Section 2

Heat and

Thermodynamics

Key Concepts

• Why is conduction slower in gases than in liquids or solids?

• In what natural cycles do convection currents occur?

• How does an object’s temperature affect radiation?

• What are the three laws of thermodynamics?

Conduction

• Conduction is the transfer of thermal energy with no overall transfer of matter.

• Conduction occurs within a material or between materials that are touching.

• Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often.

Conduction is the transfer of thermal energy without transferring matter. This device, called Newton’s cradle, helps to visualize conduction. After one ball strikes the rest, most of the kinetic energy is transferred to one ball on the end.

Thermal Conductors

• A thermal conductor is a material that conducts thermal energy well.

• A wire rack in a hot oven can burn you because the metal conducts thermal energy so quickly. Pots and pans often are made of copper or aluminum because these are good conductors.

The arrows show howthermal energy is conducted awayfrom the heat source in a metalfrying pan

Thermal Insulators

• Why is it safe to pick up the wooden spoon

• Wood heats up slowly because it is a poor conductor of thermal energy

• A material that conducts thermal energy poorly is called a thermal insulator.

Convection

• Convection is the transfer of thermal energy when particles of a fluid or gas move from one place to another.

• A convection current occurs when a fluid circulates in a loop as it alternately heats up and cools down.

• Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior.

The arrows show convectionof air in an 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.

A heating coil on astove radiates thermal energy.The changing color of the redarrows indicates that the fartheryou are from the coil, the lessradiation you receive.

Thermodynamics

• The study of conversions between thermal energy and other forms of energy is called thermodynamics.

First Law of Thermodynamics

• The first law of thermodynamics states that 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.

You can consider the bicycle pump, thetire, and the air inside of both to be a system. The person does work on the system by pushing on the pump. Some of the work is converted into thermal energy, which heats the air in the pump and the tire.

Third Law of Thermodynamics

• The third law of thermodynamics states that absolute zero cannot be reached.

The third law ofthermodynamics states thatabsolute zero cannot be reached.This physicist is adjusting a laserused to cool rubidium atoms to3 billionths of a Kelvin aboveabsolute zero. This record lowtemperature was producedby a team of scientists at theNational Institute of Standardsand Technology.

Reviewing Concepts

• 1. Why is conduction in gases slower than conduction in liquids or solids?

• 2. Give three examples of convection currents that occur in natural cycles.

• 3. What happens to radiation from an object as its temperature increases?

• 4. State the first law of thermodynamics.• 5. In your own words, what is the second law of

thermodynamics?• 6. State the third law of thermodynamics.• 7. Why does a metal spoon feel colder than a wooden

spoon at room temperature?• 8. Why is solar energy transferred to Earth by radiation?