Download Teacher`s notes 22 Specific Heat Capacity of a solid

Physics
Electricity and Heat
Sensors:
Loggers:
Temperature, Voltage, Current
Any EASYSENSE
Logging time: EasyLog
Teacher’s notes
22 Specific Heat Capacity of a solid: Experiment to
measure the specific heat capacity of a metal
block
Read
This investigation will measure the specific heat capacity of a solid.
A block of metal of known mass is heated by known number of degrees, the energy required to create this
temperature rise, in this mass of metal is used to calculate the specific heat the metal.
The heat capacity,C of a body is the heat needed to produce unit temperature rise.
ΔQ = C Δθ
ΔQ = heat supplied
C = heat capacity
Δθ = temperature change
Heat capacity can also be described as the ratio of the heat supplied compared to the rise in energy
C= ΔQ/ Δθ
Specific heat capacity,c also known simply as specific heat is the measure of the heat energy required to
raise the temperature of 1kg of a substance by one degree.
So c = C/m
And
ΔQ = mc Δθ
m = mass
c = specific heat capacity
The term specific in the physical sciences often refers to quantities divided by a specified reference quantity.
When specific heat capacity is used, the term usually means mass-specific, or "per unit of mass." For
example, water has a mass-specific heat capacity of about 4,184 joules per degree Celsius per kilogram. Or,
put another way, it takes 4,184 joules of energy to raise the temperature of 1 kg of water by one degree.
The energy input into the metal block will be calculated from the voltage and current used to power a heater
immersed into the water. To ensure heat transfer from the heater to the metal block, the space surrounding
the heater is filled with a heat conduction material with a high boiling point. Glycerol is suggested. The
software is used to calculate the electrical power from the voltage and current graphs. The area under the
Power vs. Time curve will produce watts per second which will be equal (in this case) to the energy.
Apparatus
1.
2.
3.
4.
5.
6.
An EASYSENSE logger.
A Smart Q Temperature sensor
A Smart Q Current sensor ±10 A
A Smart Q Voltage sensor ±12 or 20 V
d.c. power supply.
Rheostat,
Electricity and Heat
T22 - 1(V2)
7. Metal block containing water / glycerol and a heating coil.
8. Lagging,
9. Electronic balance.
Set up of the software
The timing of the investigation is open ended so use EasyLog. This will record data until the user intervenes
and stops the logging.
Notes
It is possible to make your own heating block, but the resources required make it quite a technical job. For
this reason it is best to purchase a block and heater that are matched. Ideally several blocks are required,
aluminium, brass and iron.
Glycerol is mentioned as the heat conductive medium to surround the heater this reduces the risk or fire. Oil
with a very high boiling point and flash point can be used. Good quality engine oil can be used; oil of this
type is designed to operate in high temperatures. Take guidance from any literature provided with the block
and heater.
A temperature rise of about 10 degrees is worth aiming for, in the end it does come down to how much time
you have available. Do factor in the time lag between turning the power off and the maximum temperature
being reached.
The current drawn by a heater can be large; this is why a ±10 A Current sensor is suggested. Experience of
your apparatus may mean you can substitute this sensor for a lower ranged device.
Results
The electrical energy supplied may be calculated by the software by finding the area under the power curve
in a Power versus Time graph.
To produce the electrical power vs. time graph
1. Click on Tools and select Post-log Function.
2. From the Preset function drop down list select Electricity then Power.
3. Select the data sets that contain the Voltage and Current data.
4. Check the value of the Current sensor and make a correction to the multiplier if necessary (to
convert mA to A), use the text in the information box to assist. Click Finish.
Graph of Voltage, Current and Power.
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T22 - 2(V2)
To find the area under the power time curve, select Area from the toolbar. Click on the first point of the graph
that will describe the start of the area and drag the box out to the end of the area.
The area under the curve will now be shaded and the value of that area will appear in data boxes at the top
of the graphing area. This number in W.s, is in fact the number of joules of heat given to the water by the
heating coil.
N.B. the time of heating can also be read from the horizontal axis of the graph.
The energy transferred to the metal block can be calculated initially as W.s and converted to joules.
With the mass of the block the, joules per kg can be calculated and finally the joules per degree per kg
calculated, the last calculation giving the specific heat capacity.
A data book will provide the accepted value of the metal being used; students can then work out the % error
from theory and practical and explain the sources of error.
Electricity and Heat
T22 - 3(V2)