Download Enter o to this page the details for the document

Physics
Electricity and Heat
Sensors:
Loggers:
Voltage
An EASYSENSE capable of fast recording
Logging time: 500 ms
Teacher’s notes
42a Induction of a voltage in a coil
Read
This activity is a demonstration of Faraday's experiments into induction. It advances the work in 41 which
uses a simple straight length of conductor.
The investigation uses a Voltage sensor; this simply produces better results in terms of the graph shape and
size. The movement from a Current to a Voltage sensor may create confusion. You can easily add a Current
sensor to show the traces of current and voltage. Use a low range Current sensor e.g. a ±100 mA. In the
previous investigation a Current sensor was used of ±100 mA range was used to capture the very low output
from the single wire.
Apparatus
1. An EASYSENSE capable of fast logging.
2. A Smart Q Voltage sensor ± 1V
3. Wire coil of 500 turns (e.g. one coil from a Helmholtz pair), with a diameter of about 10 cm, and
depth of 2 cm, or the equivalent. See note below.
4. Bar magnet
5. Laboratory stand and clamps
Set up of the software
Use the setup file 42a Induction coil.
If you wish to set up the logger or software manually use the information shown below
Recording
method
Total
recording
time
intersample
time
Trigger
Channel
Trigger
Condition
Trigger
Level
PreTrigger
Graph
500 ms
200 μs
Voltage
Rises above
30 mV
50%
Notes
We used a large flat coil of 500 turns around a 10 cm diameter former. The depth of the coil was 2 cm.
Other coils such as coils from demountable transformers and intact coils from discarded ticker timers can be
used, and work well. They may require some form of a guide to get the magnet through the centre, a roll of
paper will normally be sufficient.
If you are constructing your own coil the number of turns and the size of the hole are not critical. A large
number of turns increases the size of the current induced. The central hole needs to be large enough for a
magnet to easily pass through. A large hole would mean that there is no need for a guide for the falling
magnet. The coil needs to be compact; a long length to the coil gives a much flattened trace.
The magnet needs to be quite strong. An Alnico magnet is recommended but any metal bar magnet will
induce a current. A rough test of magnetic strength is to see how many standard crocodile clips can be lifted
by the magnet. The magnet needs to be able to lift 4 linked crocodile clips firmly for the experiment to be
successful.
Electricity and Heat
T42a - 1 (V2)
A soft landing for the magnet needs to be provided. Repeated hard blows to a magnet will reduce its
strength.
Using pre - trigger allows data to be collected but not recorded until the trigger is reached. The polarity of
voltage induced depends upon which pole of the magnet is moving through the coil. In this experiment the
trigger is 30 mV. If the coil is "upside down" or the magnet is dropped "wrong pole" first the recording may
only capture half the event. Using pre-trigger, the full event is captured as 50% of the data will come from
before the trigger event.
The variability of coils and magnets may require these settings to be altered to get the best results.
Results
The above screen shows a typical set of data using a bar magnet - Zoomed In has been used to enlarge the
area of the graph. A Voltage sensor was used, sometimes background noise created by e.m.f will trigger a
Current sensor early.
Extension
1. Does it make a difference which pole of the magnet passes through the coil first?
2. Does turning the coil over make a difference?
3. Does the strength of the magnet have an effect?
4. Does increasing the number of turns in the coil have any effect?
5. Investigate the relationship between the velocity with which the magnet passes through the coil and
the induced voltage.
6. Investigate the change in magnetic field and the induced voltage / current. Use a Magnetic field
sensor mounted alongside the coil.
7. Investigate the relationship between the height the magnet is dropped from and the induced voltage.
Hint
For this a length of tubing marked in cm divisions can be used. If the magnet is always dropped from
the top of the tube, the height from the top of the tube to the top of the coil can be altered to give a
range of heights. The use of Overlay will allow results to be directly compared.
If using a coil with a small hole in the middle e.g. a demountable transformer coil, a sheet of A4
paper rolled to form a tube will provide a good guide for the magnet through the coil. If the paper is
wound into a roll and allowed to "unwind" inside the coil, this will provide a guide. Distance marks on
the paper will allow the paper to be moved up and down in the coil.
Electricity and Heat
T42a - 2 (V2)