Favourite Demonstrations in Electromagnetism

Transcription

1 Favourite Demonstrations in Electromagnetism 1. Electroscope (Electric Charge Detector) An electroscope is an instrument used to determine the presence of an electrostatic charge; its magnitude; and sign (+ or -). Aim: This simple project makes a cheap but highly sensitive electroscope. : 1. MPF-102 N-Channel JFET (Field-Electric Transistor) Ohm resistor 3. Large LED (light emitting diode) 4. 9 V battery 5. Battery connector (9 V) 6. Selection of objects to test: comb, fur, foil, sticky tape, balloon etc. 7. Soldering iron and flux core solder : The electroscope has only 3 parts an LED, the FET and a resistor (that also acts as an antennae). The diagram shows the wiring setup with the flat side of the FET facing up. Initially connect all of the parts with alligator clips and wires then test. Once it s working, then commit to connecting everything permanently with solder. short(-) LED long (+) black(-) FET red(+) RESISTOR Battery Connector Tips: FET s are easily damaged by static charges. Use tools (tweezers, pliers etc) with insulated handles. Discharge hands on something earthed before handling. Store your finished electroscopes in a static insulated bag of the sort that computer components retail in. Method: 1. Connect the battery and the LED should go on. If it doesn t, the device may need resetting (do this whenever required by touching your finger to the antennae). 2. Bring a charged object close to your electroscope. Negatively charged objects turn the LED off. It lights again as the object is removed. Positively charged objects make the LED brighter. It darkens as the object is removed. The electroscope works best in a dry atmosphere. 2. Electromagnetic Induction Lenz s Law Magnetism and electricity are different aspects of the same phenomenon electromagnetism. Once it was discovered that a magnetic field could induce an electric current each of the following were created within a very short period of time the electromagnet; the transformer; the electric motor; the electric generator. In 1883 Heinrich Lenz established that an induced electric current always opposes the change that produces it now called Lenz s Law. Aim: To demonstrate Lenz s Law x Spherical neodymium magnet (approx 1 cm dam) 2. 1 x steel ball bearing (same size as magnet) 3. Copper plumbing pipe (1-1.5 m long; diameter slightly larger than the magnet) 4. Clear plastic tube (1-1.5 m long; same diameter as copper pipe) 1

2 Electromagnetic Induction Lenz s Law (cont) Drill a series of holes along one side of the copper pipe spaced several centimetres apart. Remove any burrs so that the inside of the pipe is smooth. Hold the pipe vertically and drop the ball bearing or the magnet into the pipe. Note the time it takes for each to drop through the pipe (neodymium magnets are brittle. Do not allow them to hit a hard surface). Remove the enamel coating from one wire tail completely using the sandpaper. The other tail gets treated differently on this tail only remove half of the enamel coating (do so on a firm surface such as a tabletop). Assemble the motor as shown in the diagram. The paperclips form supports for the copper wire armature (coil). exposed wire enamel You will find the magnet takes considerably longer to travel through t he pipe than the ball bearing. The falling magnet is surrounded by a magnetic field that induces an electric current in the metal tube. This current, in turn, induces a magnetic field that opposes the magnetic field of the falling magnet slowing its descent. 3. The Worlds Simple Electric Motor revisited! armature magnet rubber band D Cell bent paper clip This is revisited in the sense that you may have encountered this kind of motor before perhaps as a kit from a science supplier. This version uses common materials to do the same as the kit version while also extending the idea somewhat. Method: globe Aim: To make a working electric motor using simple materials cm length of magnet wire (enamelled; approx. 0.8 mm diameter) 2. 2 x large paper clips 3. 1 x rubber band (#64) 4. 1 x Disc magnet 5. 2 wires with alligator clips 6. 1 x light bulb (1.5V) 7. 1 x Light socket (1.5 V) 8. 1 x D Cell 9. Fine sand paper Wrap at least 8 turns of the magnet wire around the D Cell, leaving a tail of a few centimetres long at each end of the coil. Remove the coil from the D Cell and wind 2 turns around the coil at opposite sides to lock hold the coil together (as per diagram). 1. Place the armature in the supports such that the armature is able to spin close to the magnets. 2. Give the armature a gentle spin and that should set the motor rotating of its own accord. If it doesn t work, recheck the sanding. 3. Connect the light bulb across the motor as shown in the diagram. The light bulb should flicker hence showing the on-off nature of the motor i.e. the electric current flows preferentially through the path of least resistance. When the current is able to flow through the armature, the light bulb will turn off. When the current is not able to flow through the armature, the light bulb will turn on. Have the motor turning at low speed for best results. It takes some time for the bulb to cool between turns of the motor so don t expect it to go on and off completely; it is more likely to flicker. 2

3 4. Homopolar Motor The electric motor as invented in 1821 by Michael Faraday was a homopolar motor comprised of a conducting disk in the presence of a permanent magnet that is free to rotate. There are many designs for homopolar motors of which this is one. Aim: To make a working motor using only limited parts. : 1. 1 x AA Dry Cell 2. 1 x ferrous screw (4 cm length works well) 3. 1 x 10 cm length insulated copper wire 4. 1 x nedymium disc magnet (approx 2cm diameter; 1-2 mm in width) : Strip the insulation from both ends of the copper wire. Piece together the items as per the diagram and the magnet and screw should start spinning. AA Cell Screw Disc magnet touch the ends of the wire to both the cell and the disc magnet 5. Alternating Current vs. Direct Current The terms AC and DC are commonly used but do students understand what they mean? Cells and Batteries produce direct current in which electrical charges flow in one direction. The mains current that is used in our homes, however, is alternating current oscillating at 50 cycles per second. Aim: To make a device that illustrates AC vs DC : 1. 1 x Bi-colour LED (for example, red and green) 2. 1 x 0.5 Watt resistor approx. 450 Ohms x 1m Speaker cable (2 conductor) 4. 1 x 9V AC/DC power pack 5. 2 x Alligator clips 6. Heat shrink tubing 7. Soldering iron plus flux-core solder : Strip the insulation from both ends of the lamp cord. On one end attach 2 x alligator clips this is for attaching directly to the power pack. To the other end attach the resistor and the LED in series. Insulate the wires both ends with heat shrink tubing (or electrical tape) When electrical current flows through one direction in the bicolour LED, the LED glows one colour. When the current flows in the other direction, it glows with the second colour. LED Resistor Connect to source speaker output Caution Neodymium magnets are brittle and easily broken (particularly the disc magnets). They can fly apart as they fragment - so eye protection is merited. They are also strongly attracted to each other as well as to anything ferrous - and can pinch and even draw blood if not handled appropriately. When working in groups it is advisable for one person to be responsible for clenching the magnet in their fist when not in use. Always use spacers when collecting and storing the magnets. 3

4 Alternating Current vs. Direct Current (cont.) Method Demonstration of DC Connect the bi-colour LED to the 9 V DC output from the power pack. The LED with either glow red or green. Reverse the alligator clips on the power pack now the LED will glow with the alternative colour as the current now flows in the opposite direction. Demonstration of AC Connect the bi-colour LED to the 9V AC output from the power pack. As the current flows through the LED in one direction it glows red; when the current moves in the other direction it glows green. Because of persistence of vision it can be difficult to distinguish the different colours and the LED may appear a yellow colour. When no current is flowing the light is off i.e. no colour. Clear some space around you, darken the room and spin the LED cord in a large circle. You will see a circle of bright light with alternating red and green dashes separated by a short dark space. 6. A Motor acting as a Loudspeaker 1. Radio with speaker outputs that accept raw speaker wire 2. Speaker wire (2 conductor; 1-2 m) 3. Electric motor (1.5 V) 4. Wood dowel (10 cm long; 4 mm diameter) 5. A variety of surfaces and containers to amplify the sound. Strip the insulation from both ends of the speaker wire and connect one end to the radio speaker outputs; the other to the electric motor (some motors work better than others experiment). Turn up the volume of the radio and you will feel the motor vibrating. The sound can be amplified by pressing the motor against a variety of different objects e.g. a plastic cup, table, cardboard box etc. Attach the wooden dowel to the rotor on the motor (drill a hole small enough so that it fits snugly). Clench the dowel between your teeth and you will be able to clearly hear the sounds of the radio the motor causes the dowel, then your teeth and skull to vibrate such that the vibrations reach your inner ear. Dowel Small DC motors and speakers have the same basic components a coil and a permanent magnet. When an electric current flows through the voice coil of a speaker it interacts with the permanent magnet and moves back and forwards. When a constant current passes through the coil of a motor, it interacts with the permanent magnet and spins. When electric current pulses through the coil of a motor, each pulse causes a slight movement of the coil if the electrical pulses are coming from a radio, it will cause the coil to vibrate such that it produces faint sounds. Aim: To demonstrate that both a DC motor and a speaker have similar properties (that arise from their similar construction). Connect to speaker outputs Motor Connect using solder or alligator clips 4

5 7. A Motor becomes a Generator The input and output energy of a motor can be reversed. I.e. an electric motor normally takes in electrical energy and transforms it into mechanical energy. However, the opposite is also true i.e. take in mechanical energy and output electrical energy. Table Aim: To show that an electric motor can also be used as an electric generator (due to their similar construction) x 1.5 V electric motors 2. 2 x insulated copper wires (each approx 50 cm long) 3. 1 x Ice cream stick 4. Masking tape or rubber tubing (a few cm s) Connect the motors with the wire as shown in the diagram. Attach the ice-cream stick to one of the rotors so that it can be easily seen when the rotor is turning. Wrap the masking tape around the rotor of the other motor (alternatively, attach the rubber tubing) the aim being to increase the friction when rubbing the rotor against another surface (As a variation, you may also try attaching a motor to a 1.5 V lightbulb). Rub the rotor of the generator motor (with tape or tubing attached) rapidly along a surface. This will spin the coil inside the motor, hence creating an electric current sufficient to turn the rotor in the other motor. 8. Pulsating Current Visualiser Aim: To construct a device that lets us visualise the electrical signal from the speaker outlets in a radio 1. 1 x LED 2. 1 x resistor (0.5 W; 220 Ohms) 3. Speaker wire (2 conductor) 4. Heat shrink tubing 5. Soldering iron plus flux-core solder 6 Radio (with speaker output that accepts bare speaker cables) Construct the circuit as shown... LED Connect to source speaker output Resistor Icecream stick/straw light 5

6 Pulsating Current Visualiser (cont) Method 1. Turn the radio to a clear station with talking or percussive music. 2. Plug the visualiser into the speaker output of the radio. 3. Turn up the volume until the LED blinks. 9. Current Generating Tube Whenever a conductor moves through a magnetic field an electric current is induced in the conductor. Aim: In this activity we pass a magnet through a coil of wire and detect the resulting small electric current using LED s. Because an electric current can flow in one direction only through an LED, it is a useful way of determining the direction of current flow x cow magnet 2. Plastic tube (30 cm; diameter slightly larger than the cow magnet) 3. 1 x large red LED 4. 1 x Large Blue LED 5. 2 x heavy cardboard pieces 6. Copper wire (120 g; 0.4 mm diameter; enamelled) 7. Soldering iron plus flux core solder 8. Electrical Tape 9. Fine sand paper Cut the cardboard pieces into washers that fit snugly over the plastic tube. Secure the cardboard washers in place using glue or tape such that there is about a 5 cm gap between them. Wind 1,500 turns of the copper wire in the space between the cardboard washers ensure the turns are tight; (they don t need to be neat). Make sure that several centimetres of both ends are exposed after winding. Clean the enamel coating off both ends of the wire (using sandpaper) and connect the LEDs as shown in the diagram. Each LED has a long and a short wirethe long wire of one LED and the short wire of the other LED are attached to each end of the coil. Don t allow the wires of the LED s to tough each other. A few turns of electrical tape around the coils will help keep the device together. Place the cow magnet into the tube. Hold your hand over each end and tilt the tube back and forth. As the magnet moves through the tube the LED s will light. Cow Magnet LEDs Coil 10. Worlds Simplest Loudspeaker tube A speaker consists of both a permanent magnet and an electromagnetic voice coil. When a pulsating electric current passes through the voice coil, the strength of its magnetic field varies, causing fluctuations as it interacts with the permanent magnet. Aim: To make a loudspeaker : 1. 2 x neodymium magnets (flat discs or cylinders) 2. Copper wire (enamelled; mm diameter) 3. 2 m Speaker wire (2 conductor) 4. Radio with speaker outputs that accept bare speaker wire 5. Soldering iron and solder with flux core 6. Selection of objects to act as speakers Make a coil of 50+ turns of copper wire with a diameter a little larger than that of the magnet. Keep track of both ends of the wire (allow for 10 cm each end). Wrap the coil tightly in electrical tape the tighter the coil the better. Strip the insulation from both ends of the speaker wire. Solder the 2 wires from one end to both ends of the coil. Connect the other ends to the speaker output from the radio. Turn on and tune the radio. Bring the coil close to the magnets and you will feel a vibration and hear the sound coming from the surface of your simple speaker. Experiment by attaching your magnets and coil to different surfaces for amplification. 6

7 Worlds Simplest Loudspeaker (cont) Wrapping the magnet in some disposable paper to protect your teeth and for reasons hygiene. Sources for All of the electrical items used in these demonstrations can be purchased from Jaycar or Tandy. I ve enjoyed purchasing magnets (including ceramic, neodymium and cow magnets) from suppliers on e-bay. There are a wide range of types available, at cheap prices (including delivery). You will need to experiment with different types of magnet type, size and shape. Other sundry materials such as solder and copper piping are available from hardware stores. Safety When undertaking any of these activities you will need to be the responsible party for evaluating any risks and take all prudent safety procedures. Activities should only be undertake n under adult supervision. The author accepts no responsibility. 7