Download Teacher`s notes 21 Specific Heat Capacity for a liquid

Physics
Electricity and Heat
Sensors:
Loggers:
Temperature, Current, Voltage
Any EASYSENSE
Logging time: EasyLog
Teacher’s notes
21 Specific Heat Capacity for a liquid: Experiment
to measure the specific heat capacity of water
Read
This investigation will measure the specific heat capacity of a liquid (water).
A sample of water of known mass is heated by known number of degrees, the energy required to create this
temperature rise, in this mass of water is used to calculate the specific heat the water.
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 1 kg 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 4184 joules of energy to raise the temperature of 1 kg of water by one degree.
The energy input into the calorimeter will be calculated from the voltage and current used to power a heater
immersed into the water. To prevent heat retention in the apparatus an open wire coil made from nichrome
wire 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.
An EASYSENSE logger.
A Smart Q Temperature sensor.
A Smart Q Current sensor ±10 A
A Smart Q Voltage sensor ±12 or 20 V
Electricity and Heat
T21 - 1(V2)
5.
6.
7.
8.
9.
d.c. power supply.
Rheostat.
Calorimeter containing water and a heating coil.
Lagging,
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
If you do not have access to a commercial calorimeter you can make one from a metal can or even glass
beaker. Results will not be quite as accurate but this can be used to get students to explain sources of error.
A heater can be crafted out of nichrome wire. A doubled strand about 20 cm coiled makes a good heater but
it does draw a large current, make sure the power supply and the sensor can cope with the large currents
required.
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.
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.
3. Select Power.
4. Select the data sets that contain the Voltage and Current data.
5. 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.
6. Click Finish
Graph showing Voltage, Current and Power.
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T21 - 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 a 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 the data boxes above
the graphing area, as shown. 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.
Name
Calculated from
Mass of the
calorimeter
mcal
=
Mass of the
calorimeter plus the
water
m1
=
Mass of the water
mw
m1 – mcal
Initial temperature
of water
θ1
=
Final temperature
θ1
=
Rise in temperature
Δθ
Electrical Energy
reading from graph
IVt
θ2 – θ1
Value
=
=
Area under the
Power vs. Time
graph
Given that the specific heat capacity of the calorimeter ccal is known, the specific heat capacity of water cw
can be calculated from the following equation:
Electrical energy supplied = energy gained by water + energy gained by calorimeter
IVt
=
mw cw Δθ + mcal ccal Δθ
Electricity and Heat
T21 - 3(V2)