Using Heat Part 2

Science Journal Entry 32 Explain the advantages and disadvantages of thermal expansion.

Science Journal Entry 33 Explain the difference between a heat engine and a heat pump.

Looking Back In the previous power point, we talked about heat engines, the internal and external combustion engines that are used in steam engines and automobiles. We also went over how the four-stroke engine works. Then we talked about three ways to heat homes: fireplaces, central heating and hot-water heating. In this lesson, we will be dealing with more ways that homes are heated but also air conditioning and refrigeration.

Steam Heating Steam heating runs much the same way as hot-water heating only uses steam instead of hot water. Steam heating runs much the same way as hot-water heating only uses steam instead of hot water. Steam heating is used in older buildings or when there are many buildings to heat from one central location. Steam heating is used in older buildings or when there are many buildings to heat from one central location. 10: :24

Steam Heating System

Electric Baseboard Heating Electrical energy is converted to thermal energy by a conductor that is much like the heating element in a stove. The hot coil heats the air by conduction and radiation. Then convection circulates the warm air to heat the room.

Radiant Heaters A radiant heater is simply a “space heater”. They are usually small portable units. They operate much like electric baseboard heating but on a much smaller scale. Some radiant heaters circulate heat with a fan. A radiant heater is simply a “space heater”. They are usually small portable units. They operate much like electric baseboard heating but on a much smaller scale. Some radiant heaters circulate heat with a fan.

Forced-Air Heating A forced-air heating system uses fans to circulate warm air. The warm air enters through ducts in the floor and rises upward toward the ceiling. Then it cools and goes down the other side of the room through vents to return to the furnace. The circulation of heat by convection creates a convection current. One advantage of forced- air heating is that the air is cleaned as it passes through filters located in the furnace. A forced-air heating system uses fans to circulate warm air. The warm air enters through ducts in the floor and rises upward toward the ceiling. Then it cools and goes down the other side of the room through vents to return to the furnace. The circulation of heat by convection creates a convection current. One advantage of forced- air heating is that the air is cleaned as it passes through filters located in the furnace.

Forced-Air Heating System

Reversing the Flow According to the 2 nd Law of Thermodynamics, work has to be done to make heat that naturally flows from a substance at a higher temperature to a substance at a lower temperature to do the opposite and flow from lower to higher temperatures. According to the 2 nd Law of Thermodynamics, work has to be done to make heat that naturally flows from a substance at a higher temperature to a substance at a lower temperature to do the opposite and flow from lower to higher temperatures. A heat pump is a device that reverses the normal flow of thermal energy. A heat pump is a device that reverses the normal flow of thermal energy. 3:57 3:57

Cooling Through Evaporation Cooling Through Evaporation When a liquid evaporates, it absorbs heat in the process. You can see this when your sweat evaporates from your skin and your skin becomes cooler. You could also see this if you wipe alcohol on your skin. Sense alcohol evaporates at room temperature, it quickly evaporates leaving your skin cooler. When a liquid evaporates, it absorbs heat in the process. You can see this when your sweat evaporates from your skin and your skin becomes cooler. You could also see this if you wipe alcohol on your skin. Sense alcohol evaporates at room temperature, it quickly evaporates leaving your skin cooler. Heat pumps must do work on a refrigerant which is a fluid that vaporizes and condenses inside the tubing of a heat pump. Heat pumps must do work on a refrigerant which is a fluid that vaporizes and condenses inside the tubing of a heat pump.

How a Air Conditioner Works The compressor raises the temperature and pressure of the refrigerant turning it into a hot, high-pressure gas. The temperature of the condenser coil is higher than the outside air temperature, so heat flows spontaneously from the coil to the outside air. A fan increases the rate at which heat flows. The compressor raises the temperature and pressure of the refrigerant turning it into a hot, high-pressure gas. The temperature of the condenser coil is higher than the outside air temperature, so heat flows spontaneously from the coil to the outside air. A fan increases the rate at which heat flows. As thermal energy is removed from the coil, the refrigerant cools and condenses into a liquid. As thermal energy is removed from the coil, the refrigerant cools and condenses into a liquid.

Warm Air in Cold Air Out Warm Air in Cold Air Out The liquid refrigerant flows through the expansion valve and decreases in temperature. The liquid refrigerant flows through the expansion valve and decreases in temperature. As the cold refrigerant flows through the evaporator coil, it absorbs thermal energy from the warm room air. The fan sends the cold air back into the room. The refrigerant becomes a vapor and the process starts all over again. As the cold refrigerant flows through the evaporator coil, it absorbs thermal energy from the warm room air. The fan sends the cold air back into the room. The refrigerant becomes a vapor and the process starts all over again. 2:53 2:53

Air Conditioner

Central Cooling System

Basic Diagram of a Refrigerator

Refrigerator is a Heat Pump A refrigerator transfers thermal energy from the cold food compartment to the warm room. The motor moves refrigerant through the refrigerator walls. A refrigerator transfers thermal energy from the cold food compartment to the warm room. The motor moves refrigerant through the refrigerator walls. There are hot coils on the back and under a refrigerator that are moving thermal energy not only from the refrigerator compartment but also the waste heat that the motor generates. There are hot coils on the back and under a refrigerator that are moving thermal energy not only from the refrigerator compartment but also the waste heat that the motor generates. 6:05 6:05