Heat & Thermodynamics Chapter 16

Heat Heat is the transfer of thermal energy from one object to another because of a temperature difference. Heat flows spontaneously from hot objects to cold objects. Heat is Not a form of matter!!!

Temperature Temperature is related to the average kinetic energy of the particles in an object due to their random motions through space.

Temperature Temperature is a measure of how hot or cold an object is compared to a reference point. On the Celsius scale, the reference points are the freezing and boiling points of water. On the Kelvin scale, absolute zero is defined as a temperature of 0 kelvins. What is 20 C in Kelvins?

Temperature As an object heats up, its particles move faster, on average. The average kinetic energy of the particles increases. One way that heat flows is by the transfer of energy in collisions. On average, high-energy particles lose energy. Low-energy particles gain energy. Overall, collisions transfer thermal energy from hot to cold objects. This is why things heat up!!

Thermal Energy Thermal energy is the total potential and kinetic energy of all the particles in an object. Thermal energy depends on the mass, temperature, and phase (solid, liquid, or gas) of an object.

Thermal Energy Thermal energy depends on temperature. Compare a cup of hot tea with a cup of cold tea. In both cups, the tea has the same mass and number of particles. The average kinetic energy of particles is higher in the hot tea, so it has greater thermal energy.

Thermal Energy does not Equal Temperature Thermal energy depends on mass and temperature. The tea is at a higher temperature than the lemonade. The lemonade has more thermal energy because it has many more particles.

Expansion Thermal expansion is an increase in the volume of a material due to a temperature increase. Occurs when particles of matter move farther apart as temperature increases. Opposite for Thermal contraction

Thermometers As temperature increases, the alcohol in a thermometer expands, and its height increases in proportion to the increase in temperature.

Specific Heat Specific heat is the amount of heat needed to raise the temperature of one gram of a material by one degree Celsius. The lower a material’s specific heat, the quicker it becomes “heated”

Specific Heat In this formula, heat is in joules, mass is in grams, specific heat is in J/g•°C, and the temperature change is in degrees Celsius.

Solve! How much heat is absorbed by a 750-g iron skillet when its temperature rises from 25°C to 125°C? Answer: Q = m * c * ∆T = (750 g)(0.449 J/g•°C)(125°C – 25°C) = (750 g)(0.449 J/g•°C)(100°C) = 34 kJ

Calorimeter A calorimeter is an instrument used to measure changes in thermal energy. A sample is heated and placed in the calorimeter. The temperature change is observed.

Conduction Newton’s cradle helps to visualize conduction. One ball strikes the rest, and most of the kinetic energy is transferred to one ball on the end.

Conduction Conduction is the transfer of thermal energy with no overall transfer of matter. Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often. Occurs within a material or between materials that are touching.

Conduction Thermal Conductors A thermal conductor is a material that conducts thermal energy well. Tile is a better conductor than wood. A tile floor feels colder than a wooden floor when both floors are at room temperature. The tile transfers thermal energy more rapidly away from your skin.

Conduction Thermal Insulators A material that conducts thermal energy poorly is called a thermal insulator. Air is a very good insulator. Wool garments and plastic foam cups use trapped air to slow down conduction.

Convection Convection is the transfer of thermal energy when particles of a fluid move from one place to another. Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior.

Convection A convection current occurs when a fluid circulates in a loop as it alternately heats up and cools down. Air at the bottom of an oven heats up, expands, and becomes less dense. The hot air rises. Rising hot air cools as it moves away from the heat source. As a result, the coolest air is at the top of the oven.

Radiation Radiation is the transfer of energy by waves moving through space. All objects radiate energy. As an object’s temperature increases, the rate at which it radiates energy increases.

Laws of Thermodynamics The study of conversions between thermal energy and other forms of energy is called thermodynamics. James Prescott Joule (1818-1889) carefully measured the energy changes in a system.

1st Law The first law of thermodynamics states that energy is conserved. Energy cannot be created or destroyed, but it can be converted into different forms. Pushing on the pump does work on the system. Some of the work is converted into thermal energy, which heats the air in the pump and the tire.

2nd Law The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. Thermal energy flows spontaneously only from hotter to colder objects. A refrigerator must do work to transfer thermal energy from the cold food compartment to the warm room air.

Spontaneous = Disorderly Spontaneous changes will always make a system less orderly, unless work is done on the system. For example, if you walk long enough, your shoelaces will become untied. But the opposite won't happen; shoelaces don't tie themselves. Disorder in the universe as a whole is always increasing.

3rd Law The third law of thermodynamics states that absolute zero cannot be reached. This physicist uses a laser to cool rubidium atoms to 3 billionths of a Kelvin above absolute zero.

Heat Engines The two main types of heat engines are the external combustion engine and the internal combustion engine.

External Combustion External Combustion Engine A steam engine is an external combustion engine—an engine that burns fuel outside the engine. When the valve in a steam engine slides, steam is trapped in the cylinder. The steam expands and cools as it pushes the piston to the left. Hot steam in Slide valve Piston Cylinder Exhaust steam out Valve rod Piston rod

Internal Combustion Internal Combustion Engine An internal combustion engine is a heat engine in which the fuel burns inside the engine. Intake stroke Compression Stroke Power stroke Exhaust stroke Cylinder Piston Air-fuel mixture Intake valve Spark plug Exhaust valve Exhaust gases About one third of the energy in a gasoline engine is converted to work.

Heating your house Several Methods: Central Heating System The hot air rises as cooler, denser air in the room sinks. Several Methods: Central Heating System Hot-Water Heating Steam Heating Electric Baseboard Heating Forced Air Heating Cool air sinks Return vent Duct Furnace Hot air rises Supply vent Chimney

Cooling your house A heat pump is a device that reverses the normal flow of thermal energy. Heat pumps must do work on a refrigerant in order to reverse the normal flow of thermal energy. A refrigerant is a fluid that vaporizes and condenses inside the tubing of a heat pump.

Types of Cooling Refrigerators Air Conditioners Warm air out Warm air in Evaporator coil Liquid absorbs heat to become vapor. Cold air out Compressor Expansion valve Pressure drops, causing liquid refrigerant to become cold. Condenser coil Vapor cools to liquid as heat is removed.