PHYS 12A Lecture

What is thermal energy?

The total microscopic energy associated with the atoms and molecules in an object.

What two factors generally affect an object's thermal energy?

The amount of matter in the object and its temperature.

How can energy be transferred to an object?

By performing work on it or transferring heat to it.

What is temperature?

A measure related to the average kinetic energy of the particles in a substance.

What is the difference between thermal energy and temperature?

Temperature describes average particle kinetic energy, while thermal energy depends on both temperature and the amount of matter.

Can a colder object have more thermal energy than a warmer object?

Yes. A cold lake can have more thermal energy than a warm person because the lake has far more matter.

What are the freezing and boiling points of water on the Celsius scale?

Water freezes at 0°C and boils at 100°C.

What are the freezing and boiling points of water on the Fahrenheit scale?

Water freezes at 32°F and boils at 212°F.

What is absolute zero?

The lowest possible temperature, where particle motion reaches its minimum possible value.

What is absolute zero in Celsius?

Approximately -273°C.

What is absolute zero in kelvin?

0 K.

How do you convert Celsius to kelvin?

T(K) = T(°C) + 273.15.

At what temperatures does water freeze and boil in kelvin?

Water freezes at approximately 273 K and boils at approximately 373 K.

Does the kelvin scale use degree symbols?

No. Write K, not °K.

What is heat?

The transfer of thermal energy between objects caused by a temperature difference.

What symbol represents heat?

Q.

In what direction does heat naturally flow?

From an object at higher temperature to an object at lower temperature.

When does net heat transfer stop?

When the objects reach the same temperature, called thermal equilibrium.

What is thermodynamics?

The study of thermal energy, heat, work, and energy transfer.

What are the three methods of heat transfer?

Conduction, convection, and radiation.

What is conduction?

Heat transfer through interactions and collisions between neighboring particles.

What materials are usually good thermal conductors?

Metals.

What is a thermal conductor?

A material through which heat moves easily.

What is a thermal insulator?

A material that resists the transfer of heat.

What materials are generally good thermal insulators?

Wood, air, gases, and many liquids.

Why are oven mitts and wooden pan handles useful?

Their low thermal conductivity slows heat transfer to your hand.

What is the law of conduction?

Q/Δt = kA(Thigh - Tlow)/L.

What does k represent in the conduction equation?

The material's thermal conductivity, measured in W/(m·K).

What does A represent in the conduction equation?

The cross-sectional area through which heat flows.

What does L represent in the conduction equation?

The thickness or distance through which heat must travel.

How does thermal conductivity affect the conduction rate?

A larger k produces faster heat transfer.

How does area affect the conduction rate?

A larger area produces faster heat transfer.

How does thickness affect the conduction rate?

A greater thickness produces slower heat transfer.

How does temperature difference affect the conduction rate?

A larger temperature difference produces faster heat transfer.

What is the thermal-resistance relationship shown in the lecture?

R = L/k; a thicker material or lower conductivity gives greater resistance.

What is convection?

Heat transfer through the bulk motion of a fluid, such as air or water.

Why does warm fluid usually rise?

Heating causes the fluid to expand and become less dense than the cooler surrounding fluid.

What is a convection current?

Circulation in which warm, less-dense fluid rises while cool, denser fluid sinks.

What causes a daytime sea breeze?

Land heats faster than water, so warm air over land rises and cooler air moves inland from the sea.

What happens to the direction of the sea breeze at night?

Land cools faster than water, so air circulation reverses and surface air moves from land toward the sea.

What is radiation?

Heat transfer by electromagnetic waves; it does not require matter or direct contact.

What are examples of electromagnetic radiation?

Visible light, infrared, ultraviolet, and radio waves.

How does energy from the Sun reach Earth?

By radiation through space.

What is true of good emitters of radiation?

Good emitters are also good absorbers.

Why do black objects heat strongly in sunlight?

They are effective absorbers of electromagnetic radiation.

What is emissivity?

A number ε from 0 to 1 that measures how effectively a surface emits thermal radiation.

What is the Stefan-Boltzmann radiation equation?

Q/Δt = εσA(Tobj⁴ - Tenv⁴).

What is the Stefan-Boltzmann constant?

σ = 5.670 × 10⁻⁸ W/(m²·K⁴).

What temperature scale must be used in the Stefan-Boltzmann equation?

Kelvin.

When is the net radiation rate positive in Q/Δt = εσA(Tobj⁴ - Tenv⁴)?

When the object is hotter than its environment.

What is one mole?

An amount containing Avogadro's number of particles.

What is Avogadro's number?

NA = 6.02 × 10²³ particles/mol.

How are number of particles and number of moles related?

n = N/NA, or N = nNA.

What is molar mass?

The mass of one mole of a substance.

How are sample mass, molar mass, and moles related?

n = msample/Mmol.

What is the ideal-gas law?

PV = nRT.

What does R = 8.31 J/(mol·K) represent?

The universal gas constant.

What assumptions are made for an ideal gas?

The particles occupy negligible volume and do not significantly interact with one another.

What temperature scale must be used in the ideal-gas law?

Kelvin.

What is the microscopic form of the ideal-gas law?

PV = NkBT.

What is Boltzmann's constant?

kB = 1.38 × 10⁻²³ J/K.

How are R, kB, and Avogadro's number related?

R = NAkB, so NA = R/kB.

What is the continuity equation?

A₁v₁ = A₂v₂.

What happens to fluid speed when a pipe becomes narrower?

The speed increases to maintain a constant volume flow rate.

What is Bernoulli's equation?

p₁ + ½ρv₁² + ρgh₁ = p₂ + ½ρv₂² + ρgh₂.

According to Bernoulli's equation, what happens to pressure when speed increases at the same elevation?

Pressure decreases.

According to Bernoulli's equation, what happens to pressure when elevation increases with no other compensation?

Pressure decreases.

Why did the water level decrease when a sinking iron mass was moved off a floating wooden block and placed in the water?

On the block, the iron required displacement equal to its weight; when submerged, it displaced only its smaller physical volume.

What is a common mistake when solving thermal-energy problems?

Assuming the object with the higher temperature must have more total thermal energy without considering its mass.

What is a common mistake in radiation and ideal-gas calculations?

Using Celsius instead of absolute temperature in kelvin.