LABORATORY WORK I

CONTENTS

# DEMONSTRATION - PRODUCTION OF ALTERNATING CURRENT

Plan, chose equipment or resources for and perform a first-hand investigation to demonstrate the production of an alternating current.  (Syllabus 9.3.3, column 3, dot point 1)

Use hand-cranked generator or dynamo set to AC output so that the slip rings are in use.  Attach CRO probe from input A of CRO to the generator brushes to measure the AC output voltage when the armature is rotated by hand.  (Note - Do not attach the CRO probe to any output terminals on the generator if these exist.  You will almost certainly not get a very good AC signal if you do so.  The signal will be mixed with a lot of noise.  For a good strong, clean signal you must attach the CRO probe directly to the brushes of the generator!)

Set CRO to AC signal input & start with 1 V/div and 5 ms/div.  Note the nature of the output waveform.  It is a typical AC signal – sinusoidal and extending in both the positive & negative voltage directions (vertical scale).

If you have the facility to do so, it is also very useful to demonstrate a DC output.  Set the generator to DC output so that the split ring commutator is now in use.  Attach CRO probe to the brushes in contact with the commutator.  Compare the voltage output this time with the AC one.  The output is a typical DC voltage signal – sinusoidal but all positive.

The diagram below shows the basic set-up.  Most laboratory generator models consist of two coils each with many turns of wire and use permanent magnets rather than the electromagnet shown.  The diagram is not drawn to scale.

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# DEMONSTRATION - PRODUCTION OF SECONDARY VOLTAGE IN A TRANSFORMER

Perform an investigation to model the structure of a transformer to demonstrate how secondary voltage is produced.  (Syllabus 9.3.4, column 3, dot point 1)

Set up the transformer coils as shown below.  Use CRO.  Attach primary coil to AC power and to input A of CRO.  Attach secondary coil to input B of CRO.  Settings on CRO – start with 5 V/div and 5 ms/div.  Observe output voltage waveform from secondary coil and compare this with voltage waveform input to primary coil.  Both voltages are clearly AC in nature and for a step-up transformer (more turns of wire on secondary than on primary), the secondary voltage will be larger than the primary.

Note that the diagram below is not drawn to scale.

Remember to use a soft iron core in the centre of the primary coil to intensify the magnetic field threading through the secondary.  See following diagram.

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# DEMONSTRATION - PRINCIPLE OF AC INDUCTION MOTOR

Perform an investigation to demonstrate the principle of an AC induction motor.  (Syllabus 9.3.5, column 3, dot point 1)

Note that the basic principle of operation of an AC induction motor is that a changing magnetic field in the stator induces a current in the rotor and then drags the resultant induced magnetic field of the rotor around with it as it (the stator field) rotates.  This, in turn drags the rotor around as well.  I’m not altogether convinced that the Syllabus writers fully appreciated what they were asking when they asked for this to be demonstrated to students in a school laboratory.

So, I think it will suffice to show that a moving magnetic field in the vicinity of a solid metal (non-magnetic) conductor can cause the conductor to move.

To do this, hang a hollow copper cylinder from a retort stand using fishing line, as shown in the following diagram.  Mark a clearly visible coloured dot on the cylinder to make movement obvious.  Hold two neodymium (rare earth) magnets (very powerful) on opposite sides of the cylinder with the north pole of one facing the south pole of the other.  Show that the copper cylinder is not attracted by the magnets.  Hold one magnet still.  Rotate the other magnet in a circular motion in the vertical plane always keeping the same pole facing the copper.  You should observe the copper cylinder start to rotate about its vertical axis.  Try both directions of rotation of the magnet.

Also demonstrate that the rotation of the cylinder is not caused by moving air currents due to your hand.  Put the magnets down and repeat the circular motion movements with your hand in the same vicinity of the cylinder as previous.  Cylinder should not rotate (we hope).

Explain that in this demonstration the moving magnetic field induces eddy currents in the copper cylinder, which produce magnetic fields, which then interact with the moving field of the magnets to produce a net rotation of the cylinder.  Explain also the differences between this demonstration and what happens in a real AC induction motor.

Note that in order to satisfy the Syllabus statement it is NOT sufficient to simply show that relative movement between a conductor and a magnetic field can induce a current.  Yes, that is a demonstration of induction.  However, if that is what the Syllabus Committee required for this dot-point, that is what they should have asked for.  Instead the Syllabus specifically asks for a demonstration of the principle of an AC induction motor not a simple demonstration of induction.  The two are distinctly different in level of complexity.

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