Objective:
To play with different
materials and compare their thermal conductivities Equipment:
Three metal cans, Rods of
different materials but same geometry, small immersion heater,
three thermometers
Introduction:
Thermal conductivity of a material is one of the key factors
which decides how much heat will pass through it if the two ends
are maintained at different temperatures. For a cylindrical
object, the equation governing the rate of heat flow is
introduced in class 11 as
where
K is the thermal conductivity of the material, A is the area of
cross section perpendicular to the heat flow,
DT
is the temperature difference across a length of
DL
along the heat flow. In the informal lab, the students can
compare the thermal conductivities of different materials in a
simple manner. Procedure:
Suppose we wish
to compare the conductivities of copper and steel. You have to
procure a rod of each material. It could be say 10 cm long and
0.2 cm in radius. The dimensions are not crucial but two rods
should be similar except for the material. The cross section
also need not be circular. Take three metal cans A, B and C.
The cans A and C should be identical, B may or may not be
identical to others.
Drill a hole in A and C and two holes in B. The holes should be
such that the rods may be fitted in them and they should be at
the same height from the bottom. Fit the copper rod between the
cans A and B, and the steel rod between B and C. Close any gap
between the rod and the can at the hole by an adhesive, say
araldite.
Fill some water in the cans. The amount of water in A and C
should be the same. The ends of the rods should be immersed in
water. Put thermometers in A, B and C. Put immersion heater in
the can B and switch it on. Be careful, if the heater is faulty
it can send electrical current in water and can. Do not touch
anything when the switch it on.
In few minutes
water in can B will start boiling. Read temperatures in all the
cans. Steel is poorer conductor of heat as compared to copper.
The temperature rise in A should be more than in B. By comparing
the temperature rise in the two cans in a given time interval,
you can have an estimate of the ratio of the thermal
conductivities of the two materials. Discussion:
The equation for heat flow given above may not be usable in the
conditions described. There are losses to the atmosphere at
every stage. That is why I insist on having all other conditions
identical except the materials of the rod. If the student wishes
he/she may modify the procedure to get more accurate results.
Hazards:
As mentioned above, if the heater is faulty it can send
electrical current in water and can. Do not touch anything when
the switch it on.
When to do:
Class 11, after you have done thermal conductivity.
Instrument
cost and availability :About Rs.
100
