Heat & measurement of Temperature Physics Notes GCE Study Buddy

Effects of heat Heat is a form of energy. Its SI units is joule, J. Expansion: Occurs when things are heated. Heat energy is gained by the body and results in an increase in size. Contraction: Occurs when things are cooled. Heat energy is lost by the body and results in a decrease in size.

Expansion of Solids Solids expand when heated Different solids expand at different rates

Expansion of liquids Liquids expand when heated. When heating water in a beaker, you will initially notice a dip in the water level before the water level rises. Different liquids expand at different rates.

Expansion of gases Gases expand when heated. For a same rise in temperature, gas expands more than liquid, and liquid expands more than solid.

Expansion and contraction of water As temperature increases from 4OC to 100 OC, water expands. But from 0OC to 4OC, water contracts, that is, its volume decreases. At 4OC, water has the lowest volume and therefore it has the highest density (since density = Mass/Volume and mass is constant).

Expansion problem in solids Railway – gaps are left along the rails to allow for expansion. Bridge – one end on the bridge is fixed while a gap is left at the other end supported by rollers. Pavement – small gaps between cement sections are filled with soft black bitumen.

Expansion problem in liquids and gases Bottles containing liquids are never completely filled. This allow space for expansion when the surrounding temperature is higher. Aerosol cans carry a warning - to be stored in cool places.

Application of expansion and contraction Riveting Rivets are applied when heated. Once cooled, they contract and bind the metal sheets together tightly. Bimetallic strip Bimetallic strips works on the principle that different metal expands / contracts at different rate for the same increase / decrease in temperature.

Application of bimetallic strip Heating element control Fire alarm

Application of expansion and contraction Hot air balloons Hot air balloons depend on the low density of the hot air to float. The air in the balloon is heated up which causes its volume to increase. Therefore the density of the hot air decreases. As the air in the balloon is less dense than the surrounding air, the balloon floats

Temperature, Melting and Boiling points Melting point is the temperature where solid changes to liquid Boiling point is the temperature where liquid changes to gas Temperature is a measure of how hot or cold a body is. Temperature of a substance is commonly measured in degree Celsius (ºC). SI unit : Kelvin (K) The temperature 0K is called the absolute zero because there is no other temperature below it At 0K, all types of motion of particles in matter will stop and the kinetic energy is zero

Thermometers Thermometers make use of a measurable physical property that varies with temperature. A Thermometer measures temperature in degree Celsius (oC). Ice point is the temperature at which pure ice melts at 0OC Steam point is the temperature at which pure water boils under normal conditions at 100OC. Liquid-in-glass – clinical thermometer Purpose – Measure body temp Range : 35oC – 42oC A constriction serves to prevent the mercury thread from going back into the bulb after the thermometer is removed from the mouth. Liquid-in-glass – laboratory thermometer Purpose – For Laboratory Use Range : -10oC – 110oC

Other thermometers Resistance thermometer Thermocouple thermometer Purpose: Lab use, resistance of wire changes with changes in temperature Range: -200°C to 1200°C Thermocouple thermometer Purpose: Lab use, strength of current varies with temperature Range: 100°C to 1100°C

Advantages of thermocouple thermometer The temperature range of a thermocouple depends on the type of metallic wires used Generally, thermometers have a very wide range, from -200°C to 1700°C The junctions have a low heat capacity so that they can respond quickly to a temperature change

Other thermometers Constant volume gas thermometer Pressure of the gas at constant volume varies with changes in temperature Range: 20 K to 1300 K Bimetallic thermometer Purpose – Measure hot oven/ molten metal Ranges: 0 to 400OC

Other thermometers Liquid crystal thermometer Chemical changes which bring about colour changes

Calibrating a thermometer Calibration means making a scale To calibrate a liquid-in-glass thermometer, the following steps are taken: Mark the lower fixed point, 0°C. The thermometer bulb is placed in pure melting ice. When the mercury meniscus is steady, its position is marked as 0°C Mark the upper fixed point, 100°C. The thermometer bulb is placed in a hypsometer so that it is surrounded by steam from boiling water at ordinary atmospheric pressure. When the mercury meniscus is steady, its position is marked as 100°C Divide the fundamental interval. The distance on the stem of the thermometer between 0°C and 100°C is measured and divided into 100 equal parts. Each part represents 1°C

Features of the laboratory thermometer The round glass of the stem acts as a magnifying glass, enabling the temperature to be read easily The sensitivity of the thermometer can be increased by The glass of the bulb can be made thin so that the rate of heat exchange between the liquid and its surroundings can be increased The narrow bore allows a big change in the length of the mercury thread when there is a slight change in volume of the liquid in the bulb The bulb is made bigger to increase the volume of liquid stored in it. This increase in volume leads to greater thermal expansion or contraction Liquids having a large thermal expansion like mercury and alcohol are used

Mercury vs Alcohol thermometers Silver Colourless, usually dyed red High thermal conductivity – can respond very quickly to temperature changes Low thermal conductivity – takes a longer time to reach its surrounding temperature Uniform expansion as the temperature increases uniformly Non-uniform expansion despite a uniform increase in temperature Does not wet glass Wets glass and gives inaccurate readings Boiling point at 360°C – high upper limit, able to measure high temperatures Boiling point at 78°C – low upper limit, unable to read high temperatures Freezing point at -39°C – unable to measure low temperatures Freezing point at -115°C – able to measure low temperatures Poisonous liquid Safe liquid Expensive Cheap Very dense liquid Light liquid Small thermal expansion – scale markings on the stem are very close to one anther, making reading difficult Large thermal expansion – scale markings on stem are far apart, making reading easy

Properties of clinical thermometer A short temperature range, from 35°C to 42°C A constriction in the capillary tube just above the bulb prevents the mercury in the thread from falling back into the bulb to allow reading

Question The length of the mercury thread in a thermometer is 2cm when it is in pure melting ice and 22cm when it is in steam above boiling water at ordinary atmospheric pressure. When the thermometer is placed in liquid X, the length of the mercury thread is 16cm. What is the temperature of the liquid X? Solution If the temperature of the liquid X is m, m = (14 x 100°C) / 20 = 70°C

Questions Resistance thermometers are normally used to measure the Boiling point of water Freezing point of water Melting point of metal Boiling point of tap water 2. A liquid-in-glass thermometer is more sensitive if it has shorter length Smaller bulb Thicker walled glass bulb Thinner walled glass bulb