Determination of specific heat capacity by
Electrical Method
Determination of specific heat capacity of
liquid by continuous flow Method
Electrical energy supplied (E1) = V1I1t1 = m1c(θ2 - θ1 ) + H
Electrical energy supplied (E2) = V2I2t2 = m2c(θ2 - θ1) + H
Specific heat capacity of the liquid (c) = [E2 – E1]/(m2 –
m1)(θ2 - θ1)
Determination of Specific Latent Heat of
Fusion of ice by Electrical Method
(M-m)x L = Ivt
Where M is mass of ice melted
when heater is switched on in t
seconds
m is mass of ice melted before the
heater is switched on in t seconds
(M-m) is the mass of ice melted due
Determination of Specific Latent Heat of
Vaporisation of water by Electrical Method
Numericals
1. A kettle is rated as 2.3 kW. A mass of 750 g of water at 20 °C is poured
into the kettle. When the kettle is switched on, it takes 2.0 minutes for
the water to start boiling. In a further 7.0 minutes, one half of the
mass of water is boiled away.
(a) Estimate, for this water,
(i) the specific heat capacity, specific heat capacity. [4600 Jkg-1K-1]
(ii) the specific latent heat of vaporisation [5] [2.6*106 Jkg-1]
(b) State one assumption made in your calculations, and explain whether
this will lead to an overestimation or an underestimation of the value for
the specific latent heat. [2]
1.
A jeweller wishes to harden a sample of pure gold by mixing it with
some silver so that the mixture contains 5.0% silver by weight. The
jeweller melts some pure gold and then adds the correct weight of
silver. The initial temperature of the silver is 27 °C. Use the data
given below to calculate the initial temperature of the pure gold so
that the final mixture is at the melting point of pure gold. [1483 K]
1. A mass of 24 g of ice at –15 °C is taken from a freezer and placed in a
beaker containing 200 g of water at 28 °C. Data for ice and for water are
given below
(i) Calculate the quantity of thermal energy required to convert the ice at –
15 °C to water at 0 °C. [3]
[756+7920 = 8700 J]
(ii) Assuming that the beaker has negligible mass, calculate the final
temperature of the water in the beaker [160C]
1. Define specific latent heat of fusion.Use the kinetic theory of matter to explain
why melting requires energy but there is no change in temperature. [2+3=5]
a. A block of ice at 0 °C has a hollow in its top surface, as illustrated in Fig. A
mass of 160 g of water at 100 °C is poured into the hollow. The water has
specific heat capacity 4.20 kJ kg–1 K–1. Some of the ice melts and the final
mass of water in the hollow is 365 g. [328 kJ kg–1 ]
a. Assuming no heat gain from the atmosphere, calculate a value, in kJ kg–1,
for the specific latent heat of fusion of ice. [3]
b. In practice, heat is gained from the atmosphere during the experiment.
This means that your answer to (i) is not the correct value for the specific
latent heat. State and explain whether your value in (i) is greater or smaller
than the correct value [2]
Temperature and thermometer
Temperature is average K.E. of molecules. It
measure the degree of hotness of a body. Measure
by an instrument called thermometer
In order to measure temperature quantitatively
some sort of numerical scale must be defined. Two
temperature scales Fahrenheit and Celsius are used
for general purpose and Kelvin scale for the
scientific purpose.
Temperature Scales
Operation of all types of thermometers depends on some
properties of matter that changes with temperature called
thermometric properties. These changes can be used to measure
the temperature. (e.g. length, resistance, pressure, voltage etc).
The values of the property such as length are measured at 2 fixed
temperatures the freezing point and a boiling point of water at
standard atmospheric pressure.
the interval between the 2 fixed temperatures is divided into a
number of subintervals (empirical scale). The Celsius and Kelvin
scale has 100 units between these points, while Fahrenheit scale
has 180 units. This is known as calibration of thermometer.
Common Temperature Scale
Three common types
– Celsius scale (oC)
– Fahrenheit scale (oF)
– Kelvin Scale (K)
Establish the Relationship between three temperature scales
Example
1. If C = 30 oC then F = 84 oF
2. What is the temperature for which Celsius and Fahrenheit
thermometer shows the same reading?
Common Temperature Scale
Different type of thermometers
1. Liquid-in-glass thermometer
2. Resistance thermometer
3. Thermocouple thermometer
4. Constant Volume gas thermometer
5. Pyrometer
Liquid-in-Glass Thermometer
Liquid-in-glass thermometer
1. mercury-in-glass
2. alcohol-in-glass
– Why not use water ???
• A relatively large bulb at the lower portion of
the thermometer hold the major portion of the
liquid, which expands when heated and rise in
capillary tube upon which appropriate scale
marking are fixed. The size of the capillary
depends on the size of the sensing bulb, the
liquid, and the desired temperature range for
the thermometer.
•
At the top of the capillary tube another bulb is
placed to provide a safety feature in case the
temperature range of the thermometer is
accidentally exceeded.
Alcohol
range
-115oC
to 78oC
Mercury
range
-39oC to
357oC
Operation of liquid-in-glass thermometer
• Operate when the bulb is exposed to the environment
whose temperature is to be measured.
• A rise in temperature causes the liquid to expand in
the bulb and rise in the capillary, thereby indicating
the temperature.
• It is important to note that the expansion registered
by the thermometer is the difference between the
expansion of the liquid and the expansion of the glass
Finding Temperature
Value of ( X 100 ) and ( X 0 ) of the
temperature measuring property are found at
steam point and at ice point.( X 100  X 0 ) is called
fundamental interval of the scale. If ( X  ) is
the value of property at unknown
temperature θ in ⁰C is given by

X  X 0

100 X 100  X 0
Mercury-in-glass thermometer
Advantages
– Expands evenly on
heating
– Responds quickly to
temperature
– A high boiling point,
so used in hot places
– It does not wet
Disadvantages
– Poisonous
– Expensive
– A high freezing point,
so not used in cold
places
Alcohol-in-glass thermometer
Advantages
– Expands about six
times of mercury
– Expand evenly on
heating
– A low freezing point,
so used in very cold
places
– It is safe
– It is cheap
Disadvantages
– It is dyed
– It wet the tube
– It does not respond
quickly with
temperature
– A low boiling point,
so not used in hot
places
Clinical thermometer
A specialized
mercury in glass
thermometer to
take body
temperature
A constriction to
prevent mercury
from falling
range a few
degrees above
and below 37oC
Liquid in glass Thermometer
Inaccuracies arise in thermometer from
Advantages
1. Non uniformity of the bore
of the capillary tube
Simple, Cheap, Portable
Direct reading, Everyday
2. The gradual change in zero
use, Clinical use and
owing to the bulb shrinking
weather recording
for number of years after
manufacture
3. The mercury in the stem
Disadvantages
not being at the same
Not very accurate
temperature as that in the
Small range (-39 to 500)
bulb
Questions
1. Length of mercury column at 0oC is 12 cm length
of mercury column at 100oC is 22 cm. What is the
temperature if the length of the column is 16 cm?
2. Length of mercury column at 37oC is 12.0 cm
length of mercury column at 100oC is 26.1 cm.
What is the length of the mercury column at
temperature 0oC ?
Resistance Temperature Detector (RTD)
Quite accurate method of temperature measurement.
Consists of some type of resistive element which is
exposed to the temperature to be measured.
The temperature is indicated through a measurement
of the change in resistance of the element.
Several types of materials may be used such as
platinum, nickel, iron(alloy), copper and tungsten.
Among them platinum is preferred
Platinum Resistance Thermometer
Why Platinum ?
1. It has high melting point
2. It is chemically stable

R  R0

100 R100  R0
3. Its resistance changes uniformly over the
wide range of temperature
Platinum Resistance Thermometer
Advantages
1. Wide range (-200 to 1200)
2. Very accurate
3. Best for small steady
temperature difference
4. Zero of this thermometer
does not change because
the resistance is always
same at the same
temperature
Disadvantages
1. Unsuitable for rapidly
changing temperature
due to large heat capacity
and low thermal
conductivity of the
container.
2.
A small amount of
impurities in platinum
can produce great error
in the temperature
3. Is not direct reading type
Thermocouple
When two metals are joined end to end to form a junction & these
junction are kept at different temperature , a small emf is produced
& current flows through the metals. The emf is called thermo emf &
the effect is called thermoelectric or Seebeck effect. The pair of
dissimilar metals forming the junction is called thermocouple which
is the sensing junction. The most common electrical method of
temperature measurement uses the thermocouple.
Thermocouple, contd…
• The magnitude of the thermo emf depends
on the types of metals used for the wires and
amount of temperature difference between
the junction.
Thermoelectric series (Seebeck series)
• Sb,Fe,Cd,Zn,Ag,Au,Pb,Mn,Cu,Pt,Co,Ni,Bi
Thermocouple Thermometer
Advantages
Disadvantages
1. Is not so accurate as
1. Wide range (-250 to 1500)
platinum resistance
2. Fairly accurate
thermometer for
3. Cheap and can easily be
measuring temperature
constructed
below 1000 oC
4. Suitable for rapidly changing 2. Different thermocouple are
temperature because
to be used for different
junction is small so it
ranges.
absorbs very small heat
3. It is difficult to maintained
5. Robust (Strong) and
cold junction at a constant
compact
temperature.
Constant-Volume Gas Thermometer
• The physical property
used in this device is
the pressure variation
with temperature of a
fixed-volume gas.
• The volume of the gas
in the flask is kept
constant by raising or
lowering the reservoir
B to keep the mercury
level at A constant
28
Constant Volume gas Thermometer
• Advantages
• Disadvantages
1. Wide range (-270 to 1500) oC
2. Very accurate and sensitive
3. Cheap and can easily be
constructed
4. Used as a standard to
calibrate other practical type
1. It is bulky
2. Slow to respond
3. Not direct reading
Absolute Zero
• The thermometer
readings are virtually
independent of the
gas in the flask.
• If the lines for various
gases are extended,
the pressure is always
zero when the
temperature is –
273.15o C
• This temperature is
called absolute zero
30
Kelvin Temperature Scale
The Kelvin temperature scale is now based on two new fixed points. Adopted in
1954 by the International Committee on Weights and Measures. One point is
absolute zero and the other point is the triple point of water.
Triple point of water This is the single
Temperature and pressure at which ice,
water, and water vapor can coexist in
thermal equilibrium.
The triple point of water occurs at 0.01o C and
4.58 mm of mercury .This temperature was set
to be 273.16K on the Kelvin temperature scale.
The Kelvin is defined as 1/273.16 of the
temperature of the triple point of water
31
Thermistor
Semiconductor device that has a negative temperature coefficient of
resistance, in contrast to the positive coefficient displayed by most
metals. NTC means, the resistance decreases as the temperature
rises.
Resistance at 25oC for typical commercial units ranges from 100 to
over 100M.
Very sensitive device and consistent performance within 0.01oC
however it is highly nonlinear behavior.
Since the resistance of the Thermistor is very high, the error due to
resistance is small. In addition, the high resistance of the thermistor
means the smaller current required for measurement.
Thermistor resistance decreases very nonlinearly with increasing temperature.
Suppose we take
two resistance
readings and
average them.
250 Kohms
50 Kohms
Avg = 150 Kohms
Average resistance
gives correct
average
temperature?
No!! Must convert to temperature before averaging.
Thermistor-based probes can contain electronics to give a linear voltage
output with temperature.
Pyrometers
•
•
•
•
Pyrometer is used:
To measure temperature of a very hot body
such as are encountered in furnaces.
Where thermometers cannot brought into
contact or
Where hot bodies are moving
Measurements done by measuring energy
radiated by a hot body or by comparison of
colour
Pyrometers