Favourite Demonstrations in Electromagnetism 1. Electroscope (Electric Charge Detector) Tips: FET’s are easily damaged by static charges. Use tools (tweezers, pliers etc) with insulated handles. An electroscope is an instrument used to determine Discharge hands on something earthed before han- the presence of an electrostatic charge; its dling. Store your finished electroscopes in a static magnitude; and sign (+ or -). insulated bag – of the sort that computer compo- nents retail in. Aim: This simple project makes a cheap but highly sensitive electroscope. Method: 1. Connect the battery and the LED should go on. Materials: If it doesn’t, the device may need resetting (do this 1. MPF-102 N-Channel JFET whenever required by touching your finger to the (Field-Electric Transistor) antennae). 2. 1000 Ohm resistor 3. Large LED (light emitting diode) 2. Bring a charged object close to your electroscope. 4. 9 V battery Negatively charged objects turn the LED off. It lights 5. Battery connector (9 V) again as the object is removed. Positively charged 6. Selection of objects to test: comb, fur, foil, objects make the LED brighter. It darkens as the sticky tape, balloon etc. object is removed. The electroscope works best in a 7. Soldering iron and flux core solder dry atmosphere. Construction: The electroscope has only 3 parts – an LED, the FET and a resistor (that also acts as an antennae). The 2. Electromagnetic Induction – Lenz’s Law diagram shows the wiring setup with the flat side of Magnetism and electricity are different aspects of the FET facing up. Initially connect all of the parts the same phenomenon – electromagnetism. Once with alligator clips and wires – then test. Once it’s it was discovered that a magnetic field could induce working, then commit to connecting everything an electric current – each of the following were cre- permanently with solder. ated within a very short period of time – the elec- LED tromagnet; the transformer; the electric motor; the electric generator. In 1883 Heinrich Lenz established that an induced electric current always opposes the FET change that produces it – now called Lenz’s Law. Aim: To demonstrate Lenz’s Law. short(-) RESISTOR long (+) Materials 1. 1 x Spherical neodymium magnet (approx 1 cm dam) red(+) 2. 1 x steel ball bearing (same size as magnet) black(-) 3. Copper plumbing pipe (1-1.5 m long; diameter slightly larger than the magnet) Battery 4. Clear plastic tube (1-1.5 m long; Connector same diameter as copper pipe) 1 B BANHAM 2012 1 Electromagnetic Induction – Lenz’s Law (cont) Remove the enamel coating from one wire “tail” completely using the sandpaper. The other tail gets Construction treated differently – on this tail only remove half of Drill a series of holes along one side of the copper the enamel coating (do so on a firm surface such as pipe – spaced several centimetres apart. Remove a tabletop). Assemble the motor as shown in the any burrs so that the inside of the pipe is smooth. diagram. The paperclips form supports for the copper wire armature (coil). Hold the pipe vertically and drop the ball bearing or the magnet into the pipe. Note the time it takes for exposed wire each to drop through the pipe (neodymium magnets are brittle. Do not allow them to hit a hard surface). enamel You will find the magnet takes considerably longer to travel through t he pipe than the ball bearing. armature The falling magnet is surrounded by a magnetic field that induces an electric current in the metal tube. magnet bent This current, in turn, induces a magnetic field that paper clip opposes the magnetic field of the falling magnet – slowing its descent. rubber band 3. The Worlds Simple Electric Motor – D Cell revisited! 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 com- globe mon materials to do the same as the kit version – while also extending the idea somewhat. Method: Aim: To make a working electric motor using simple 1. Place the armature in the supports such that the materials. armature is able to spin close to the magnets. Materials 2. Give the armature a gentle spin and that should set the motor rotating of its own accord. If it doesn’t 1. 100 cm length of magnet wire (enamelled; work, recheck the sanding. approx. 0.8 mm diameter) 2. 2 x large paper clips 3. Connect the light bulb across the motor as shown 3. 1 x rubber band (#64) in the diagram. The light bulb should flicker – hence 4. 1 x Disc magnet showing the on-off nature of the motor i.e. the elec- 5. 2 wires with alligator clips tric current flows preferentially through the path of 6. 1 x light bulb (1.5V) least resistance. When the current is able to flow 7. 1 x Light socket (1.5 V) through the armature, the light bulb will turn off. 8. 1 x D Cell When the current is not able to flow through the ar- 9. Fine sand paper mature, the light bulb will turn on. Have the motor turning at low speed for best results. It takes some Construction time for the bulb to cool between turns of the mo- Wrap at least 8 turns of the magnet wire around tor – so don’t expect it to go on and off completely; the D Cell, leaving a tail of a few centimetres long at it is more likely to flicker. 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). B BANHAM 2012 23 4. Homopolar Motor 5. Alternating Current vs. Direct Current The electric motor as invented in 1821 by Michael The terms AC and DC are commonly used – but do Faraday was a homopolar motor – comprised of students understand what they mean? Cells and a conducting disk in the presence of a permanent Batteries produce direct current in which electrical magnet that is free to rotate. There are many de- charges flow in one direction. The mains current signs for homopolar motors of which this is one. that is used in our homes, however, is alternating current – oscillating at 50 cycles per second. Aim: To make a working motor using only limited parts. Aim: To make a device that illustrates AC vs DC Materials: Materials: 1. 1 x AA Dry Cell 1. 1 x Bi-colour LED (for example, red and green) 2. 1 x ferrous screw (4 cm length works well) 2. 1 x 0.5 Watt resistor approx. 450 Ohms. 3. 1 x 10 cm length insulated copper wire 3. 1 x 1m Speaker cable (2 conductor) 4. 1 x nedymium disc magnet (approx 2cm 4. 1 x 9V AC/DC power pack diameter; 1-2 mm in width) 5. 2 x Alligator clips 6. Heat shrink tubing Construction: 7. Soldering iron plus flux-core solder Strip the insulation from both ends of the copper wire. Piece together the items as per the diagram Construction: and the magnet and screw should start spinning. 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 tub- ing (or electrical tape) When electrical current flows AA Cell touch the ends through one direction in the bicolour LED, the LED of the wire to glows one colour. When the current flows in the both the cell other direction, it glows with the second colour. and the disc magnet LED Connect to source Screw speaker output Disc Resistor magnet 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 responsi- ble for clenching the magnet in their fist when not in use. Always use spacers when collecting and storing the magnets. 23 B BANHAM 2012 3 Alternating Current vs. Direct Current (cont.) Materials 1. Radio with speaker outputs that accept “raw” Method speaker wire 2. Speaker wire (2 conductor; 1-2 m) Demonstration of DC 3. Electric motor (1.5 V) Connect the bi-colour LED to the 9 V DC 4. Wood dowel (10 cm long; 4 mm diameter) output from the power pack. The LED with 5. A variety of surfaces and containers to amplify either glow red or green. Reverse the alliga- the sound. tor clips on the power pack – now the LED will glow with the alternative colour as the Construction Strip the insulation from both ends of the speaker current now flows in the opposite direction. wire and connect one end to the radio speaker outputs; the other to the electric motor (some mo- Demonstration of AC tors work better than others – experiment). Turn up Connect the bi-colour LED to the 9V AC the volume of the radio and you will feel the motor output from the power pack. As the cur- vibrating. The sound can be amplified by pressing rent flows through the LED in one direction the motor against a variety of different objects – e.g. a plastic cup, table, cardboard box etc.