Propagation of Temperature Waves along a Bar
Introduction
The aim of this experiment is to firstly, construct and calibrate two thermocouples
and use them to measure an absolute zero temperature. The second aim of this
experiment is to measure the propagation of variations of temperature using an
aluminium bar. Pulses of heat will be supplied to one end of the bar and the other will
be left open to the atmosphere. Two thermocouples will be placed at different
positions on the bar to measure the propagation of variations of temperature.
Theory
A thermocouple is a junction between two dissimilar metals. It generates an e.m.f.
dependant on the junctions’ temperature when you connect two in a series. There
are two different types of junctions within this setup. They are the ‘reference junction’
and the ‘sensor junction’. Having these two junctions allows you to obtain an
absolute temperature by keeping the reference junction at a known temperature (e.g.
in an ice bath of 0ºC). The absolute temperature is measured by the sensor junction.
This is a diagram of a copper/constantan thermocouple:
Fig 1. A diagram of a copper/constantan thermocouple.
When the reference junction is held at a constant temperature of 0ºC the following
e.m.f. equation can be used:
𝐸 = 𝑎 + 𝑏𝜃 + 𝑐𝜃 2
E - E.m.f. (V)
θ – Temperature (ºC)
The letters a, b and c are constants, a is expected to equal 0.
Experimental 1 – Constructing and Calibrating two Thermocouples
Instrumentation
1) Copper wire, 0.5m, 32 or 34 s.w.g.
2) Constantan wire, 0.5m, 32 or 34 s.w.g.
3) Soldering iron
4) Cored solder
5) Ice cubes
6) Ice crusher
7) Glass beakers full of tap water
8) Ice/water baths
9) Conical flask
10) Bunsen burner
11) Dewar vessel
12) Liquid nitrogen
Plan of Measurements
Construct two thermocouples:
1) Remove insulation from ends of wires
2) Twist the bare ends of the wires and solder
3) Calibrate the thermocouples using:
a. Ice point
b. Steam point
c. Boiling of liquid nitrogen fixed point
a. Ice point
I.
Make two water baths from crushed ice cubes and water
II. Immerse the thermocouple junctions in the separate baths
III. Measure the e.m.f.
b. Steam point
I.
Put an inch depth of water in the conical flask
II. Heat the flask over a Bunsen burner until boiling
III. Put one of the thermocouples junctions in the steam and the other leave in
the ice bath
IV. Measure the e.m.f.
c. Boiling of liquid nitrogen
I.
Half full the Dewar vessel with liquid nitrogen
II. Put the sensor junction in the liquid nitrogen
III.
Measure the e.m.f.
Experiment 2 - measure the propagation of variations of
temperature using an aluminium bar
Instrumentation
1) Aluminium alloy, 0.3m long
2) Heating coil
3) Drill
4) White heat-sink compound
5) Cotton-insulated 34 s.w.g. constantan resistance wire
6) DC supply
7) Digital multimeter
8) Switch
9) Beaker of melting ice
10) Polystyrene sheets
11) Stop watch
Plan of measurements
1) Obtain a suitable length of cotton-insulated 34 s.w.g. constantan resistance
wire for the heater. Connect it to a 5V DC supply with a length that makes the
heating coil dissipate at 3-4 watts. Wind the heater firmly on the metal rod.
2) The two thermocouples should be connected in series with the switch
provided. Drill two small holes in the metal rod and stick one junction of each
into each hole.
3) Lag the sides and end of the heater bar with layers of polystyrene and secure
with sellotape.
4) Connect the heater to a power supply via a switch and warm the heater up for
ten minutes.
5) Turn the power on and off every three minutes, measuring the thermocouple
readings every five or ten seconds.