Heat and Thermo Dynamics 16.2

Conduction

A 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 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.

Conduction occurs within a material or between materials that are

touching.

In conduction, collisions between particles transfer thermal energy,

without any overall transfer of matter.

Thermal Conductors A thermal conductor is a material that conducts thermal

energy well.

Metals are good thermal conductors.

When a frying pan is on a hot stove, the bottom of the metal

pan heats first and the metal handle last. The flames do not

directly heat the handle.

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.

Thermal Insulators A material that conducts thermal energy poorly is called a

thermal insulator.

o 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.

A convection current occurs when a fluid circulates in a loop as it

alternately heats up and cools down.

o Air at the bottom of an oven heats up, expands, and becomes

less dense. The hot air rises.

o Rising hot air cools as it moves away from the heat source.

o 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.

o When you stand to the side of a charcoal grill, heat reaches you

without convection or conduction.

o The sun warms you by radiation on a clear day. The space

between the sun and Earth has no air to transfer thermal energy.

o Heat lamps used in restaurants are another example of radiation.

o The heating coil on a stove radiates thermal energy.

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. Joule's

system included a falling weight that turned a paddle wheel in a container of water.

Joule found that the work done by the falling weight almost exactly equaled the thermal energy

gained by the water.

First Law of Thermodynamics The first law of thermodynamics states that

Energy cannot be created or destroyed, but it can be converted into different forms.

o Added energy increases the thermal

energy of a system or does work on the

system. In either case, energy is conserved.

Second Law of Thermodynamics 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.

o A refrigerator must do work to transfer thermal energy from the cold food

compartment to the warm room air.

o The thermal energy is released by coils at the bottom or in the back of the

refrigerator.

A heat engine is any device that converts heat into work.

The efficiency of a heat engine is always less than 100 percent.

Thermal energy that is not converted into work is called waste heat. Waste heat is lost to the surrounding environment.

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.

Third Law of ThermodynamicsThe efficiency of a heat engine increases with a greater difference between the high

temperature inside and the cold temperature outside the engine.

A heat engine could be 100 percent efficient if the cold outside environment were at

absolute zero (0 Kelvin). This would violate the third law of thermodynamics.

The third law of thermodynamics sta tes that absolute zero cannot be reached.