Heat and Heat Technology Chapter 10

What is Temperature? Temperature- A measure of the average kinetic energy of the particles in an object. Temperature measures the average kinetic energy of the object because individual particles are moving at different velocities and have different kinetic energies All particles are in constant motion (even if they don’t appear to be) The faster the particles are moving, the more kinetic energy they have because KE = mv2, the more velocity, the higher KE 2 The more kinetic energy they have, the higher the temperature. Solid vs. Liquid vs. Gas

Measuring Temperature Using a thermometer- As a substance’s temperature increases, it’s particles move faster and spread out. There is more space between them, and the substance expands. The only direction the molecules can expand is up Temperature is measured in three ways: Fahrenheit – Used in the US, based on salt water Celsius- Used in rest of the world. The freezing point and boiling point of water is separated into 100 even intervals Kelvin- used in science 0 K is the lowest temperature called Absolute Zero– all molecular motion stops. Lowest possible temp.

Measuring Temperature oC to oF oF = (9/5 x oc) + 32   See # 1 oF to oC oC = 5/9 x (oF – 32) See # 2 oC to K K = oC + 273 See # 3 K to oC oC = K – 273 See # 4

Temperature Conversions Convert 45 oC to oF Convert 68 oF to oC Convert 45 oC to K Convert 32 K to oC Convert 98.6oF to oC Convert 35oC to oF

More About Thermal Expansion Expansion joints in highway bridges Bridges heat up and expand – this can cause the bridge to break so joints are installed. Hot air balloons When the air inside is heated, it expands and becomes less dense. The balloon rises. Other examples

10.2 What is Heat? HEAT IS TRANSFERRED THERMAL ENERGY Thermal Energy- The energy transferred between objects that are at different temperatures. Objects can have the same temperature but different amounts of heat because HEAT is the TOTAL amount of kinetic energy but temperature is the AVERAGE amount of heat Which has more heat? 90oF cup of water or 90oF tub of water

Thermal energy is transferred Total kinetic energy of the particles that makes up the substance Energy is always transferred from the higher temperature to the lower temperature until they are the same temp. All drinks will always become room temp eventually. Hot tea and iced cold soda… Explain what happens to the tea… Explain what happens to the soda… Stethoscopes “feel” cold because energy is transferred away from your back to the stethoscope. They are room temp, but your body is way hotter. Heat flows out of your body- making it feel cold.

3 Ways to Transfer Thermal Energy 1. Conduction Thermal Conduction is the transfer of thermal energy from one substance to another substance through direct contact. Molecules need to be touching! which spoon would you rather touch? Thermal Conductor- Substances that can conduct thermal energy very well. They allow heat to flow through easily…Ex: Iron, tile…. Thermal Insulator- Substances that do not conduct thermal energy well. They don’t let heat flow through easily Ex: Plastic, styrofoam, wood

Ways to Transfer Thermal Energy Convection The transfer of thermal energy by the movement of a liquid or a gas. Warm air or liquid rises to the top because it’s less dense (high energy) At the surface, the warm air/liquid cools, condenses, and sinks back to the bottom. This cycle is called a convection current Ovens…

Ways to Transfer Thermal Energy Radiation The transfer of energy by electromagnetic waves such as visible light and infrared waves. This can occur in empty space. How energy gets from the sun to earth Does NOT need matter to transfer through

Radiation and the Greenhouse Effect Earth’s atmosphere acts like the windows of a greenhouse It allows the sun’s visible light to pass through it. A greenhouse also traps heat, keeping the inside warm. What are the greenhouse gases? What produces them? Is it colder on a clear night or a cloudy night?

Specific Heat Specific Heat-The amount of energy needed to change the temperature of 1 kg (or g) of the substance by 1°C The lower the specific heat, the less energy that is needed to change the temperature The higher the specific heat, the more energy that is needed to change the temperature Sand vs. water. Metal vs. water Wood vs. metal Air vs. water

Heat can’t be measured directly We need to use a formula. heat = mass x constant x change in temperature Q = heat (J) m = mass (g) c = specific heat (J/g·°C ) ΔT = change in temperature (°C) T2 – T1

Specific Heat Calculations How many joules does it take to heat 20. g of water from 10.0 to 40.0 oC? (Cp (water) = 4.184 J/g oC ) How much heat is required to heat 75 g of Iron (Cp = 0.444 J/gCo) from 15.5 to 57.0 oC? How much heat is required to heat 75 g of Iron (Cp = 0.444 J/gCo) from 15.5 to 57.0 oC

Specific Heat Calculations How many joules does it take to change the temperature of 15 g of water 25oC? (Cp (water)_ = 4.184 J/g oC ) Using the data table, how much heat is needed to change the temp of 25g of iron from 10oC to 90oC?

6.3 Matter and Heat States of Matter – the physical forms in which a substance can exist. The states of matter depend on the energy of the particles. Gases = high Kinetic energy = fast moving Liquids = medium Kinetic energy =slide past each other Solids = low kinetic energy – vibrate in place

Changes of State Change of State A change of a substance from one state of matter to another state of matter. PHYSICAL CHANGE!!! Include: melting, freezing, vaporization, condensation An ice cube that melts and then turns to a gas can be graphed on a temperature vs. energy graph.

Heat and Chemical Changes Calorimeter-a device that measures heat When one object transfers thermal energy to another object, the energy of the warmer object is absorbed by the other object. A calorimeter uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature. These devices are sealed to prevent heat from escaping. When a hot piece of metal is placed inside it will lose heat, when the temp of the water and metal are the same, heat will STOP flowing .

calorie- food is burned, energy released is transferred to the water, temp change of the water can be measured. 1 calorie of food =4.184J of energy