Chapter 9 Heat 9 1 Temperature and Thermal

Chapter 9 Heat

9. 1 Temperature and Thermal Equilibrium Defining Temperature We often associate temperature with how hot or cold something feels to us. Determining an object’s temperature with precision requires a standard definition of temperature. A car engine for example operates at a temperature that would make water boil. This is normal for an engine.

Adding or removing energy usually changes temperature If you want to increase the temperature of a heating element you turn the knob to a higher setting. This allows more electricity to flow through the element causing it to get warmer. Since electricity is energy, it makes the element hotter.

Temperature is proportional to the kinetic energy of atoms and molecules Mater is made up of atoms and molecules. The faster these vibrate/move/rotate, the more kinetic energy they will have. Temperature is just the average kinetic energy of all of these particles. This atomic motion is known as internal energy. Our symbol for this energy is “U”. Video

Temperature is meaningful only when it is stable Imagine a can of juice immersed in ice water. Eventually the two will be the same temperature. To do this the can cools way down and the ice water warms up slightly. This final temperature or condition is known as thermal equilibrium. Thermometers do this as well. The bottom of thermometer changes to the temperature of whatever we are measuring.

Matter expands as its temperature increases Increasing the temperature of a gas causes its volume to increase. The same is true for liquids and gasses as well. Their increase in size is known as thermal expansion. Bridges are built so they can change their length from winter to summer. The quantity they expand by is their coefficient of volume expansion. The coefficients are largest for gasses and smallest for solids. Video

Measuring Temperature Thermometers work because they are able to thermally expand over a large range of temperatures. To do this they can use mercury or colored alcohol. The change in length of the liquid column is proportional to the temperature.

Calibrating thermometers requires fixed temperatures Thermometers are calibrated by marking their scales to correspond to known reference points. The most useful and universal points are the ice point and steam point. On the Celsius scale, these points are 0 degrees and 100 degrees. The spaced units, called degrees, match the linear expansion of the liquid.

Temperature units depend on the scale used The three most commonly used temperature scales are the Fahrenheit, Celsius and Kelvin. The US is one of the few places that still uses the Fahrenheit scale. Most nations that have adopted the metric system use the Celsius scale. The Kelvin scale is mostly used in science laboratories.

The Kelvin Scale uses the same increments as the Celsius scale but changes the starting point so there are no negative values. The starting point for the Kelvin scale is known as absolute zero. This is calculated by graphing the pressure of an ideal gas. 273. 15 degrees Kelvin is the same as 0 degrees Celsius.

Practice A Temperature Conversion What are the equivalent Celsius and Kelvin temperatures of 50. 00 F? First we convert Fahrenheit to Celsius… Answer 10. 00 C 283. 2 K

Questions kinetic 1. Temperature is the average ______ energy of all of the moving particles. 2. When two objects become the same temperature, they thermal are in _______ equilibrium. 3. When solids, liquids and gasses are heated, they expand. This is known as thermal _____. expansion 4. Thermometers are calibrated by marking their scales to reference correspond to known ____ points. 5. The starting point for the Kelvin scale is known as _____ zero. absolute

9. 2 Defining Heat and Energy To understand thermal process, we need to consider what is happening at the atomic level. As this juice cooled, the ice water warmed slightly until both were at the same temperature. This transfer of energy (from the juice to the ice water) is known as heat. Heat is also known as the process by which energy is transferred because there is a temperature difference.

Energy is transferred between substances as Heat With gravity we go from high to low, the same applies to heat. Energy will flow from a warmer object to a cooler one. Think of heat (energy transfer) like gravity, things go down hill. If you leave the refrigerator door open, you are not letting the cold out, you are letting the warm in.

At the atomic level, we see the atoms in the juice can (shown) are moving with a lot of kinetic energy, high temperature. These moving atoms make the aluminum sides of the can vibrate. The vibrating can walls then cause the ice water atoms to start to move faster (increasing their kinetic energy). The transfer of energy as heat is in one direction. Video

The transfer of energy as heat alters an object’s temp Thermal equilibrium can be understood in terms of energy exchange between two objects at equal temperature. The net energy transferred is zero “ 0”. The atoms of the water and juice are in continuous motion and therefore have some temperature. Because temperature is an average, some atoms may be moving faster than others. The amount of heat moving into this can is the same as the heat moving out.

Energy transfer depends on the temperature difference between two objects. The greater the temperature difference, the greater the rate of energy transfer between them as heat. Example (a) shows a difference of 25 degrees. Example (b) shows the same temperature difference or the same transfer of energy as heat.

Quick Experiment How warm is your hand? How warm is the air in this room? Now move your hand real quick, does it feel cooler? Why? What do you think the temperature of the metal chair leg is below you? Hold on to it, does it feel cold?

Heat has the units of energy Heat is like work, it is energy in transit. Because of this its units can be converted into joules. “U” will represent internal energy, similar to KE and PE. “Q” will represent heat. “W” will still be the same for work.

Thermal Conduction The handle of the skillet will become warmer as one atom absorbs and passes the energy of the flame onto the next atom and so on. Because these atoms are in close proximity to each other they pass energy slowly outward from the source. This known as thermal conduction. Some materials like iron skillets conduct heat well. The hotmit on his hand is a poor conductor. How can poor conductors useful?

The rate of thermal conduction depends on the substance. In general, metals are good thermal conductors. On the other hand, cork, ceramic and cardboard are good thermal insulators. They slow down or stop heat.

Convection and radiation also transfer energy Convection transfers energy within a moving fluid. As liquids or gasses are heated, their volume changes and therefore makes them buoyant. Warmer materials move upward, cooler dense materials sink back down.

Electromagnetic radiation can transfer energy without a medium like convection and conduction require. Radiation can even travel through the vacuum of space. You have probably noticed the long wave radiation of a fire, or the visible wavelengths sunshine.

Heat and Work is done in pulling a nail out of wood. The nail encounters friction as its pulled. The increase in internal energy raises the nail’s temperature. Bending metal takes energy. The internal friction will cause the bending metal to warm up.

Total energy is conserved We learned earlier that when friction is involved, mechanical energy was not conserved. We also saw that objects colliding inelastically did not conserve kinetic energy either. This is due to energy being absorbed internally by the objects, such as a nail warming up as its pulled. Total energy is a universally conserved property, the sum must equal zero when we include internal energy. Video

Practice B Conservation of Energy As we saw earlier, this arrangement was used to demonstrate energy conservation. As gravity (PE) pulled the ropes, they turned the paddles moving the water (KE). As the water was agitated, atomic friction caused the internal energy to increase, raising the water’s temperature. By using this formula, how much did the water’s internal energy increase in joules?

m = 11. 5 kg h = 1. 3 m Answer 150 J g = 9. 81 m/s 2

Questions 1. The transfer of energy is known as __A___. 2. Energy will flow from a warmer object to a __B___ one. 3. ___C___ equilibrium can be understood in terms of energy exchange between two objects at equal temperature, the net energy transferred is zero. 4. For thermal ____D____ to occur, the materials must be touching each other. 5. __E____ can only occur in liquids.

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