Hoo Sze Yen
Physics SPM 2008
C HA HAP PTER 4: HEA HE A T A ND ENERG Y 4.1
The rma l Eq Eq uilib rium
Heat Object 1
Object 2 Heat
Thermal equilibrium is reached between two objects in contact when: • The net transfer of heat is zero (exchange rate of heat are equal) • The two objects have the same temperature
4.1.1 Thermo me tric tric p rop erties erties When an object is heated, the temperature of the object rises. The change in the physical properties in the object due to the change in temperature is known as thermometric properties. Thermometers are built based on the thermometric properties: Property Volume
Thermometer Mercury thermometer Alcohol thermometer Fixed-volume gas thermometer Resistance thermometer Thermocouple thermometer
Pressure of gas Resistance of metal wire Electromotive force
4.1.2 Calibration Calibration of a thermom eter To calibrate a thermometer, two extreme points must be chosen to mark its scale. These points must be able to be reproduced accurately. In the Celsius scale, the two fixed points po ints of temperature are: (i) ice point (0°C) – temperature of pure ice melting under standard atmospheric pressure (ii) steam point (100°C) – temperature of pure water boiling under standard atmospheric pressure To calculate the scale of a thermometer, given the two fixed points: θ x
Chapter 4: Heat and Energy
−
θ 0
=
l x l 100
−
l 0
−
l 0
×
(θ 100
θ 0
−
) Page 1 of 10
Hoo Sze Yen
Physics SPM 2008
where l 1 = length on scale based on lower fixed point (typically 0°C) l 2 = length on scale based on higher fixed point (typically 100°C) l x = length on scale based on temperature of object to be measured θ 1 = temperature of lower fixed point (typically 0°C) θ 2 = temperature of higher fixed point (typically 100°C) θ x = temperature to be calculated Note: You can replace the “length” with other thermometric properties, i.e. resistance, pressure, or e.m.f.
4.1.3 Liquid thermo m eter in g lass •
Typical liquid thermometers in glass use mercury or alcohol.
Design Shape Characteristics Glass bubble with the thin wall that reinforces it Small bubble size
Fine tube with a small diameter Thick and curved walls of the glass tube
Work Principles
The thin wall allows faster heat flow Small quantity of liquid in the small bubble allows the liquid to react quickly towards the change of temperature Small change of temperature will cause a large movement of liquid in the fine tube. Acts as a magnifying glass so that the meniscus of liquid column can be clearly seen
Mercury Expends evenly when heated Does not stick to glass Coloured (opaque and easily visible) Reacts quickly to changes in temperature Maximum temperature can be measured = 357°C
Minimum temperature that can be measured = -39°C Expensive Poisonous
Chapter 4: Heat and Energy
Purpose
Increases the sensitivity of the thermometer Increases the sensitivity of the thermometer
Increases the sensitivity of the thermometer Makes reading the temperature easier
Alcohol Does not expend evenly when heated Sticks to glass Colourless (Commonly coloured with a red dye) Reacts slowly towards changes in temperature
Maximum temperature that can be measured = 78°C. Not suitable to measure high temperatures Minimum temperature that can be measured = 115°C. More suitable to be used during winter Cheap Safe
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Hoo Sze Yen
Physics SPM 2008
4.1.4 Fixed- volume ga s thermo me ter
•
• •
This thermometer is built based on the principle of th e pressure of air increasing when temperature increases, provided the volume is constant ( see Pressure Law). A big volume of air is necessary to indicate a significant change in the pressure. Unsuitable for fluctuating temperatures because time is taken for mercury to stabilize.
4.1.5 Resistanc e thermom eter
Resistance thermometer •
•
Resistance thermometer circuit
Resistance thermometers have large ranges (from -260°C to 1700°C) so they are typically used to measure the temperatures of oven s and furnaces. Suitable for measuring fast-changing temperatures.
4.1.6 Thermoc ouple thermo me ter
Chapter 4: Heat and Energy
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Hoo Sze Yen •
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Physics SPM 2008
Electromotive forces (e.m.f.) induced in thermocouples is directly proportional to the temperature gradient (temperature difference between the hot a nd cold junctions). The cold junction is maintained at 0°C so that the millivoltmeter reading is zero. The hot junction is placed at the object to be measured. The hot junction is small, so it can be used to measure objects in places that are hard to reach. Thermocouples have ranges from -200°C to 1700°C so they can be used to measure the temperature of molten metal in furnaces. Very sensitive and suitable for fast-changing temperatures.
4.2
Hea t Capa c ity a nd Spe c ific Hea t Capa c ity
Heat capacity: An object that has a large heat capacity requires a bigger quantity of heat to raise the temperature to 1°C Specific heat capacity: The amount of heat energy needed to raise the temperature of 1 kg of a material by 1°C
Q = mcθ where Q = heat energy [J] m = mass [kg] -1 -1 c = specific heat capacity of the material [J kg °C ] θ = change in temperature [°C]
4.2.1 Applications •
•
•
•
Water is used as a coolant in car engines because ¾ Specific heat capacity is large, ¾ Easily obtained and cheap, ¾ Does not chemically react with the materials in the engine. Cooking utensils (woks, pots) are usually made of material with low specific heat capacity to ensure temperature increases quickly when heated. Handles are made of material with high specific heat capacity and are poor conductors. Clay pots are made of clay with high specific heat capacity and are poor conductors. When removed from heat, the soup inside the pot will continue to boil as heat is still being received from the pot.
Chapter 4: Heat and Energy
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Hoo Sze Yen
4.3
Physics SPM 2008
La tent Hea t a nd Sp ec ific La tent Hea t
Latent heat: Heat that is absorbed during the change of state of the material (solid liquid ↔ gas)
↔
Specific latent heat: The amount of heat needed to change the state of 1 kg of a material
Latent heat of vapourization
Latent heat of fusion
Q = mL where Q = heat energy [J] m = mass [kg] -1 L = specific latent heat of the material [J kg ]* * Specific latent heat of fusion – for melting/freezing Specific latent heat of vapourization – for boiling/condensation
4.3.1 Applications • • • •
Cooling drinks with ice Ice packages to keep cooled food cold Cooking by steaming Heating drinks using steam
Chapter 4: Heat and Energy
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Hoo Sze Yen
4.4
Physics SPM 2008
Eva p oration a nd Boiling
Evaporation Boiling Change of state from liquid to gas at any Change of state from liquid to gas at temperature below boiling point boiling point Happens only at the surface of the liquid Happens throughout the entire liquid Drop in temperature No change in temperature No physical signs Bubbles formed in liquid
4.4.1 Fac tors that affec t eva po ration Factor
The surface area of liquid exposed increases Temperature of liquid increases The air moisture of the surrounding environment reduces Air or wind blows on the surface of the liquid Air pressure reduces
Evaporation Rate Increasing
Increasing
Increasing
Increasing Increasing
Explanation
More molecules at the water surface can be released to the air. Molecules contain more kinetic energy so it is easier to break the bonds and be released to the air as vapour Less vapour molecules in the air restrict movement of liquid molecules from the liquid to the air Molecules that are released from the liquid surface are carried to the air Liquid molecules are easily released to the air
4.4.2 Fac tors that a ffec t b oiling Factor
Foreign objects are diluted into the liquid Air pressure increases Heating rate increases
Effects to boiling Boiling point increases
Boiling point increases Boiling rate increases
Chapter 4: Heat and Energy
Explanation
Some of the heat is absorbed by the foreign objects, so less heat is absorbed by the liquid to enable boiling to happen Harder for molecules to move from liquid to air Energy is supplied to the molecules at a higher rate. More bonds between the molecules can be broken per second
Page 6 of 10
Hoo Sze Yen
Physics SPM 2008
4.4.3 Ap plic ations of evap oration •
•
•
Refrigerator ¾ Freon liquid flows through an expanding valve and expands into an evaporation pipe. The expansion causes the temperature of Freon o liquid to drop about -30 C. the temperature in a ¾ Because o refrigerator (-10 C) is higher than the o Freon liquid temperature (-30 C), the Freon liquid evaporates at a higher rate and absorbs heat from air around the refrigerator. This cools the area within the refrigerator. ¾ Freon vapour flows to a compressor pump that is below the refrigerator. ¾ The pump compresses Freon vapour and pushes it to a cooling strip on the outside of the refrigerator. ¾ The hot compressed Freon vapour releases heat to the surrounding air and is condensed again into liquid. ¾ The Freon liquid flows to an expending valve to repeat the same process again. Air-conditioner Evaporator in a room Evaporation of Freon liquid (to) vapour Latent heat is absorbed Freon liquid expends Cold air is released
Condenser outside a room Freon Vapour Condenser (to) liquid
Latent heat is released Freon vapour is compressed Hot air is released
Reduce temperature of people with fever
Chapter 4: Heat and Energy
Page 7 of 10
Hoo Sze Yen
Physics SPM 2008
4.4.4 Ap plic ations of bo iling •
•
•
Fractional distilling ¾ Petroleum is made from a mixture of hydrocarbon that is complex and can be separated to different parts through the process of distilling by stages because every part has a different boiling point. staged distilling, ¾ During petroleum is heated in a staged distilling tower. ¾ Vapour with a lower boiling point will rise to the top portion of the tower, then condensed and separated. ¾ Vapour with a higher boiling point will collect at the lower portion of the tower and condense to form a liquid. Distilling under high pressure ¾ When the pressure of water is 2 times the atmospheric pressure, water boils at a o temperature of about 120 C. ¾ In a pressure cooker, water that is boiled at a very high temperature allows the food inside to be cooked in a shorter amount of time. ¾ Water that is boiled at a temperature that is higher in an autoclave is used to sterilise surgical equipment at a hospital. Boiling under low pressure ¾ Lower atmospheric pressure at high altitudes causes water to boil at a temperature o that is lower than 100 C. A lower quantity of heat energy is required to boil water when the boiling point is reduced. ¾ At a sugar producing factory, a sugar dilution is created by mixing sugar cane o with water. This liquid is boiled at 60 C under low pressure until sugar crystals are obtained. This method saves energy and cost. Boiling at a lower temperature does not cause sugar to change in colour. ¾ In the process of making evaporated milk, water is evaporated from the milk under low temperature. Boiling at a lower temperature ensures that the nutrients and vitamins in the milk are not destroyed.
Chapter 4: Heat and Energy
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Hoo Sze Yen
4.5 •
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Physics SPM 2008
Gas Laws
A closed container containing gas has: ¾ Fixed number of molecules ¾ Constant mass The gas behaviour is dependant on three variables: ¾ Pressure ¾ Volume ¾ Temperature
Note: For all gas law equations, the temperature involved must be absolute, i.e. in Kelvin T = θ + 273 where T = temperature [Kelvin] θ = temperature [°C]
4.5.1 Boyle ’s La w Boyle’s Law: For a gas of fixed mass, the pressure is inversely proportional to its volume if the temperature is constant.
PV = k P 1V 1 = P 2V 2 where P = pressure of the gas [Pa] 3 V = volume of the gas [m ]
4.5.2 Charles’ Law Charles’ Law: For a gas of fixed mass, the volume is directly proportional to its temperature if the pressure is constant.
V T
=
k V 1 T 1
=
V 2 T 2
3
where V = volume of the gas [m ] T = temperature of the gas [K]
Chapter 4: Heat and Energy
Page 9 of 10
Hoo Sze Yen
Physics SPM 2008
4.5.3 Pressure Law Pressure Law: For a gas of fixed mass, the pressure is directly proportional to its temperature if the volume is constant.
P
= k T P 1 P 2
T 1
=
T 2
where V = volume of the gas [m3] T = temperature of the gas [K]
4.5.4 Universal Gas Law Combining all three gas laws: PV
= k T P 1V 1 P 2V 2
T 1
=
T 2
4.5.5 Ap plic ations of ga s laws •
•
•
•
Bicycle pump ¾ Gauge is pushed inwards – air is compressed, so pressure increases in cylinder ¾ Air pressure in cylinder is greater than in bicycle tyre, so air flows into the tyre Car tyre ¾ Car tyre manufacturers recommend cold air pressure in tyres as 220 kPa although the tyre functions optimally at 280 kPa ¾ This is because as the car moves, friction heats up the air within the tyres ¾ As the temperature increases, the pressure increases Sponge cake ¾ When placed in an oven, a cake expands to twice its original size ¾ There are plenty of air bubbles in the cake ¾ As the temperature in the oven increases, the air bubbles expand Bubble wrap ¾ Bubble wrap is full of pockets of air bubbles ¾ As the air bubbles are compressed by the packaged items, the pressure increases ¾ The additional pressure is able to support and protect the fragile items
Chapter 4: Heat and Energy
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