Heat Transfer & Greenhouse Effect – Lecture Review

Question-and-answer flashcards covering conduction, convection, radiation, surface properties, and the greenhouse effect.

What are the three primary mechanisms of heat transfer?

Conduction, convection, and radiation.

At the molecular level, how does heat transfer occur during conduction?

Faster, higher-temperature molecules collide with slower, cooler molecules, transferring kinetic energy from the hot region to the cold region.

Why are metals generally excellent thermal conductors?

They contain many free electrons that roam through the lattice and rapidly carry energy from hot to cold regions.

Why are materials such as wood and cloth poor conductors of heat?

They lack free electrons, so energy transfer relies only on slower atom-to-atom collisions.

What name is given to a material that resists the flow of heat?

A thermal insulator.

How does the phase of a substance influence its thermal conductivity?

Gases are poorest conductors, liquids and solids conduct better, and metals (solid) conduct best because their particles are closer together and— in metals— free electrons aid transfer.

Define thermal conductivity.

A material property that quantifies how readily heat is conducted; higher thermal conductivity means more efficient heat transfer.

What is convection?

Heat transfer by the bulk movement of a fluid caused by density differences that arise from temperature gradients.

Why does warm air or fluid rise during natural convection?

Heating causes it to expand, lowering its density relative to the surrounding cooler fluid, so buoyancy forces it upward.

Give an atmospheric example of natural convection.

Sea breezes (day) and land breezes (night) produced by differential heating of land and water.

What is forced convection?

Convection enhanced by an external device—such as a pump or fan—that moves the fluid to improve heat transfer.

How does a car’s cooling system illustrate forced convection?

A pump circulates hot radiator fluid through the engine, while a fan draws cooler air through the radiator, carrying heat away from the fluid.

In heat transfer, what is radiation?

The transfer of energy by electromagnetic waves (e.g., visible light, infrared) without the need for a material medium.

Can radiant heat transfer occur through a vacuum?

Yes; radiation does not require matter, so it can cross empty space (e.g., sunlight reaching Earth).

Roughly at what temperature does a solid begin to glow red-hot?

≈1000 K.

What is a blackbody?

An ideal surface that absorbs (and therefore can emit) all incident electromagnetic radiation, regardless of wavelength.

How does a black, rough surface compare with a shiny silver surface in absorbing sunlight?

Black, rough surfaces absorb about 97 % of incident radiation; shiny silver surfaces absorb only ~10 % and reflect the rest.

Why do dark-colored clothes feel hotter than light-colored clothes in sunshine?

Dark fabric absorbs more solar radiation and re-emits half of it inward toward the body, increasing perceived warmth.

When an object is in thermal equilibrium with its surroundings, how are radiant absorption and emission related?

The rate of energy absorbed equals the rate emitted, so the object’s temperature remains constant.

What is the greenhouse effect?

Partial trapping of Earth’s outgoing infrared radiation by atmospheric gases, causing the planet’s surface temperature to rise.

Which two atmospheric gases most strongly contribute to the natural greenhouse effect?

Water vapor and carbon dioxide.

Without the greenhouse effect, what would Earth’s average surface temperature be?

About 250 K (–23 °C).

Approximately how much warmer does the greenhouse effect make Earth?

Around 40 K.

How have atmospheric CO₂ levels changed in the past century, and why is this significant?

They have risen by more than 20 % (about 4 % per decade) mainly due to burning fossil fuels, potentially leading to several-kelvin global warming and climate impacts.