil to the positive terminal of the battery and the other end of the wire to the negative terminal, as seen in the diagram below (you may find it convenient to tape down one end). 4. You can measure the strength of your electromagnet by counting the number of paperclips it can hold at once. Investigating the properties of electromagnets: Remove the nail from the centre of the coil does the electromagnet still function? Will a non-magnetic metal have the same effect? (Try using a piece of stainless-steel). You should find that your magnet is much weaker with the nail removed. In the same way that the needle was magnetised by the bar magnet in experiment 2, the copper coil magnetises the iron nail, turning the nail itself into a magnet and increasing the strength of the magnetic field of the entire system. Investigate how altering the copper coil changes the strength will gaps between the coils lower the strength? Will adding a second layer of coils double the electromagnet s strength? Will a third layer triple it? Will different sized batteries have any effect on the electromagnet? Try AAA, AA, C or D batteries. Increasing the current flowing through the coil will improve the strength of the magnetic field larger batteries should allow you to pick up more paperclips.

7 4. Making an electric motor In an electromagnet, an electric current flowing through a wire can generate a magnetic field we can use this property to create an electric motor. If we set up an electric circuit so that current is flowing through a piece of metal, we can generate movement in that metal by placing it in the presence of an external magnetic field (a permanent magnet). In this demonstration we will create a small electric motor using a battery and a button magnet. Equipment and consumables: Battery (AAA, AA or C work well) Copper wire, approximately 20 cm (the wire used in part 3 is suitable Small screw (about 4 cm) Button/disc magnet, small enough to fit on the head of the screw and strong enough to suspend the screw as seen in the photo below. Available in some electronics stores but most easily found online. Often sold in bulk, though you will only need one to construct the motor. ($7.70 for 100 magnets at Surplustronics.co.nz tinyurl.com/knnu4h5). SAFETY NOTE: Small, strong magnets present a serious hazard to small children if swallowed. The magnets attract each other through the walls of the stomach and intestine, creating the risk of perforating organs. The magnets may require surgery to remove. Method: 1. Take a 20 cm piece of copper wire and strip approximately 1 cm of the coating from each end. For wire with a plastic coating, this may be achieved using wire-strippers, scissors or a knife. For laminated wire you can use sandpaper, or a cigarette lighter to burn off the coating (wipe away the soot afterwards). 2. Place the button magnet on the head of the screw. 3. Hold the battery in the air and suspend the screw from the lower terminal (see the picture below). 4. Connect one end of the wire to the top terminal. Take the other end of the wire and brush lightly against the magnet. The screw will quickly start spinning! But take care that the battery does not become too hot.

8 This type of device is called a homopolar motor the earliest type of electric motor ever discovered and first demonstrated by Michael Faraday in The motor works by creating an electrical current in the screw which generates a magnetic field. The magnetic field of the screw interacts with the (much stronger) field of the button magnet, causing the screw to rotate. Overall, the energy in the battery is converted to movement. This process is completely reversible; in principle you could hold the screw and rotate it in the opposite direction to charge the battery (although this would take an extraordinary amount of time to achieve by hand). Investigating the properties of electric motors: Does the screw always rotate in the same direction? What happens when you hang the screw from the other end of the battery? The screw itself does not determine the direction of rotation a nail should work equally well, but will make it more difficult to distinguish the spinning motion. Does the screw rotate at different speeds using different sized batteries? The screw should rotate faster with higher current (from a larger battery), although this may be difficult to observe. There are several different homopolar motor designs that make suitable class demonstrations. The design described above is one of the most simple. Examples of more complex, designs such as those shown in the photos below, can be found in this youtube video (tinyurl.com/homopolar-motor). These designs largely use the same materials as the motor you assembled above; however you may find it preferable to use a thicker wire to construct the rotors.

9 Useful Websites: (An in depth description of magnets and magnetism) www-istp.gsfc.nasa.gov/education/imagnet.html (A history of our understanding of magnetism, provided by NASA)