Experiment one: The Motor Effect
For the HSC Physics syllabus dot-points:
- Perform a first-hand investigation to demonstrate the motor effect
- 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
- Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current
For the first dot-point, we wound wires five times, threading each loop through the region between two strong permanent (neodymium) magnets. We then applied a current of 3 amps through the wire (our power supply was auto current-limiting so no excessive heat was produced). With 5 loops threaded, each with 3 amps, the total effective current was 3×5 = 15amps, and a movement of the wires were observed. The direction of force on the wire was as predicted by the right-hand push rule (aka right-hand palm rule).
For the second dot-point, we tested each condition (distance, strength and speed of magnet) on a coil connected to an ammeter and we observed a direct correlation between strength of magnet and induced current, a direct correlation between speed of magnet and induced current, and an inverse correlation between distance of magnet and induced current.
For the final dot-point, we moved a magnet in and out of a coil connected to an ammeter. The ammeter needle’s direction of movement continually reversed, indicating that an alternating current was produced. We also used a hand-wound AC generator to power an incandescent light bulb.
Experiment two: Transformers and Induction motors
For the HSC Physics ( http://www.duxcollege.com.au/ ) syllabus dot-points:
- Perform an investigation to model the structure of a transformer to demonstrate how a secondary voltage is produced
- Perform an investigation to demonstrate the principle of an AC induction motor
For the first dot-point, we constructed a model transformer using a 300 turn coil as the primary and a 600 turn coil as the secondary. Primary voltage was 18V AC and the secondary voltage was measured (by an AC-capable voltmeter) to be approx 35V AC when using laminated iron cores. The secondary voltage was observed to fall significantly (around 22V) when laminated iron core was replaced with a solid bar of iron (more eddy currents were possible with unlaminated iron core, decreasing efficiency of the magnetic flux transfer). We then switched the primary and secondary coil so that the primary was now 600 turns and the secondary was 300 turns. The secondary voltage was measured to be 8.5V AC. The percentage inefficiency was calculated to be the same in the step up and step down versions.
For the secondary dot-point, we constructed a single phase squirrel cage induction motor. Single phase induction motors generate initial torque due to the presence of shading coils. These coils delay the flux transfer at carefully chosen parts of the surrounding stator so that the initial change in flux produces a torque on the rotor. The squirrel cage core is not connected to any electricity — it moves only due to Lenz Law — it chases the external rotating magnetic field. We observed that the speed of the induction motor was at its highest if using a laminated core. When we changed the orientation of the rotor so that flux has to pass through a section of air, we observed the motor slow down significantly. When we bridged this gap with a solid iron bar, the rotor sped up slightly, but was still slower than its original speed. This again illustrates that air and iron bars are not as effective in transferring magnetic flux as laminated iron.
HSC Biology first-hand investigations coming soon!