Chapter Eleven: Heat

Chapter Eleven: Heat • 11.1 Heat • 11.2 Heat Transfer

11.1 What is heat? • Heat is thermal energy that is moving. • Heat flows any time there is a difference in temperature. • Because your hand has more thermal energy than chocolate, thermal energy flows from your hand to the chocolate and the chocolate begins to melt.

11.1 What is heat? • Heat and temperature are related, but are not the same thing. • The amount of thermal energy depends on the temperature but it also depends on the amount of matter you have.

11.1 Units of heat and thermal energy • The metric unit for measuring heat is the joule. • This is the same joule used to measure all forms of energy, not just heat.

11.1 Heat and thermal energy • Thermal energy is often measured in calories. • One calorie is the amount of energy it takes to raise the temperature of one milliliter of water by one degree Celsius.

11.1 Specific heat • The specific heat is a property of a substance that tells us how much heat is needed to raise the temperature of one kilogram of a material by one degree Celsius. Knowing the specific heat of a material tells you how quickly the temperature will change as it gains or loses energy.

11.1 Why is specific heat different for different materials? • Temperature measures the average kinetic energy per particle. • Energy that is divided between fewer particles means more energy per particle, and therefore more temperature change. • In general, materials made up of heavy atoms or molecules have low specific heat compared with materials made up of lighter ones.

11.1 The heat equation

Solving Problems • How much heat is needed to raise the temperature of a 250-liter hot tub from 20°C to 40°C?

Solving Problems • Looking for: • …amount of heat in joules • Given: • V = 250 L, 1 L of water = 1 kg • Temp changes from 20°C to 40°C • Table specific heat water = 4, 184 J/kg°C • Relationships: • E = mCp(T2 – T1) • Solution: • E = (250L × 1kg/L) × 4,184 J/kg°C (40°C - 20°C) = 20,920,000 J • Sig. fig./Sci. not. 20,920,000 J = 2.1 x 107 J

11.2 Heat transfer • Heat conduction is the transfer of heat by the direct contact of particles of matter. • Conduction occurs between two materials at different temperatures when they are touching each other. Where is the heat energy conducted to and from in this system?

11.2 Heat transfer • Thermal equilibrium occurs when two bodies have the same temperature. • No heat flows in thermal equilibrium because the temperature is the same in the two materials.

11.2 Thermal conductors and insulators • Materials that conduct heat easily are called thermal conductors and those that conduct heat poorly are called thermal insulators. Is a down coat a conductor or an insulator?

11.2 Convection • Convection is the transfer of heat through the motion of matter such as air and water. • The hot water at the bottom of the pot rises to the top and replaces the cold water.

11.2 Convection • Convection is mainly what distributes heat throughout a room.

11.2 Thermal radiation • Heat from the Sun is transferred to Earth by thermal radiation. • All the energy the Earth receives from the Sun comes from thermal radiation. • The higher the temperature of an object, the more thermal radiation it emits.

11.2 Thermal radiation • Thermal radiation is also absorbed by objects. • The amount of thermal radiation absorbed depends on the surface of a material. • Dark surfaces absorb most of the thermal radiation they receive. • Silver or mirrored surfaces reflect thermal radiation.

11.2 Heat transfer, winds, and currents • A thermal is a convection current in the atmosphere. • When a surface, like a road absorbs solar radiation, it emits energy as heat. • The warmed air molecules gain kinetic energy and rise. • Colder air is forced aside and sinks.

11.2 Thermal radiation • There are giant convection currents in Earth’s atmosphere. • The global wind patterns and Earth’s rotation also cause surface ocean currents to move in large circular patterns.

Chapter Eleven: Heat