Heat and Temperature

Heat and Temperature Chapter 14 General Science Chapter 14

Temperature • What do you know about temperature? • Hot and cold can be used to describe temperature. • Heat is related to temperature, but they are not the same thing. General Science Chapter 14

Heat • The energy that flows from something with a higher temperature to something with a lower temperature. • Always flows from warmer to cooler • Heat is measured in joules General Science Chapter 14

Tiny moving particles • All matter is made up of tiny particles that are in constant motion. (Kinetic Theory) • The particles have kinetic energy. • The faster they move, the more kinetic energy they have. • Temperature is a measure of the average kinetic energy of the particles in a sample of matter. General Science Chapter 14

Temperature • Which particles are moving faster, those in a hot cup of coffee or those in a bowl of ice cream? • The coffee – higher temperature means more kinetic energy, which means that the particles are moving faster General Science Chapter 14

Temperature • We use thermometers to measure changes in temperatures. • Absolute Zero: Lowest temperature that an object can have (-273.15°C) • Common units for temperature are • Kelvin SI unit • Celsius • Fahrenheit General Science Chapter 14

Temperature conversions K = C + 273 C = K - 273 General Science Chapter 14

Heat and Phase Changes • Heat is the transfer of energy from a warmer temperature to a cooler temperature. • In some cases this temperature change will cause the substance to go through a phase change General Science Chapter 14

Phase Changes • Solid to a liquid ---- Melting • Liquid to a gas ---- Evaporation • Gas to a liquid ---- Condensation • Liquid to a solid ---- Freezing • When sufficient heat is added or removed from a substance (which slows or increases the motion of the particles) a phase change will occur General Science Chapter 14

Phase Changes • The temperature at which the phase occurs is called the melting point or freezing point, etc. • Melting point and Freezing point occur at the same temperature • It just depends on whether the substance is gaining heat or losing heat • Therefore evaporation point/boiling point and condensation point occur at the same temperature General Science Chapter 14

Heating curve for water General Science Chapter 14

Heating Curve • Important information for the heating curve • What is the independent variable? • What is the dependent variable? • Can you identify what phase a substance is in when looking at the heating curve? • Can you identify where phase changes occur on the heating curve? General Science Chapter 14

Discussion #1 • Why do ice cubes melt when you put them in your drink? • What is absolute zero and what is the value of absolute zero? • What does temperature measure? • What are the 3 temperature scales? General Science Chapter 14

Discuss #1 • What happens to molecules during a phase change? • Explain the parts of the heating curve. • Explain what is happening on the constant sloped lines on the heating curve. • What is a phase change and what are 3 examples? General Science Chapter 14

Conduction • The transfer of energy by direct contact of particles. • When particles collide, the faster moving one gives some of its energy to the slower moving one. General Science Chapter 14

Conduction • Can transfer energy through a given material or from one material to another. • Example: holding a metal spoon with one end in boiling water. • Can take place in solids, liquids, or gases. • Solids usually conduct heat better • particles are closer together General Science Chapter 14

Conductors • Good heat conductors – materials that transfer heat easily. • Metals • Poor heat conductors – don’t conduct heat easily • plastic • wood • glass • fiberglass General Science Chapter 14

Reducing heat flow • Good heat insulators reduce the flow of heat. • They are bad heat conductors • Gases such as air • Wood • Plastic • Glass • fiberglass General Science Chapter 14

Air pockets • Many insulating materials make use of small pockets of air inside them. The pockets are too small to allow convection currents to form, so they are good insulators. • Styrofoam coolers • Down blankets • Insulation for your house General Science Chapter 14

R-value • Resistance to heat flow • Higher values mean it is a better insulator. • You should use higher R-value materials in roofs and ceilings because more heat escapes upward by convection currents. General Science Chapter 14

Double-paned windows • Heat is lost through glass windows. • By putting a thin layer of air between two panes of glass gives a window a higher R-value. • High R-values keep heat inside in the winter and outside in the summer. General Science Chapter 14

Thermoses • Also called vacuum bottles. • Contain a double glass wall with a vacuum in the middle to prevent heat transfer. General Science Chapter 14

Fluid • Any material that can flow • Liquid or gas • Air is a common example General Science Chapter 14

Convection • The transfer of energy by the movement of matter • The particles move from one place to another, carrying the energy with them. • When a fluid is heated, the particles move faster. Since they can move, they do – and they spread out. • Fluids expand when heated. General Science Chapter 14

Heating water • When the water at the bottom gets hot, it expands, and becomes less dense. • The cooler, more dense water above it sinks and pushes the warm water up. • As the water rises, it becomes cooler and more dense, and moves towards the bottom again. General Science Chapter 14

Convection currents • This movement creates convection currents that transfer energy from warmer to cooler parts of the fluid. General Science Chapter 14

Radiation • The transfer of energy in the form of invisible rays. • Does not require matter to be present. • Radiant energy – energy that travels by radiation General Science Chapter 14

Energy from the sun • Radiant energy • it travels through mostly empty space to reach us. • Shiny materials reflect radiant energy, while dull materials absorb it. • Dark-colored materials absorb more radiant energy than light-colored materials. General Science Chapter 14

Discussion #2 • List the 3 types of heat transfer • What is a convection current? • What is the difference between an insulator and a conductor? • List 3 insulators and 3 conductors. • What does an R-value tell us? • What is the advantage of a double pane window? General Science Chapter 14

Discuss #2 • What is conduction? • What is radiation? • What is convection? • Why do birds fluff their feathers and mammals fluff their fur to keep warm? General Science Chapter 14

Specific Heat • Different materials require different amounts of energy to produce the same temperature change. • The specific heat (Cp) of a material is the amount of energy it takes to raise the temperature of 1 kg of the material 1 degree Celsius. • Units are J/kg∙°C • Also called heat capacity General Science Chapter 14

Specific heat • Water has a high specific heat, so it takes a lot of energy to raise its temperature. • That’s why the temperature of a lake or unheated swimming pool is always cooler than the temperature of the air around it. General Science Chapter 14

Using Specific heat • We can’t measure change in thermal energy directly. • However, we can measure the change in temperature and use the specific heat to calculate the change in thermal energy. General Science Chapter 14

Change in Thermal Energy General Science Chapter 14

Delta • The Greek letter D (delta) means “change in” • DT means change in temperature • Always take final temperature minus initial temperature. • When DT is positive, the object has increased in temperature and taken in heat. • When DT is negative, the object has decreased in temperature and given off heat. General Science Chapter 14

Example • Calculate the thermal energy change when 230 g of water warms from 12 °C to 90 °C. • Q = m∙Cp∙DT • Q = (0.230 kg)(4184J/kg∙°C)(90 °C – 12 °C) • Q = (0.230 kg)(4184 J/kg∙°C )(78 °C) • Q = 75061 J General Science Chapter 14

You try • A 3.1 kg block of aluminum cools from 35°C to 20 °C. What is the change in its thermal energy? • Q = (3.1 kg)(-15 °C)(920 J/kg∙°C) • Q= - 42780 J General Science Chapter 14

Discussion #3 • What do each of the symbols in the equation Q = m∙Cp∙DTstand for? • What substance has the larger specific heat? Water or Aluminum • Would 750 grams of Iron (Cp = 500) or 750 grams of Zinc (Cp = 800) require more energy to warm up? Explain • Would your answer above change if the substances were cooling down? Why? General Science Chapter 14

Stoves or fireplaces • Simplest heating systems • Fuel is burned in the area to be heated • Heat transferred to surrounding air by conduction, convection, and radiation. General Science Chapter 14

Radiator • Device with a large surface area • Heats surrounding air by conduction • Convection currents carry warm air throughout the room. General Science Chapter 14

Radiator (Hot water or Steam) • A fuel is burned to heat water. • The hot water travels through pipes to the radiator. • Or, the water is boiled and the steam travels through the pipes • The cooled water or steam returns to the furnace to heat again. General Science Chapter 14

Forced air • Fuel is burned to heat air. • A blower forces the warm air into a room. • Convection currents carry the warm air throughout the room. • Cool air returns to the furnace to heat again. General Science Chapter 14

Radiant Heat (electric or water) • Heating coils in the floor or ceiling are heated by electricity or with water. • Nearby air is heated by conduction. • Materials in the room are heated by radiation. General Science Chapter 14

Radiant Energy Examples • Pictures from Zach Wiltse General Science Chapter 14

Heat Pump • Heat energy is collected with a network of coiled pipes on the outside of the home. • Heat from the ground evaporates a liquid inside the coils. • The vapor moves to a condenser which increases the pressure to increase the temperature of the vapor further. General Science Chapter 14

Heat Pump • The heated vapor warms the surrounding air. • The heated air is moved through the building with a blower, similar to a forced air system. • As the vapor cools, it condenses to a liquid and is returned to network of pipes on the outside of the home to repeat the process General Science Chapter 14

Geothermal picture General Science Chapter 14

Heat Pump General Science Chapter 14

Solar Heating • Uses energy from the sun. • Two kinds of solar heating • passive • active General Science Chapter 14

Passive solar heating • Windows allow solar energy in. • It is absorbed as thermal energy by materials in the room. • Later, it is released to the room. General Science Chapter 14

Heat and Temperature