Temperature and Energy

• Temperature is a measure of how hot (or cold) something is.
• Specifically, it is a measure of the average kinetic energy of the particles in an object. As the average kinetic energy of an object increases, its temperature will increase.
• A thermometer is an instrument that measures and indicates temperature.
• Fahrenheit and Celsius are common scales used for measuring temperatures.
• On the Fahrenheit scale, water freezes at 32ºF and boils at 212ºF.
• The Celsius scale, which is widely used in science, gives a value of 0ºC to the freezing point of water and a value of 100ºC to the boiling point of water at standard atmospheric pressure.
• Fahrenheit-Celsius Conversion Equations
• A degree Celsius is 1.8 times as large as a degree Fahrenheit. Also, the temperature at which water freezes differs for the two scales by 32 degrees.

T= T f-32/1.8 T F = 1.8t = 32

T F = Fahrenheit temperature

t = Celsius temperature

• The Kelvin scale is based on absolute zero.
• Absolute zero is the temperature at which molecular energy is at a minimum (0 K on the Kelvin scale or –273.16ºC on the Celsius scale).
• Celsius-Kelvin Conversion Equation
• T K = t + 237

• Temperature changes indicate an energy transfer.
• Heat is the energy transferred between objects that are at different temperatures.
• The transfer of energy as heat always takes place from a substance at a higher temperature to a substance at a lower temperature.
• For example, if you hold a glass of ice water in your hands, energy will be transferred as heat from your hand to the glass.

The transfer of heat energy from a hot object can occur in three ways:

• Thermal conduction is the transfer of energy as heat through a material.
• Convection is the movement of matter due to differences in density that are caused by temperature variations.
• Radiation is the energy that is transferred as electromagnetic waves, such as visible light and infrared waves.

Thermal Conduction

• Conduction involves objects in direct contact.
• Conduction takes place when two objects that are in contact are at unequal temperatures.

Convection

• Convection results from the movement of warm fluids.
• During convection, energy is carried away by a heated fluid that expands and rises above cooler, denser fluids.
• A convection current is the vertical movement of air currents due to temperature variations.

Radiation

• Radiation is energy transferred as heat in the form of electromagnetic waves.
• Unlike conduction and convection, radiation does not involve the movement of matter.
• Radiation is therefore the only method of energy transfer that can take place in a vacuum.
• Much of the energy we receive from the sun is transferred by radiation.

• Any material through which energy can be easily transferred as heat is called a conductor.
• Poor conductors are called insulators.
• Gases are extremely poor conductors.
• Liquids are also poor conductors.
• Some solids, such as rubber and wood, are good insulators.
• Most metals are good conductors.
• Specific heat describes how much energy is required to raise an object’s temperature.
• Specific heat is defined as the quantity of heat required to raise a unit mass of homogenous material 1 K or 1°C in a specified way given constant pressure and volume.

Energy = (specific heat) X (mass) X (temperature change)

Heating and Cooling

• Heating a house in the winter, cooling an office building in the summer, or preserving food throughout the year is possible because of machines that transfer energy as heat from one place to another.
• These machines operate with two principles about energy that you have already studied:
• The first law of thermodynamics states that the total energy used in any process—whether that energy is transferred as a result of work, heat, or both—is conserved.
• The second law of thermodynamics states that the energy transferred as heat always moves from an object at a higher temperature to an object at a lower temperature.
• Most heating systems use a source of energy to raise the temperature of a substance such as air or water.
• The human body is a heating system. Some of the energy from food is transferred as heat to blood moving throughout the human body to maintain a temperature of about 37°C (98.6°F).
• In central heating systems, heated water or air transfers energy as heat.
• Solar heating systems also use warmed air or water.
• In the solar system, a solar collector uses panels to gather energy radiated by the sun.
• This energy is used to heat water that is then moved throughout the house.
• This is an activesolar heating system because it uses energy from another source, such as electricity, to move the heated water.
• In a passive solar heating system, energy transfer is accomplished by radiation and convection.
• In this example, energy from sunlight is absorbed in a rooftop panel.
• Pipes carry the hot fluid that exchanges heat energy with the air in each room.
• When energy can be easily transformed and transferred to accomplish a task, such as heating a room, we say that the energy is in a usable form.
• After this transfer, the same amount of energy is present, according to the law of conservation of energy. Yet less of it is in a form that can be used.
• In general, the amount of usable energy always decreases whenever energy is transferred or transformed.
• Insulation minimizes undesirable energy transfers.
• In all cooling systems, energy is transferred as heat from one substance to another, leaving the first substance with less energy and thus a lower temperature.
• A refrigerant is a material used to cool an area or an object to a temperature that is lower than the temperature of the environment. During each operating cycle, the refrigerant evaporates into a gas and then condenses back into a liquid.

Heat Engines

• A heat engine is a machine that transforms heat into mechanical energy, or work.
• Internal combustion engines burn fuel inside the engine.
• An automobile engine is a four-stroke engine, because four strokes take place for each cycle of the piston.
• The four strokes are called intake, compression, power, and exhaust strokes.
• Internal combustion engines vary in number of pistons.