(A)heat and temperature
Heat is a form of energy, called thermal energy that is normally transferred from one body to another as a result of temperature difference between the two bodies while temperature is the measure of degree of hotness or coldness of a body.
(B) construction and graduation of liquid in glass thermometer
The first step in construction and graduating of a liquid in glass thermometer is to get a capillary tube of the same cross sectional area throughout its length. One end of the tube is then sealed, and a bulb is blown at the same end. Thereafter, the tube is left to cool, and using a funnel, Mercury is poured into the tube kept vertically. Trapped air in the tube is expelled by gently warming the bulb, after which the same procedure is repeated until more Mercury is poured into the bulb. Finally, all air is expelled out of the glass tubing by boiling the Mercury. The glass is then maintained at a temperature a little above the highest at which it will be used, and the top is sealed.
(I) the thermometer must be placed vertically both in the hypsometer and funnel of ice, to ensure accurate marking of the points.
(II) the thermometer must be placed well above water, this is to ensure that the temperature of water is not recorded which depends on the temperature of impurities contained in water.
(III) The pressure of steam must be 76cmhg otherwise the marking would require a minor correction since steam point is the temperature of steam at pressure of the Mercury.
(C) thermometer and their thermometric substances
|Thermometers||Thermometric substance used||Physical properties of the thermometric substance|
|(I) Mercury in glass thermometer||Mercury||Increase in volume of Mercury with increase in temperature|
|(II) Gas thermometers||Gas||Pressure of gas at constant volume or volume of gas at constant pressure|
|(III) Thermo-electric||Current flowing||One junction of two dissimilar metals is heated|
|(IV) Resistance thermometers||Resistance of a metal||Increase in the resistance of a metal wire when the wire is heated|
Note: upper fixed point is the temperature of steam from pure water boiling at normal atmospheric pressure while the lower fixed point is the temperature of a mixture of pure ice and water at normal pressure.
(D) working principles of various thermometers
Liquid in glass thermometer: most widely used is the Mercury in glass thermometer; examples are the clinical thermometer and maximum thermometers.
Clinical thermometer is used in determining the temperature of the human body.it range lies between 35°C to 45°C as the normal body temperature of human beings ranges between the above range.
A clinical thermometer
Maximum and minimum thermometers; the maximum and minimum thermometers record the maximum and minimum temperatures over a period of time.
Combined maximum and minimum thermometer
Gas thermometers are basically used for very accurate temperature measurements. The operation utilised the fact that its temperature changes for a large volume change of gas.
Simple gas thermometer
Note: the temperature =
O°C O°C = P o – Po / P100 – Po × 100/1
Where Po = pressure at 0°C
P100 = pressure at 100°C
o = pressure at O°C
The working principle is based on the fact that for a metallic conductor, the electrical resistance changes proportionally with it’s temperature. One example is the Platinum resistance thermometer.
The temperature scale for this resistance is defined as;
O°C=R o – R0 / R100 – R0
R0= resistance of the Platinum coil at 0°C (ice point)
R100= resistance of the Platinum coil at 100°C (steam point)
o= resistance of the Platinum coil at temperature of O°C (unknown temperature)
Thermo-electric thermometers are used in industry for measuring very high temperatures.
Note: pyrometer is another type of thermometer, that utilizes the laws governing radiation from a hot body.
(E) temperature scales of thermometers and conversion from one temperature scale to another
Conversion from one temperature scale to another
Note: C – 0/ 100 – 0 = F – 0/ 100 – 0
(B)effects of heat on matter
Heat is a form of energy which flows due to temperature difference.
Effects of heat include the following;
- Chemical changes
- Change in physical property
- Change in temperature of the body,
- Change of state of the body,
- Expansion of the body,
- Change in pressure of the body
- Thermionic emission occurs (means emission of electron from metallic surface).
Note: other forms may change in size or assume a different form (e.g, one can change from solid to liquid, and liquid to vapour).
(F) thermal expansivity of solid and liquid
Experimental proof of solid expansion
Ball and ring experiment
Note: when the metal ball is heated for some minutes and made to pass through the ring, it is found that, it would no longer pass through the same ring it passed before heating. This is due to the expansion of the ball. If it is left to cool, and it is once more passed through the ring, it passes easily.
Effects of expansion in everyday life
- Railway lines and bridges
- Wires (telephone, PHCN, TV,etc).
Railway lines and bridges; gaps are left in between sections of rail on a railway line, it is to allow for expansion and contraction when the temperature increases or decreases respectively.
Expansion of a beige
Buildings; expansion or contraction of galvanized iron sheets used in roofing of some buildings, generates cracking noise, which is heard when one is under the roof of such Building.
In telegraph wires: to give room for expansion, the wires are given some allowances to allow for the expansion as well as contraction.
Applications of expansion
One of the most important applications of expansion is seen in a bimetallic strip, which is used in the construction of a thermostat, a device for maintaining a steady temperature. The thermostat is used in electric laundry irons, in refrigerators and hot water storage.
The bimetallic strip; it consists of two different metals riveted together
The bimetallic strip
Note: it is used in bimetallic strip thermometer, thermostat applied in laundry irons and refrigerators, the balance wheel of clocks and watches, etc.
Linear expansivity of a solid
The linear expansivity of a solid (metal) is defined as an increase in length per unit length for one degree rise in temperature. Mathematically defined as
= l2 – l1 / l1(
O2 – O1) K-1
Where l2, l1 are the final and initial lengths of the solid at final and initial temperature
O2 and O1 respectively.
Experimental determination of linear expansivity
Measurement of linear expansivity
Note: from definition of linear expansivity, one can quickly determine the value of linear expansivity, that is;
= l2 – l1 / l1(
O2 – O1)
Area and volume (cubic) expansivity of solid
The area (superficial) expansivity is given by = A2 – A1 / A1(
O2 – O1)
Similarly, the volume of cubic expansivity be given by; V2 – V1 / V1(
O2 – O1)
Note: real (absolute) cubic or volume expansivity of a liquid is defined as the increase in volume per unit volume per unit rise in temperature.
Apparent cubic or volume expansivity of a liquid is defined as the increase in volume per unit volume for a unit rise in temperature when the liquid is heated in an expansive vessel.
Heat can be transferred from one point to another, by any of these three modes: conduction, convection and radiation.
Conduction of heat: this is a process by which heat is passed along a metal object from one particle at higher temperature to another of lower temperature without the heated particle appearing to move.
Metal as a good conductor of heat
Convection: this is the process by which heat energy is transferred in a fluid (liquid or gas) by the actual movement of the heated particles.
Application of convection
- Origin of land and sea breeze
- The rise of smoke in a chimney
- House ventilation, motor car cooling system and domestic hot water system
Land and Sea breezes
Radiation is the process by which heat is transferred from its source to the body absorbing it without heating the intervening medium.
(H) Boyle’s law and its applications
Boyle’s law states that the volume of a fixed mass of gas varies inversely as its pressure, provided the temperature remains constant.
P1V1 = P2V2
Boyle’s law verification
Graph verification of. Boyle’s law
Application of Boyle’s law
- It is employed in determining the volume of a given mass of gas at constant temperature when the pressure is known
- It is also used in finding the pressure of a given mass of gas at constant temperature when the volume is known
Charles’s law and its applications
Charles’s law states that the volume of a fixed (given) mass of gas is directly proportional to its absolute temperature (T), provided the pressure remains constant.
V1 / T1 = V2 / T2
Experimental verification of Charles’s law and graph of volume against temperature at constant pressure.
Applications of Charles’s law
(I) absolute zero of temperature; this shows that as the temperature is reduced, the volume continues to diminish. Theoretically, the volume of the gas becomes zero at -273°C, but practically it is not so, for the gas liquifies before it gets to that temperature.
(II) the Kelvin temperature scale; the temperature on a Celsius scale can be converted to Kelvin simply by adding 273, thus, 0°C = 0 + 273K = 273K.
(III) cubic expansivity of a gas; it is defined as increase in volume
Original volume at 0°C × change in temperature
Pressure law or Gay lussac’s law
Pressure law or Gay lussac’s law states that the pressure of a fixed mass of a gas is directly proportional to its absolute temperature provided the volume is kept constant. P1 / T1 = P2 / T2
General gas law
The general gas law is derived from the combination of three laws; Boyle’s law, Charles’ law and pressure law
P1V1 / T1 = P2V2 / T2
(I) specific heat and thermal capacity of a body
The specific heat capacity of a substance is the quantity of heat required to raise the temperature of unit mass (1kg) of a substance through a degree rise in temperature (i.e 1°C or 1K)
Q = mc(
O2 – O1) > Quantity of heat
O2 – O1) > specific heat capacity
Heat capacity of a body is the quantity of heat required to raise the temperature of the entire body through one degree rise in temperature (1K). Unit of C is in joule per Kelvin (JK-1).
(J) latent heat
Latent heat of fusion is the heat energy required to convert a substance from its solid form to its liquid form without change in temperature.
Specific latent heat of fusion of a substance is the quantity of heat required to change the unit mass of a substance from solid to liquid without change of temperature. Unit of specific latent heat is joules per kilogram Jkg-1. Q=m×l
Where m= mass of substance (kg)
Q= quantity of heat (in joules)
The specific latent heat of vaporisation of a substance is the quantity of heat required to change unit mass of substance from liquid to vapour without change of temperature.
Factors influencing the rate of evaporation include:
(I) temperature: the rate of evaporation increases with temperature increase.
(II) pressure: the rate of evaporation decreases with increasing pressure.
(III) area of liquid surface exposed:the greater the surface area of liquid exposed, the more rapid will be the evaporation.
(IV) the nature of the liquid: different liquids evaporate at different rates; the lower the boiling point of a liquid, the greater will be the rate of evaporation.
(V) wind and dryness of air: wind blows away the water vapour around the material and causes more evaporation to take place.
If we heat a beaker containing some liquid, the liquid evaporates due to increases in temperature.
Difference between boiling and evaporation
|(I) this the change from liquid to vapour at temperature below normal boiling point.||(I) this is the change from liquid to vapour at the boiling point.|
|(II) occurs at all temperatures.||(II) occurs at fixed temperature at a given pressure.|
|(III) temperature during evaporation is not steady.||(III) temperature remains steady during boiling.|
|(IV) occurs only at the surface of liquid.||(IV) occurs throughout the whole liquid.|
(L) vapour pressure
Saturation vapour experiment
Variation of s.v.p with temperature
Note: a saturated vapour is a vapour that is in contact with its own liquid within a confined space.
Water vapour in the atmosphere
About 70% of the Earth’s surface is covered by water. Evaporation has caused the atmosphere to always contain some water vapour. Some water vapour include:
(I) dew: it is the temperature at which the water present in the air is just enough to saturate the air.
(II) mist: when moist air near the Earth’s surface and a few distance above it is cooled, the water vapour in it condenses and forms to tiny droplets which are suspended in air. These suspended water droplets are known as mist.
(III) cloud: cloud is a mass of small water droplets that float in the air. The Clouds are high up in the atmosphere.
(IV) humidity and relative humidity: the air that is moist is known as humidity while relative humidity is the ratio of the mass of water vapour present in a certain volume of air to the mass of water vapour required to saturate the same volume of air at the same temperature.
Relative humidity (R.H) =Mass of water vapour in a given volume of air ÷ mass of water vapour required to saturate the same volume of air at the same temperature
Or = s.v.p. of water at dew point
s.v.p. of water at original air temperature
1.Which of the following statements is not correct? (Wassce 1996)
A. Heat energy can be transformed into mechanical energy
B. That total heat content of a body is the sum of the kinetic energies of its molecules
C. If a body is heat, its molecules move faster
D. The total heat content of a body is the product of its specific heat capacity and its mass
E. If a body is cooled, molecular movement remains constant
2. The ice point of an ungraduated mercury in glass thermometer is X, while its steam point is 90°. This thermometer reads 60° when the true temperature is 40°C. Calculate the value of X. (Wassce 1999)
Solution: 90° steam point. 100°
60°. —————. 40°
X. —————. 0
Since the temperature is the same
= 60 – X/ 90 – X = 40 – 0/ 100 – 0
= 100(60-x) = 40(90-x)
=600-10x = 360-4x
= 600-360 = -4x+10x
x = 240/6 = 40°
3. Which of the following is the SI unit of heat capacity? (Wassce 2015)
Solution: heat capacity = H/
O = Jk-1
4. Thermal energy added or removed from a substance that changes the state of the substance is called. (Wassce 2017)
A. Latent heat
B. Heat of reaction
D. Specific heat
5. At a fixed point below a liquid surface, the pressure downward is P1 and the pressure upward is P2. It can be deduced that. (Jamb 2000)
A. P1 = P2
B. P1 > P2
C. P1 < P2
D. P1 > P2
6. 2kg of water is heated with a heating coil which draws 3.5A from a 200V mains for 2 minutes. What is the increase in temperature of the water? [Specific heat capacity of water = 4200Jkg-1K-1] (Jamb 2007)
Solution: I = 3.5A, v= 200, t= 2×60=120
O= IVT/ MC = 3.5×200×120/ 2×4200 =10°C
7. Calculate the temperature change when 500J of heat is supplied to 100g of water .[specific heat capacity of water = 4200Jkg-1K-1].(Jamb 2012)A. 12.1°C
O= Q/MC = 500/0.1×4200=1.2°C
8. The mode of heat transfer which does not require a material medium is. (Jamb 1986)A. Conduction
Solution: radiation is the transfer of heat from its source to the body. The body absorbs heat without heating a medium.
1 year ago
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