201 Engineering Fundamentals 1 Practical Assignments
Practical assignment 201/4: Demonstrate the Effect of Heat on the
Resistance of a Conductor
1 Objective references
200.51
2 Preparation
2.1 Location of test
The training centre or other venue where supervision and
appropriate working conditions will be provided.
2.2 Requirements
Power pack or battery of several cells
Ammeter with ranges of 0-10A and 0-1A
Voltmeter with a range of 0-5 V
2x Variable resistors (rheostats): one with 20 ohm maximum
resistance and another with 10 ohm
maximum resistance
Electrical circuit connectors
Filament lamp (tungsten) 2.5 V
2.3 Instructor notes
Candidates may undertake assignments in pairs,
provided results analysis is undertaken independently
by each candidate.
Candidates must be familiar with Ohm’s law, electrical
circuits and the functions of resistors, ammeters and
variable resistors.
Health and safety issues must be explained to candidates in
the context of use of electric currents. It may be useful for the
instructor to undertake a dummy run demonstration of the
procedures before allowing the candidates to proceed. It is
particularly useful to check that the filament lamp will display
non-ohmic relationship at reasonable voltage values.
The writing up of this assignment may be done outside the
two hour practical session.
28
IVQ in Electrical and Electronic Engineering 8030 (2000)
3 Candidates’ instructions
3.1
The time allowed for this assignment is 2 hours. You are
advised to read all the instructions before commencing
work. If you do not understand all the instructions then
please ask you instructor.
3.2
In this experiment you will investigate the electrical
conduction characteristics of a conductor device (eg a
filament lamp). You will use a variable resistor as a ‘voltage
divider’ to give a smooth increase in voltage from zero to
positive values.
Set up the electrical circuit as shown in Fig. 1. But DO NOT
switch on the circuit until your instructor has had a chance
to see that it is correctly set up.
3.3
Set up the ammeter to read at the higher range of 0-10A.
Switch on the circuit. If the current reading is too small
then change to the 0-1A setting. The reason for carrying
out this procedure is that ammeters are sensitive devices
which can be damaged by high through put of current.
3.4
Adjust the voltage divider so that a range of voltage
readings are produced, eg 0, 0.5V, 1.0V, 1.5V, 2.0V, 2.5V,
3.0V, 3.5V, 4.0V, 4.5V, 5.0V
For each voltage reading, record the current in amperes.
3.5
Plot the change in current with voltage.
3.6
Describe the relationship between the voltage and current
for the conductor (filament lamp) eg is it a straight line
relationship or does the graph begin to curve at the higher
voltage levels?
3.7
Refer to a textbook to identify why the filament lamp
behaves in a way that does not follow Ohm’s law at higher
voltage values.
Syllabus: 2000 edition
29
3.8
3.9
Look up textbooks (or obtain information from your
instructor) on how other types of conductors which do not
obey Ohm’s law have application in circuit design.
Write up this assignment, ensure that your name is on your
work and hand it in to the instructor.
4 Marking
4.1
Assignment completed in 2 hours (excluding
write up).
(
)
4.2
Electrical circuit set up as advised.
[
]
4.4
A range of voltage and current readings taken.
[
]
4.5
Graph of current against voltage plotted.
[
]
4.6
Relationship between current and voltage for the
conductor correctly described.
[
]
Relationship correctly identified as non-ohmic
and due to Increase in resistance due to
heating effects.
[
]
Application of non-ohmic properties in other
conductor devices in use in electrical circuits
appreciated in a very general way.
(
)
Work handed in to the instructor.
[
]
4.7
4.8
4.9
5 Assignment completion
The candidate will have satisfactorily completed this assignment
if successful in all items marked with a [ ] and at least 1 of the
items marked with a ( ).
A period of seven days must elapse before an unsuccessful
candidate may retake this assignment.
30
IVQ in Electrical and Electronic Engineering 8030 (2000)