## Ch16

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## Ch16

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ch 16 study guide

- Pages:
- 10
- School:
- University of Houston
- Course:
- Phys 1302 - Introductory to Physics II

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Chapter 16 Temperature and Heat 1 Temperature and the Zeroth Law of Thermodynamics Heat is the energy that is transferred between objects because of a temperature difference If heat can flow between two objects they are in what is called thermal contact Thermal contact is not the same as physical contact heat can be transferred without two objects physically touching When objects are in thermal contact the transfer of heat stops when the objects are in thermal equilibrium When two objects are in thermal equilibrium they have the same temperature Zeroth Law of Thermodynamics If two objects are in thermal equilibrium with a third system they are in thermal equilibrium with each other 2 Temperature Scales Celsius water freezes at 0 C and boils at 100 C Fahrenheit water freezes at 32 F and boils at 212 F Conversion between Fahrenheit and Celsius 9 TF TC 32 5 5 TC TF 32 9 L Whitehead 1 1 2 Phys 1302 Convert 40 C to Fahrenheit 9 TF 40 32 104 F 5 Experiments show that there is a lowest temperature below which it is not possible to cool an object This lowest temperature is called absolute zero The pressure of a gas has a depends on its temperature This relationship can be mapped out in the lab for different gases If you extrapolate the curve back to the point where the pressure of the gas would be zero you find that zero pressure happens at the same temperature for any gas That temperature is defined as absolute zero Kelvin The Kevin scale is set so that 0 K is exactly absolute zero The size of a Kelvin degree is the same as the size of a Celsius degree i e a change of 1 degree Kelvin is equivalent to a change of 1 degree Celsius The conversion between Celsis and Kelvin T TC 273 15 3 The degree symbol is not used with the Kelvin scale What is absolute zero in degrees Fahrenheit 0 TC 273 15 TC 273 15 C 9 TF 273 15 32 5 TF 459 67 F 3 Thermal Expansion Most materials expand when heated L Whitehead 2 Phys 1302 A rod of length L0 will expand by an amount proportional to the temperature change when heated or cooled The length change depends on the coefficient of linear expansion is different for different materials L L0 T 4 Steel rails for a train track are laid in a region subject to extremes of temperature The rails are laid in sections that are 5 2 m long How much longer is each section when the temperature is 50 C compared to when to the temperature is 10 C The change in temperature is T 50 C 10 C 60 C 60K L L0 T 12 10 6 K 1 5 2m 60K 3 7 10 3 m 3 7 mm A bimetallic strip consists of two metals bonded together to form a linear strip Two different metals will generally have different coefficients of linear expansion When the strip is heated or cooled one side of the strip will expand or contract more than the other Thus the shape of the strip the amount that is curves depends on the temperature Many thermostats have a bimetallic strip to turn on or shut off a heater If a metal square with side length L is heated the area will expand A0 L L L L L L T L L T L2 2 L L T L T 2 L2 2 L2 T 2 L2 T 2 L Whitehead 3 Phys 1302 For typical changes in temperature T is small which means the term proportional to 2 T 2 is typically negligible Let A L2 be the original area A0 L2 2 L2 T A0 A 2 A T A0 A 2 A T A 2 A T 5 The above applies to any area not just a square What about a washer a metal disk with a hole in the middle The hole actually expands along with the washer with the same coefficient of expansion Similarly the empty volume within a container expands when the container is heated The change in volume is given by V V T 3 V T 6 where is called the coefficient of volume expansion can be approximated by 3 Suppose we heat a copper container 17 10 6 K 1 of volume 150 cm3 that is completely filled with oil 0 68 10 3 K 1 by 10 C How much oil spills out The volume of the copper container and the volume of oil both expand If the volume of the oil expands more than the volume of the container oil will spill out The amount that spills will be the difference between the volume expansions T 10 C L Whitehead 4 Phys 1302 10 K Vcopper 3 V T 3 17 10 6 K 1 150cm3 10K 0 076cm3 Voil V T 0 68 10 3 K 1 150cm3 10K 1 02cm3 Vspill Voil Vcopper 0 94cm3 The density of water density is mass per volume reaches a maximum at about 4 C That means if you heat water from 0 to 4 C it actually contracts becomes more dense instead of expanding Above 4 C water expands with heating like any normal liquid When temperatures drop in winter the surface waters of a lake cool first and sink because they become more dense as they get cooler The warmer water rises to the top and gets cooled Eventually all the water is about 4 C At that point when the water gets cooler it actually rises becomes less dense So after 4 C the coolest water is on top and the top of the lake freezes first This creates an insulating layer and the water at the bottom of the lake doesn t freeze it stays at about 4 C If water didn t have this property lakes would freeze from the bottom up If it was cold enough the lake could freeze solid meaning fish wouldn t survive L Whitehead 5 Phys 1302 4 Heat and Mechanical Work Heat is just another form of energy and must be taken into account when applying conservation of energy One unit of heat is the calorie cal one kilocalorie 1 kcal 1000 cal was defined as the amount of heat it took to raise the temperature of 1 kg of water from 14 5 C to 15 5 C The mechanical equivalent of heat the equivalent mechanical work of one calorie of heat transfer is 1cal 4 186J 7 The unit that you are familiar with from nutritional information is the Calorie capital C which is equivalent to 1 kilocalorie lowercase c Another unit for measuring heat is British thermal units Btu 1 Btu is defined as the energy required to heat 1 lb of water from 63 F to 64 F 1Btu 0 252kcal 1055J 8 Suppose a 65 kg person climbs 100 stairs with each stair 20 cm high How many calories do you burn The work you are doing against gravity is converted to heat energy The total 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