Chapter 4:physics:form 4
Chapter 4:physics:form 4
Chapter 4:physics:form 4
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
x 0 =
Chapter 4: Heat and Energy
l x l0 (100 0 ) l100 l 0
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where l1 = length on scale based on lower fixed point (typically 0C) l2 = length on scale based on higher fixed point (typically 100C) lx = length on scale based on temperature of object to be measured 1 = temperature of lower fixed point (typically 0C) 2 = temperature of higher fixed point (typically 100C) x = temperature to be calculated Note: You can replace the length with other thermometric properties, i.e. resistance, pressure, or e.m.f.
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
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 = 357C Minimum temperature that can be measured = -39C Expensive Poisonous
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 = 78C. Not suitable to measure high temperatures Minimum temperature that can be measured = 115C. More suitable to be used during winter Cheap Safe
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This thermometer is built based on the principle of the 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.
Resistance thermometer
Resistance thermometers have large ranges (from -260C to 1700C) so they are typically used to measure the temperatures of ovens and furnaces. Suitable for measuring fast-changing temperatures.
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Electromotive forces (e.m.f.) induced in thermocouples is directly proportional to the temperature gradient (temperature difference between the hot and cold junctions). The cold junction is maintained at 0C 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 -200C to 1700C so they can be used to measure the temperature of molten metal in furnaces. Very sensitive and suitable for fast-changing temperatures.
4.2
Heat capacity: An object that has a large heat capacity requires a bigger quantity of heat to raise the temperature to 1C Specific heat capacity: The amount of heat energy needed to raise the temperature of 1 kg of a material by 1C Q = mc where Q = heat energy [J] m = mass [kg] c = specific heat capacity of the material [J kg-1 C-1] = change in temperature [C]
4.2.1 Applications
Water is used as a coolant in car engines because Specific heat capacity is large, Easily obtained and cheap, 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.
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4.3
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
Q = mL where Q = heat energy [J] m = mass [kg] L = specific latent heat of the material [J kg-1]* * 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
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4.4
Evaporation Change of state from liquid to gas at any temperature below boiling point Happens only at the surface of the liquid Drop in temperature No physical signs
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Condenser outside a room Freon Vapour Condenser (to) liquid Latent heat is released Freon vapour is compressed Hot air is released
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4.5
Gas Laws
A closed container containing gas has: Fixed number of molecules Constant mass The gas behaviour is dependant on three variables: Pressure Volume Temperature
Note: For all gas law equations, the temperature involved must be absolute, i.e. in Kelvin T = + 273 where T = temperature [Kelvin] = temperature [C]
V =k T V1 V2 = T1 T2
where V = volume of the gas [m3] T = temperature of the gas [K]
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P =k T P1 P2 = T1 T2
where V = volume of the gas [m3] T = temperature of the gas [K]
PV =k T P1V1 P2V2 = T1 T2
Bicycle pump Gauge is pushed inwards air is compressed, so pressure increases in cylinder Air pressure in cylinder is greater than in bicycle tyre, so air flows into the tyre Car tyre Car tyre manufacturers recommend cold air pressure in tyres as 220 kPa although the tyre functions optimally at 280 kPa This is because as the car moves, friction heats up the air within the tyres As the temperature increases, the pressure increases Sponge cake When placed in an oven, a cake expands to twice its original size There are plenty of air bubbles in the cake As the temperature in the oven increases, the air bubbles expand Bubble wrap Bubble wrap is full of pockets of air bubbles As the air bubbles are compressed by the packaged items, the pressure increases The additional pressure is able to support and protect the fragile items
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