Relate temperature to the kinetic energy of atoms and molecules. Describe the changes in the temperatures of two objects reaching thermal equilibrium. Identify the various temperature scales, and convert from one scale to another.
When you hold a glass of lemonade with ice, you feel a sharp sensation in your hand that we describe as “cold.” Likewise, you experience a “hot” feeling when you touch a cup of hot chocolate.
We often associate temperature with how hot or cold an object feels when we touch it. Our sense of touch serves as a qualitative indicator of temperature. However, this sensation of hot or cold also depends on the temperature of the skin and on the properties of the object; therefore it can be misleading. So determining an object’s temperature with precision requires a standard definition of temperature and a procedure for making measurements that establish how “hot” or “cold” objects are.
What Is Temperature? The temperature of a substance is a measure of the average kinetic energy of the particles of a substance. When energy is added to a substance its temperature increases cause the added energy is distributed among the particles of the substance.
Temperature & Particles Speed
The lower the temperature The Slower the particles of a substance The higher the temperature The faster the particles of a substance The lower the temperature The Slower the particles of a substance
Internal energy Particles of a substance move, than they have kinetic energy. Particles of a substance interact with each other (attract, repel), than they have potential energy. The sum of KE and PE of all the particles of a substance is called internal energy. The symbol for internal energy is U.
Thermal equilibrium When a warmer and a cooler substance are put in physical contact, the warmer substance cools down and the second one warms up. Eventually, both substances will reach the same temperature. This temperature will not change over the time. We say then that the substances are in thermal equilibrium with each other. The temperature of any two objects in thermal equilibrium always lies between their initial temperatures.
Thermal equilibrium is the basis for measuring temperature with thermometers By placing a thermometer in contact with an object and waiting until the column of liquid in the thermometer stops rising or falling, you can find the temperature of the object. The reason is that the thermometer is in thermal equilibrium with the object.
Thermal expansion When the temperature of a substance increases, its volume increases; this is known as thermal expansion. Different materials undergo different amounts of expansion for the same temperature change. Gases expand the fastest; solids expand the slowest.
Thermal Expansion
Bridge Sections
Train Rails
Water is an exception; it expands as it is cooled down between 4°C and 0°C.
Measuring temperature In order for a device to be used as a thermometer, it must make use of a change in some physical property that corresponds to changing temperature, such as the volume of a gas or liquid, or the pressure of a gas at constant volume. For example, a liquid (mercury, alcohol, etc.) inside a thermometer when heated, it expands.
Calibration of A Thermometer To make the scale of temperature for a thermometer, we need to choose two reference temperatures (or two fixed points). Then the range between the fixed points is divided into equal divisions or degrees.
Celsius scale (Used by the scientific committee worldwide) The lower fixed point is got when the thermometer is put in pure melting ice. This point or this temperature is taken as 0°C. The upper fixed point is got when the thermometer is put above pure boiling water. This point is taken as 100°C.
Calibrating a Thermometer
Fahrenheit scale (Commonly used in the united states) The ice point on the Fahrenheit scale 32.0 degrees
Temperature values in the Celsius and Fahrenheit scales can have positive, negative, or zero values.
Kelvin scale (Absolute temperature) Absolute temperature is proportional to the kinetic energy of the atoms in a substance. Kelvin scale has only positive values.
The graph of pressure versus temperature above is for an ideal gas. As the gas’s temperature decreases, so does its pressure. The graph suggests that if the temperature could be lowered to −273.15°C, the pressure of the sample would be zero. This temperature is designated in the Kelvin scale as 0.00 K, where K is the symbol for the temperature unit called the kelvin.
Temperatures in the Kelvin scale are indicated by the symbol T. A temperature difference of one degree is the same on the Celsius and Kelvin scales. The two scales differ only in the choice of zero point. Thus, the ice point (0.00°C) equals 273.15 K, and the steam point (100.00°C) equals 373.15 K.