Chapter 12 Celsius and Fahrenheit scales are the

Chapter 12

� Celsius and Fahrenheit scales are the two most commonly used scales. � They were both designed with reference to the freezing point and boiling point of water � Positions were marked on a thermometer for these two temperatures and graduates were made in equal increments � Celsius is used more often around the world

� The size of a Celsius degree is larger than that of a Fahrenheit degree by a factor of 9/5.

� Introduced by William Thompson (Lord Kelvin) � Not expressed with the word “degrees” � SI base unit for temperature � Based on the concept of absolute zero.

� Measure changes in thermometric properties � Several types exist including �Thermocouple �Electric resistance thermometers �Thermograph or thermogram � Type needed depends on goal of research and necessary procedure � Here, we will use a regular mercury or alcohol thermometer.

� Linear Expansion is the increase in one dimension of a solid due to an increase in temperature. � The length (Lo) of an object changes by an amount (∆L) when its temperature changes by an amount (∆T). α is the coefficient of linear expansion. α is a constant. This constant is a numerical description of the degree of thermal expansion solids will undergo when heated. Values can be obtained in your text (p. 365) or online.

� Thermal Stress – buildings using steel supports must account for expansion in order to prevent “buckling” of steel beams � Bimetallic Strip – used in coffee pots, thermostats, etc. � Expansion of Holes – A hole in a piece of solid material expands when heated and contracts when cooled, just as if it were filled with the material that surrounds it. � Holes with larger coefficients of linear expansion expand more than those in materials with smaller coefficients.

� It makes sense that if the length of an object expands when heated, the volume would too. � The volume (V 0) of an object changes by an amount (∆V) when its temperature changes by an amount (∆T). β is the coefficient of volume expansion.

� If water at 0°C is heated, its volume decreases until the temperature reaches 4°C. � Above 4°C, water behaves normally and volume will increase as temperature increases. � The density of water is greatest at 4°C. Click here to find out why volume of ice is greater than volume of water.

� Heat is energy that flows from a highertemperature object to a lower-temperature object because of the difference in temperatures. � SI Unit: joule (J) � Substances DO NOT contain heat. Instead, they contain internal kinetic energy due to the motion of their molecules or atoms. � Heat is only used to describe the energy in transit from one substance to another.

� The heat (Q) that must be supplied or removed to change the temperature of a substance of mass (m) by an amount (∆T) is • c is the specific heat capacity of the substance. • Basically, this constant describes how quickly or slowly a substance will change temperature 1 kcal = 4186 joules 1 cal = 4. 186 joules

� Remember: In a closed system, energy is neither created nor destroyed. � A calorimeter is a container used to determine the specific heat capacity of a substance. � In following the law of conservation of energy we can say that heat lost = heat gained.

� During phase changes, addition or removal of heat will not cause temperature changes. � Once the phase change is complete, the addition or removal of heat will cause a change in temperature.

� The heat (Q) that must be supplied or removed to change the phase of a mass (m) of a substance is • L is the latent heat of the substance. • m is the mass of the substance. • Q is the heat or energy of the substance. • Lf Latent Heat of F(change between solid and liquid. • Lv Latent Heat of Vaporization (change between liquid and gas) • Ls Latent Heat of Sublimation (change between solid and gas phases)

�Q is the symbol for heat. Heat is an exchange of energy. � In this chapter you were given several equations for heat (Q). � Remember that Q = Q and these equations can be set equal to one another in many cases. � Substance react differently to changes in temperature and experience different energy exchanges (heat) depending on their chemical composition.