Induced currents and Eddy Currents

Transcription

1 Induced currents and Eddy Currents There are quite a few nice experiments you can do to demonstrate Eddy currents and induced currents with relatively cheap equipment. Equipment: Rare-earth ceramic magnets: You can order small rare earth magnets online relatively cheaply (for example from ebay, $5.95 for 50 6mm x 1.5mm magnets, free postage). It is a good idea to wrap them in sticky tape before using them with students as they can crack are carcinogenous. Tubes: Aluminium and plastic tubing can be bought from hardware shops such as Bunnings (~ $10 for 1 m of aluminium tubing, plastic tubing is ~ $3 a meter). Wire and alligator clips A galvonometer. A micro-ammeter or micro-voltmeter (DC) with zero located on the middle of the scale works well A coil of wire: You can buy one or make one. Wrapping wire around a plastic tube works. Make sure you do it neatly with the turns very close together. Demo 2 will probably not work with homemade coils the coils need a large number of turns. Demo 1: An induced current Connect each end of the coil of wire to the galvanometer. Observe the needle on the meter as the magnet is pushed into and pulled out of the coil. What happens if you turn the magnet around? Notes for High School teachers: Induced Currents and Eddy currents

2 Explanation: A changing magnetic field induces an EMF in the coil of wire: ε = -Δφ/Δt where ε is the EMF (it is measured in volts), Δφ is the change in the magnetic flux (it measured in weber, Wb, φ = BA, magnetic field intensity in tesla x area in meters squared) and Δt is the change in time. The EMF is a voltage difference between the two ends of the wire. This voltage difference causes a current to flow. According to Lenz s law, the current that starts to flow will oppose the change that induces it. You galvanometer will detect either the EMF (if it is a voltmeter) or the induced current (if it is an ammeter). If you move the magnet in the opposite direction (or switch the polarity of the magnet) the EMF will have the opposite sign and the current will flow in the opposite direction. Demo 2: Movement at a distance (can be a little tricky to get working) Connect two coils together. The coils need to be well made for this one to work. Hang strong rare earth magnets from a spring through each of the coils. The springs need to be easy to extend (small spring constant). A good way to connect the magnets to the springs is to stick a bolt to the top of the magnet (it should stick due to the magnetic force) and then attach a paper clip to the bolt (again with the magnetic force), slip one end of the paper clip through the spring. The springs can be hung from a retort stand. Now move one of the magnets through one of the coils. Observer the magnet in the other coil. (It should start to move)

3 Explanation: The magnet moving through the first coil induces a current in the coil. As the coils are connected to each other this induced current flows through both coils. The change in the current in the second coil induces a magnetic field. This magnetic field interacts with the magnet in the second coil making it move. Demo 3: Eddy Current Breaking Set up three tubes (1 m long is good but shorter will do, they all need to be the same length) by clamping them to with a retort stand. One tube should be of aluminium, one of aluminium with a cut made down its length and one of plastic. Have students predict what will happen when a magnet is dropped into each of them. Then drop a rare earth magnet down each, releasing the magnets at the same time. Did the students predict correctly? Explanation: As the magnet drops down the aluminium pipe the changing magnetic field tries to induce a current. This current tries to flow in a circle perpendicular to the path of the magnet. As plastic is not a conductor no induced current can flow through it so there is no opposition to the magnet falling through the plastic tube. In the aluminium tube with the slice cut through it the current tries to flow but when it reaches the cut there is a section with a very high resistance. This prevents the induced current from flowing. Some eddy

4 currents are induced in the aluminium as the magnet moves passed so there is a slight opposition to the movement of the magnet. In the aluminium tube the current can flow around in a circular path. According to Lenz s law this induced current will oppose the movement of the magnet. This slows the magnet down. Demo 4: Eddy current breaking of Pendulum See the notes compiled by Joe Wolfe. These can be downloaded from: Syllabus References: Motors and Generators 2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage: - perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet - plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: - the distance between the coil and magnet is varied - the strength of the magnet is varied - the relative motion between the coil and the magnet is varied - explain the production of eddy currents in terms of Lenz s Law - describe generated potential difference as the rate of change of magnetic flux through a circuit 4. Transformers allow generated voltage to be either increased or decreased before it is used:

5 -perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced - gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome Notes compiled by Elizabeth Angstmann, First Year Physics Director, UNSW.