1Chapter 15Temperature, Heat and Expansion22Temperature• Temperature – the quantity that tells us how hot or cold something is• Temperature is measured in units called degrees (o). In the US, the scale we use is Fahrenheit and the metric scale is Celsius– Water freezes at 32 oFor 0 oC– Water boils as 212 oF or 100 oC• A more meaningful scale measure how hot something is to absolute zero – the coldest something can beHeat is a form of energy. Absolute zero is the point where the atoms/molecules of the material have no energy.33Heat• Heat – the energy that can be transferred from one object to another because of a temperature difference– Heat is only transferred to cooler objects– Cold is just the absence of heat• Internal energy (thermal energy) – the grand total of all the energies inside a substance– Objects do not contain heat, they contain energy– The unit for measuring this energy is called the calorie (the amount of energy needed to change the temperature of 1 g of water by 1 oC)Heat is the measurement of the average internal energy, not total energy (an object will be the same temperature even if it gains KE).Note that Calorie is 1000 calories or 1 kilocalorie.44Problem Solving: Heat• Which as more heat, a cup of hot coffee or an iceberg?–Most likely it will be the iceberg since, even though it has a lower temperature than the coffee, there is allot of it. So, having allot of something with a smaller average amount of energy often has more energy than a small amount of something with allot of energy.Remember, heat is measured from absolute zero. Since a cup of hot coffee is about 373 K and an iceberg is about 273 K, the coffee only has about 1/3 more energy than the iceberg. Therefore, it would only take 1 and 1/3 cup of iceberg to have the same energy as the cup of coffee.55Specific Heat Capacity•“The specific heat capacity of any substance is defined as the quantity of heat required to change the temperature of a unit mass of the substance by 1 degree”– Water has a specific heat of 1 calorie per gram• Specific heat is a measure of a material’s “thermal inertia”– The higher the specific heat, the slower it heats up and cools down; the more internal energy it can storeWater has one of the higher specific heat capacities.66Problem Solving: Specific Heat• Water has a higher specific heat than metal does. So, if you have equal masses of water and metal at the same temperature, which one is storing more heat?–The water is storing more energy because it takes more heat to raise a gram of water a certain amount than it does a gram of metal. So more heat went into bringing the water up to the same temperature as the metal.77Thermal Expansion• As the internal energy of an object increases, the more its atoms and molecules “jiggle” around• Because of this “jiggling,” the object tends to expand• Water has odd thermal expansion properties– Water has the least volume at 4 oC. That means that water then expands as it freezes! (That’s why ice cubes float) The “jiggling” of the atoms of a material stops at absolute zero.88Problem Solving: Thermal Expansion• What was the temperature of Lake Erie on the coldest day of the winter in 1884?–Since water expands as it freezes, we expect there to be a layer of ice on the surface, but the fishes are fine in the liquid water underneath. Water is densest at 4oC and because of this, it sinks to the bottom of the lake. Therefore, we expect the temperature at the bottom of any frozen lake to be 4oC.We also expect the temperature at the bottom of the lake to be near 4 oC in the summer too since it takes water so long to heat up and ALL of the water would have to heat up before the temperature at the bottom of the lake increases.9Chapter 16Heat Transfer1010Conduction• Conduction – the transfer of heat by direct contact• A piece of metal feels cool because it is absorbing heat from your hand. A wooden table at the same temperature does not feel as cool because it is not absorbing as much heat from your handThis is the primary method of heat transfer in solids.1111Convection• Gases and fluids transfer heat by convection, or the motion and mixing of fluids because of thermal expansion• We have experienced this before by watching water boil or noticing that the air higher up in a room is hotter than the air below• Winds are caused by convectionThis is the primary method of heat transfer in fluids and gases.1212Radiation• Heat from the sun is not brought to us by conduction or convection• Thermal energy can be carried by electromagnetic (EM) waves (light)• All materials can radiate heat this way (including you and me)• The frequency of the EM waves are proportional to the temperature of the objectT~fpeakA good emitter of radiant energy is also a good receiver.1313Problem Solving: Heat Transfer• How do thermoses prevent conduction?• How do thermoses prevent convection?• How do thermoses prevent loss of heat through radiation?–The thermos has double layer walls to keep heat from traveling directly through the wall (just like double pane windows)–There is a vacuum in between the thermos walls and a tight fitting lid to keep the steam in–Good thermoses are made of shiny metal the reflect radiation back into the substance in the thermos1414Newton’s Law of Cooling• An object will be in equilibrium with its surroundings when it has the same temperature– An object that cools in a cold room will slightly warm the room– An object that warms in a hot room will slightly cool the room• Newton discovered the the rate of cooling (or heating) depended on the temperature difference of the objects:T ~ cooling of Rate ∆Remember, it is the RATE of cooling.1515Problem Solving: Newton’s Law of Cooling• If a lukewarm cup of water is placed in the same room as a hot cup of water, we expect the hot cup to have a higher rate of cooling. Do we also expect the hot cup to reach room temperature first?–NO! The hot water is cooling faster, but it has more energy to lose than the lukewarm water. Therefore, the lukewarm water will still beat the hot water to room